FUNDAMENTALS OF OIL AND GAS PROCESSING
The oil and gas industry is usually divided into three major sectors: upstream, midstream and
downstream. The upstream sector includes the searching for potential underground crude oil and
natural gas fields, drilling of exploratory wells, and subsequently drilling and operating the wells
that recover and bring the crude oil and/or raw natural gas to the surface. The upstream segment
of the oil and gas industry contains exploration activities, which include creating geological
surveys and obtaining land rights, and production activities, which include onshore and offshore
drilling.
The midstream sector involves the transportation (by pipeline, rail, barge, oil tanker or truck),
storage, and wholesale marketing of crude or refined petroleum products. Pipelines and other
transport systems can be used to move crude oil from production sites to refineries and deliver
the various refined products to downstream distributors.
The downstream sector commonly refers to the refining of petroleum crude oil and the
processing and purifying of raw natural gas as well as the marketing and distribution of products
derived from crude oil and natural gas. The downstream sector touches consumers through
products such as gasoline or petrol, kerosene, jet fuel, diesel oil, heating oil, fuel oils, lubricants,
waxes, asphalt, natural gas, and liquefied petroleum gas.
Exploration and production together is referred to as E&P. Exploration is about finding
underground reservoirs of oil and gas (oil and gas fields), and includes structural geology studies,
prospecting, seismic surveys, drilling activities that take place before the development of a field
is finally decided.
Determination of the number and location of reservoirs, types of wells, assessment of oil
recovery mechanism, design of wells to meet production requirements, process facilities,
infrastructure facilities, terminal/export facilities, and operating and maintenance strategies is
done in the development stage.
Production is the process of producing the discovered petroleum using drilled wells through
which the reservoir’s fluids (oil, gas, and water) are brought to the surface and separated. In fact,
bringing the well fluids to the surface and preparing them for use in refinery or processing plants
are called production.
Hydrocarbon fluids and gases are processed and separated into marketable products or feedstock
for the petrochemical industries in crude-oil refineries and gas-processing plants. More than
2500 refined products are generally produced from crude oil in the petroleum refining industry.
Service providers provide storage facilities at terminals throughout the oil and gas distribution
systems. These facilities are most often located near producing, refining, and processing facilities
and are connected to pipeline systems to facilitate shipment when product demand must be met.
While petroleum products are held in storage tanks, natural gas tends to be stored in underground
facilities. In the other words, storage is used by all sectors of the petroleum industry. Liquid
petroleum products may be stored in above-ground or underground steel or concrete tanks or in
underground salt domes, mined caverns, or abandoned mines.
Crude oil and gas are transported to processing facilities and from there to end users by pipeline,
tanker/barge, truck, and rail. Pipelines are the most economical transportation method and are
most suited to movement across longer distances, e.g. across continents.
Tankers and barges are also employed for long-distance transportation, often for international
transport. Rail and truck can also be used for longer distances but are most cost-effective for
shorter routes.
Figure 1: Crude oil transportation by barge
Crude oil is categorized using two qualities: Density and sulfur content.
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Density is measured by API gravity, and ranges from light (high API gravity/low density)
to heavy (low API gravity/high density).
Sulfur content ranges from sweet (low sulfur content) to sour (high sulfur content).
Light and sweet crude oil is usually priced higher, and therefore more sought-after, because it is
easier to refine to make gasoline than heavy and sour crude oil. Oil volume is measured in
barrels (bbl), which equals 42 gallons. Western Uganda has approximately 6.5 billion barrels of
oil reserves, with at least 1.4 billion estimated to be economically recoverable. China National
Offshore Oil Corporation (CNOOC) and Total Energies co-own all of Uganda's existing oilfields
alongside the state-run Uganda National Oil Company (UNOC). At peak, Uganda plans to
produce about 230,000 barrels of crude oil per day.
Natural gas is found in both associated formations, meaning it is formed and produced with oil,
and non-associated reservoirs. Gas can either be dry (pure methane), or wet (exists with other
hydrocarbons like butane). Although wet gas must be treated to remove the other hydrocarbons
and other condensates before it can be transported, it can increase producers' revenues because
they can sell those removed products.
In the petroleum industry, locating underground or underwater oil reserves characterizes the
upstream process. The upstream process in petroleum involves bringing oil and gas to the
surface. Extraction wells represent an example of a structure operating in this stage in the
process. Upstream is a term for the operations stages in the oil and gas industry that involve
exploration and production. Upstream firms deal primarily with the exploration and initial
production stages of the oil and gas industry.
Oil and gas wells produce a mixture of hydrocarbon gas, condensate or oil; water with dissolved
minerals, usually including a large amount of salt; other gases, including nitrogen, carbon
dioxide (CO2), and possibly hydrogen sulfide (H2S); and solids, including sand from the
reservoir, dirt, scale, and corrosion products from the tubing. The purpose of oil and gas
processing is to separate, remove, or transform these various components to make the
hydrocarbons ready for sale.
For the hydrocarbons (gas or liquid) to be sold, they must be:
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Separated from the water and solids
Measured
Transported by pipeline, truck, rail, or ocean tanker to the user
Oil and gas production is one of the most capital intensive industries: It requires expensive
equipment and highly skilled labors. Once a company identifies where oil or gas is located, plans
begin for drilling. Many oil and gas companies contract with specialized drilling firms and pay
for the labor crew and rig day rates.
Drilling depths, rock hardness, weather conditions and distance of the site can all affect the
drilling duration. Tracking data using smart technologies can help with drilling efficiency and
well performance by providing real-time information and trends. While every drilling rig has the
same essential components, the drilling methods vary depending on the type of oil or gas and the
geology of the location.
Oil and gas process overview
1. Facilities; Offshore and onshore.
2. Main process sections; Wellheads, Manifolds/gathering, Separation, Gas compression,
Metering, storage and export.
3. Utility systems; these are systems which does not handle the hydrocarbon process flow,
but provides some utility to the main process safety or residents.
OIL EXPLORATION
Introduction:
*Origin and formation of crude
*Composition and types of crude
*Petroleum and its products
*Oil formations and conditions
The practice of locating or identifying the area of available sources of natural gas and petroleum
is called exploration. Hydrocarbon exploration is the search by petroleum geologists and
geophysicists for deposits of hydrocarbons, particularly petroleum and natural gas, in the Earth
using petroleum geology.
Petroleum literally means rock oil. It is a natural organic compound mainly composed of HC
occurs either in gaseous or liquid state. The liquid part obtained after the removal of dissolved
gases commonly referred as crude oil. Many theories have suggested to explain the origin and
formation of petroleum/crude in nature.
They were listed as:
*Carbide theory
*Engler’s theory
*Modern theory
Theories of petroleum:
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Carbide or inorganic theory: Petroleum is produced inside the earth by the action of water
on metallic carbides. First lower HC were produced and by hydrogenation and
polymerization higher HC were produced.
ENGLER’S Theory: (animal origin) It is produced by the decomposition of marine animals
under high pressure and temperature.
Modern theory: (Vegetable origin) It is from biogenic origin, they are produced by a mother
substance kerogen or shales.
OIL FORMATION
Accumulation of organic material and its environmental conditions and good heating event and
trapping mechanism give the capability of oil formation. Three components were needed for oil
formation:
*Source & Source rock *Migration path *Reservoir rock *Trap *Cap rock (seal)
And mainly kerogens are present in cap rocks (limestone, sandstone, shales) and so they are
found in sedimentary rocks.
COMPOSITION AND TYPES OF CRUDE
They contain (hydrocarbon) HC and non-HC compounds:
*HC compounds includes paraffins, napthenes, aromatics, asphaltic, olefins.
*Non- HC includes sulphur, nitrogen, oxygen, and some other metals.
Crudes are grouped into:
(i) Paraffinic base (ii) Hybrid or naphthenic base (iii) Asphaltic base (iv) Intermediate base.
• Petroleum is a mixture of various hydrocarbons and varies in colors from light brown to dark
brown or black.
• Its products were classified on three basis: light, middle and heavy distillates.
LIGHT DISTILLATES
*LPG (natural gas, associated and dissolved gas)
*Naphthas
*Gasoline (petrol or motor sprit).
MIDDLE DISTILLATES:
*Kerosene
*Diesel
*ATF (Aviation Turbine Fuel)
*Fuel oils, lube oils.
HEAVY DISTILLATES or RESIDUES:
*Petroleum wax
*Bitumen
*Petroleum coke
EXPLORATION TECHNIQUES:
*Geological methods
*Geophysical methods
*Geochemical methods
1. Geological methods:
*Remote sensing
*Subsurface mapping
2. Geophysical techniques:
*Gravity : Respond to density
*Electrical : Respond to resistivity
*Magnetic : Respond to susceptibility
*Seismic : Respond to velocity
3. Geochemical techniques:
*Direct method
*Indirect method
The hunt for the HC is focused at the favorable or promising areas based on geological
consideration.
1. Geological Methods:
*Sub surface mapping: Selection and mapping of promising areas which satisfy the criteria of
promising areas which satisfy the criteria of being sedimentary rocks of marine origin with the
presence of anticline structure of Mesozoic, Cenozoic and Paleozoic periods. They are expressed
in terms of known geographic locations, stratigraphic markers, and rock composition at the
depth. It is given by GIS (Geographic Information system) technology. Contour maps are widely
used in this type.
*Remote sensing: The scanning of the earth by satellite or high-flying aircraft in order to obtain
information about it. Remote-sensing data are highly useful for the oil and gas industry. Remote
sensing has proven to be an integral tool for downstream and upstream oil and gas operations
through evaluation of infrastructure for well-site planning and for exploration through largescale regional reconnaissance. Spectral analysis is a key form of processing to evaluate for
surface outcrops and surface hydrocarbon.
2. Geophysical techniques:
*Gravimetric survey: It is done by gravimeter. It measures the gravitational field and this
reading correlates with the density of the region. It measures gravity in the units of acceleration
called Milligan. It provides the information regarding underlying formation of oil, salt domes,
basin shape and sedimentary thickness.
*Electrical Survey: The accuracy is mainly dependent on geological conditions. It is suitable for
wells drilled with fresh mud.
Types: *Electrical resistivity topography (ERT) * Self Potential method (SP) * Induced
polarization (IP)
*Magnetic survey: It is done by air-borne magnetometer either in ground or air.
Principle: magnetic attraction on the surface depends on the magnetic intensities of the rocks and
their distance from the surface. Aim: To locate the sedimentary rocks. They have low magnetic
properties than the other rocks.
*Seismic survey: It is conducted by sound waves. It indicates the nature of the rock and angle of
dip. It is used in onshore to detect the echoes returning from the ground sent to sea by geophones
and hydrophones.
3. Geochemical techniques:
• Direct method: It involves the presence of dispersed oil components in the form of HC or
bitumen in the soils, waters and rocks in the vicinity of oil and gas accumulations.
Types: * Gas logging * Bitumen survey * Hydro chemical techniques
• Indirect method: It is based on the detection of any chemical, physical or microbiological
changes in the soils, waters, or rocks associated with the oil and gas deposits.
Types: * Soil-salt method * Oxidation and reduction potential method * Microbiological method
* Hydro chemical technique
Petroleum is found in sedimentary rocks under high pressure but at low temperatures. If the
hydrocarbon index (HI) is lesser than 0 the oil is present if HI>0 then no oil is present. Long
chain HC are bitumen & Short chain HC are gas & medium chain HC are paraffin. Age of oil is
indicated by CPI (Carbon Preference Index)
OIL AND GAS PRODUCTION FACILITY
An oil and gas production facility is used in production process of fluids from oil wells in order
to separate out key components and prepare them for export. Typical oil well production fluids
are a mixture of oil, gas and produced water. An oil production plant is distinct from an oil
depot, which does not have processing facilities. Oil production facility may be associated with
onshore or offshore oil fields.
Onshore means on land. In the oil and gas industry, any exploration and production work done
on land with land equipment is said to be onshore. Offshore means off land, which means on the
water. Any exploration and production work done on water with marine equipment is then said
to be offshore.
a. Onshore
Onshore refers to processes that take place on land that are associated with oil, gas or condensate
production that has taken place offshore. The offshore production facility delivers oil, gas and
condensate by pipelines to the onshore terminal and processing facility. Alternatively oil may be
delivered by ocean-going tanker to the onshore terminal. The configuration of onshore oil
production facilities depends on the size of the oil field.
For simple fields comprising a single well or a few wells, an oil storage tank may be sufficient.
The tank is emptied periodically by road tanker and transferred to an oil refinery. For larger
production rates a rail tanker transfer facility may be appropriate. For larger fields a full threephase processing facility is required. Three-phase separators separate the well fluids into its three
constituent phases: oil, gas and produced water. Oil may be transferred by road or rail tanker or
by pipeline to an oil refinery. Gas may be used on the site to run gas engines to produce
electricity or can be piped to local users. Excess gas is burned in a ground flare. Produced water
may be re-injected into the reservoir.
In onshore drilling facilities, the wells are grouped together in a field, ranging from a half acre
per well for heavy crude oil to 80 acres per well for natural gas. The group of wells are
connected by carbon steel tubes which sends the oil and gas to a production and processing
facility where the oil and gas are treated through a chemical and heating process. Onshore
production companies can turn on and off rigs more easily than offshore rigs to respond to
market conditions.
Figure 2: Onshore facility
Onshore oil terminals may include large crude oil tanks for the initial storage of oil prior to
processing. Such tanks provide a buffer volume where oil is delivered by tanker. The oil tanker
delivery rate is considerably greater than the processing capacity of the plant. Crude oil tanks
also allow offshore production to continue if the export route becomes unavailable.
Onshore oil terminals generally have fired heaters to heat the oil to improve subsequent
separation. Separator vessels and coalescers stabilize the crude and remove any sediments,
produced water and allow light hydrocarbons to flash-off. Large separation vessels give the oil
an appropriate residence time in the vessel to allow effective separation to occur.
Onshore separators operate at near atmospheric pressure to release as much vapor as possible.
The oil processing plant aims to achieve an appropriate vapor pressure specification for the oil.
The associated gas is processed for export or used in the plant as fuel gas. Stabilized oil is routed
to storage tanks prior to dispatch for international sales delivery by tanker, or to a local oil
refinery for processing.
Advantages of onshore facility

Onshore drilling is cost-effective
While there are, in fact, some economic benefits to offshore drilling, the hard facts are that it
could take years for a site to actually get a drilling facility set up in the middle of the ocean. This
alone takes a big cut out of facility budgets.
With onshore drilling, shales are readily available, and equipment can easily be moved from site
to site with the help of skids and other resources that make drilling sites flexible and mobile. This
cuts down on the shipping and installation costs that would otherwise be much higher for an
offshore project.
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Onshore simply provides greater production results
The facts are that onshore production is a process that produces fabulous results for the oil and
gas industry. The reason for this is that resources are just more plentiful on land. Currently, 70
percent of the world’s oil and gas comes from onshore sites, with 30 percent being produced
offshore. Even though offshore sites can be a source for fuel, there is clearly a method between
the two that is more reliable.
b. Offshore
Offshore drilling is a mechanical process where a wellbore is drilled below the seabed. It is
typically carried out in order to explore for and subsequently extract petroleum that lies in rock
formations beneath the seabed. Most commonly, the term is used to describe drilling activities on
the continental shelf, though the term can also be applied to drilling in lakes, inshore waters and
inland seas. Offshore drilling presents environmental challenges, both offshore and onshore from
the produced hydrocarbons and the materials used during the drilling operation.
Offshore drilling uses a single platform that is either fixed (bottom supported) or mobile (floating
secured with anchors). Offshore drilling is more expensive than onshore drilling, and fixed rigs
are more expensive than mobile rigs. Most production facilities are located on coastal shores
near offshore rigs.
In the Oil & Gas industry, a Central Processing Facility (CPF) is part of the upstream process
that first transforms crude oil or raw natural gas after it exits the production wells. The CPF
separates oil from the gas, water, sand, solvents or additives it may contain.
Figure 3: Offshore oil and gas facility in Canada
Task: Which oil facility is the Tilenga project set to have?
Offshore oil drilling involves the extraction of fossil fuels from the earth’s crust below the ocean
or seabed. That’s because the ocean bed is rich in oil wells. However, locating the offshore oil
reserves and installing an oil rig on the water surface can be daunting. As a result, it makes
offshore drilling costly. An oil rig is a structure above an oil well on land or in the sea that has
special equipment attached to it for drilling and removing oil from the ground also called oil
platform.
Factors affecting the decision to recover offshore oil and gas
1. Physical Factors
 Ocean Related Factors
 Climate/Weather Factors
 Oil Related Factors
 Environmental Protection Factors
Ocean Related Factors
a) Ocean Depth
b) Ocean Currents
c) Icebergs
d) Pack Ice.
Climate/Weather Factors
a) Wind Speeds
b) Storms.
Oil Related Factors
a) Size of Reserve
b) Oil Quality.
Environmental Protection Factors
a) How does the other physical factors affect the chances of an oil spill?
b) Other resources like fish stocks, marine mammals and spawning grounds that may be affected
by an oil spill.
2. Human Factors
 Worker Safety
 Financial Factors
Worker Safety
a) How safe can the drill rig and production platform be for the workers?
Financial Factors
a) Cost of inputs like building a rig to withstand icebergs or building a rig to drill at great depths.
b) Cost of processes like transporting the oil from offshore to land, or maintaining the platforms
equipment.
c) Price of oil set by world markets.
Economic importance of off-shore oil and gas operations
Our Life style depends on energy (Oil and Gas being dominant). Oil is valued as a fuel because it
produces large amounts of heat and power per unit of mass. It is relatively easy to store, move,
and convenient as a source of energy for transportation. Also, oil is a raw material that can be
processed into refined products.
Factors affecting viability of off-shore oil industry

World oil prices:
High prices, encourage companies to develop oil production. Low prices discourage companies
due to reduced profit.
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Government policy:
Tax breaks encourage companies to develop oil production. Tax increases discourage companies
due to reduced profit.
Advantages of offshore oil drilling.
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Increases Oil Production: Since the advent of offshore drilling technology, petroleum
production has grown to sustain the rising demand.
Promotes Energy Independence: With offshore drilling, many foreign countries can now
explore the ocean for oil and gas, promoting self-reliance.
Encourages Economic Growth: Third-world nations bordering oceans can explore their
oceanic oil reserves and grow.
The drawbacks of offshore oil drilling.
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Costly and Potentially Dangerous: Offshore oil drilling is expensive as it requires
sophisticated equipment. Also, it poses significant risks to workers.
Numerous Environmental Damages: Offshore oil drilling generates mass pollution,
primarily during oil spills. If not cleared, the spills can harm marine life.
Since offshore oil drilling involves many risks, it’s essential to prioritize safety. One way to
mitigate risks at offshore drilling sites is installing blowout preventers (BOPs). A blowout
preventer is a specialized valve or similar mechanical device, used to seal, control and monitor
oil and gas wells to prevent blowouts, the uncontrolled release of crude oil or natural gas from a
well. They are usually installed in stacks of other valves.
DRILLING RIGS
A drill rig is a machine that consists of all the parts necessary to bore a pilot hole into the earth's
crust, which can then be enlarged to extract product. Drill rigs are used to locate and extract
water, oil, gas or any other product from the earth. They can be used onshore or offshore and are
configured to match the product and environment in which they operate.
A drilling rig is a device used to drill, case and cement water, oil and gas wells. The selection
and sizing of a drilling rig depends on the following:
1. The well design.
2. The anticipated loads during drilling, tripping, casing and ultimately testing and completing
the well (The maximum hole depth).
3. Comparison of the rating of the existing rigs with the well design loads (Rig availability).
4. Selection of the appropriate rig and its components (Horse power requirements & cost).
Types of drilling rig
• Onshore (land) rigs. They can drill from few to several thousand meters (1000-7500 m).
• Offshore rigs. Offshore rigs are of five basic types; each type is designed to suit a specific
offshore environment.
OFFSHORE RIGS
An offshore rig is a large structure on or in water with facilities to drill wells, to extract and
process oil and natural gas, and to temporarily store product until it can be brought to shore for
refining and marketing. In many cases, the platform contains facilities to house the workforce as
well. These are further classified into five types namely:
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Barge
Jack-Up
Fixed Platform
Submersible & Semi-Submersible
Drillship
1. Barge
The barge is a shallow draft, flat-bottom vessel equipped as an offshore drilling unit, used
primarily in swampy areas and shallow water. This type of vessel can be found operating in the
swamps of river deltas in West Africa, in coastal areas, and in shallow lakes. It can be towed to
the location and then ballasted to rest on the bottom.
2. Jack-Up
Bottom of the legs rest on the sea floor. Can drill in water depths ranging from few meters up to
120 m. Towed to the well site position by a boat
3. Fixed Platform
Fixed platforms are usually used in development drilling. Several wells can be drilled from the
same platform through multiple conductors. Most fixed platforms are located in water depths of
450 to 600 feet; some are even set in waters 800 feet deep. The standard configuration consists of
a steel jacket pinned to the seabed by long steel piles, surmounted by a steel deck which supports
equipment and accommodation buildings or modules, one or more drilling rigs, and a helicopter
deck. Some platforms are built of reinforced concrete or a combination of steel and concrete.
4. Submersible & Semi-submersible rigs
The submersible rig is floated to the well location and rests on the sea floor when it is drilling.
Semi-submersible rigs are floating rigs that have pontoons and columns that can be flooded with
water to keep the rig stable. Semi- submersible rigs utilize a spread anchoring system of radially
spaced anchors to hold their position. Some semi-submersibles are capable of setting on the sea
floor, in the same manner as submersibles. Some semi-submersible rigs can drill in water depths
over 2200 m.
5. Drillship rigs
These are ships or “floaters” specially constructed or converted for deep water drilling. They
offer greater mobility than jack-up or semisubmersible rigs, but they are not as stable when
drilling. They are capable of drilling in almost any depth of water. Modern ships are fitted with
dynamic positioning equipment which enables them to keep on-station above the borehole. They
have greater storage capacity than other types of rigs. They are often able to drill deeper wells
and operate independent of service and supply ships.
MAIN PROCESS SECTIONS
The oil and gas process is the process equipment that takes the product from the wellhead
manifolds and delivers stabilized marketable products, in the form of crude oil, condensates or
gas. Components of the process also exist to test products and clean waste products such as
produced water. Main process sections include; Wellheads, manifolds/gathering, separation, gas
compressing, metering, storage and export.
1. Reservoir and Wellheads
There are three main types of conventional wells:
 The most common well is an oil well with associated gas.
 Natural gas wells are wells drilled specifically for natural gas, and contain little or no oil.
 Condensate wells are wells that contain natural gas, as well as a liquid condensate. Natural
gas being lighter than air, will naturally rise to the surface of a well. Wellheads
What we have in wellheads
• Crude Oil
• Natural Gas
• Condensates
Crude Oil
Consists of up to 200 or more different organic compounds, mostly hydrocarbons. The higher the
API number, expressed as degrees API, the less dense (lighter, thinner) the crude. The lower the
degrees API, the more dense (heavier, thicker) the crude. Crude oil API gravities typically range
from 7 to 52 corresponding to about 970 kg/m3 to 750 kg/m3, but most fall in the 20 to 45API
gravity range. Crude oil has a specific weight of 790 to 970 kg per cubic meter.
The chemical composition is generalized by the carbon number. Though the heavy stock and the
light stock could be mixed to produce a blend with the same API gravity as the medium stock.
Heavy crude can be processed in a refinery by cracking and reforming that reduces the carbon
number to increase the high value fuel yield.
Natural gas
The natural gas used by consumers is composed almost entirely of methane. Raw natural gas
comes from three types of wells: oil wells, gas wells, and condensate wells.
This gas can exist separate from oil in the formation (free gas). Dissolved in the crude oil
(dissolved gas). Natural gas from gas and condensate wells (non-associated gas).
Natural gas processing consists of separating all of the various hydrocarbons and fluids from the
pure natural gas.
Condensates
Ethane, propane, butane, and pentanes must be removed from natural gas and it is not mean a
waste product. Include hydrocarbons, known as 'natural gas liquids' (NGL). NGL include ethane,
propane, butane, iso-butane, and natural gasoline.
Uses like; raw materials for oil refineries or petrochemical plants, as sources of energy, and for
enhancing oil recovery in oil wells. Condensates are also useful as diluent for heavy crude.
The reservoir
Oil and gas deposits form as organic material, 100 to 200 million years ago. Porous rock needs to
be covered by a non-porous layer. The hydrocarbons migrates out of the deposits and upward in
porous rocks and collects in crests under the non-permeable rock by the tectonic movement.
A young reservoir (e.g. 60 million years) often has heavy crude, less than 20 API. Seismic data
and advanced visualization 3D models are used to plan the extraction. Still the average recovery
rate is 40%, leaving 60% of the hydrocarbons trapped in the reservoir. The best reservoirs with
advanced Enhanced Oil Recovery (EOR) allow up to 70%. Modern wells are drilled with large
horizontal offsets to reach different parts of the structure
Exploration and Drilling
The main components of the drilling rig are the Derrick, Floor, Drawworks, Drive and Mud
Handling.
The control and power can be hydraulic or electric. The hydraulic or electric top drive hangs
from the derrick crown and pressure and rotational torque to the drill string. The Drill String is
assembled from pipe segments about 30 meters long. A cone bit is used to dig into the rock.
Different cones are used for different types of rock and at different stages of the well. Typical
values are 50kN force on the bit and a torque of 1-1.5 kN.m at 40-80 RPM for an 8 inch cone.
Wells can be any depth from almost at the surface to a depth of more than 6000meters. The oil
and gas typically formed at 3000-4000 meters depth. To prevent an uncontrolled blow out, a
subsurface safety valve is often installed. Different cones are used for different types of rock and
at different stages of the well.
The well
Completing a well consists of a number of steps.
• Installing the Well Casing
• Installing the wellhead
• Installing lifting equipment or treating the formation should that be required.
Well Casing
Well casing consists of a series of metal tubes installed in the freshly drilled hole. Types of
casing used depend on the subsurface characteristics of the well. Including the diameter of the
well and the pressures and temperatures experienced throughout the well. The casing is normally
cemented in place, tubing is inserted inside the casing. The production casing is typically 5 to 28
cm. Production depends on reservoir, bore, pressure etc. A packer is used between casing and
tubing at the bottom of the well.
Completion
A completion is an operation on an oil and gas well to open up the reservoir to production. An
initial completion occurs after the drilling rig is moved off location. The wellbore is isolated by
casing and cement and/or a packing device preventing any oil or gas from entering the wellbore
and flowing to surface. Consists of deciding on the characteristics of the intake portion of the
well in the targeted hydrocarbon formation. There are a number of types of completions. Open
hole completions. Conventional perforated completions. Sand exclusion completions. Permanent
completions. Multiple zone completion, Drain hole completions.
Wellhead
Wellheads can be Dry or Subsea completion.
• Dry Completion: the well is onshore on the topside structure on an offshore installation.
• Subsea wellheads are located under water on a special sea bed template. Require wellheads that
can withstand a great deal of upward pressure of up to (140 Mpa) from the escaping gases and
liquids.
The wellhead consists of three components. •
i.
ii.
iii.
The casing head
The tubing head
The 'Christmas tree’. Christmas tree composed of a master gate valve, a pressure gauge, a
wing valve, a swab valve and a choke. The casing will be screwed, bolted or welded to
the hanger. Several valves and plugs will normally be fitted to give access to the casing.
The tubing is used to position the tubing correctly in the well. Sealing also allows
Christmas tree removal with pressure in the casing.
A wellhead is the component at the surface of an oil or gas well that provides the structural and
pressure-containing interface for the drilling and production equipment. The primary purpose of
a wellhead is to provide the suspension point and pressure seals for the casing strings that run
from the bottom of the hole sections to the surface pressure control equipment.
While drilling the oil well, surface pressure control is provided by a blowout preventer (BOP). If
the pressure is not contained during drilling operations by the column of drilling fluid, casings,
wellhead, and BOP, a well blowout could occur.
When the well has been drilled, it is completed to provide an interface with the reservoir rock
and a tubular conduit for the well fluids. The surface pressure control is provided by a Christmas
tree, which is installed on top of the wellhead, with isolation valves and choke equipment to
control the flow of well fluids during production.
Wellheads are typically welded onto the first string of casing, which has been cemented in place
during drilling operations, to form an integral structure of the well. In exploration wells that are
later abandoned, the wellhead may be recovered for refurbishment and re-use.
Offshore, where a wellhead is located on the production platform it is called a surface wellhead,
and if located beneath the water then it is referred to as a subsea wellhead or mud-line wellhead.
Subsea wellhead; mechanically they are placed in a Subsea structure (template). Allows the wells
to be drilled and serviced remotely from the surface, and protects from damage. A hydraulic
power unit (HPU) provides hydraulic power to the subsea installation via an umbilical. The
umbilical is a composite cable containing tension wires, hydraulic pipes, electrical power and
control and communication signals.
Wells are also divided into production and injection wells.
• Injection wells is drilled to inject gas or water into the reservoir. Purpose of injection is to
maintain overall and hydrostatic reservoir pressure and force the oil toward the production wells.
• When injected water reaches the production well, this is called injected water break through.
Radioactive isotopes added to injection water, are used to detect breakthrough.
Artificial Lift
Production wells are free flowing or lifted. A free flowing oil well has enough down-hole
pressure to reach a suitable wellhead production pressure. •If the formation pressure is too low,
then the well must be artificially lifted. Larger wells will be equipped with artificial lift to
increase production even at much higher pressures.
Some artificial lift methods are:
• Rod Pumps
• Down-hole Pumps
• Gas Lift
• Plunger Lift
Rod Pumps
Also called donkey pumps or beam pumps. A motor drives a reciprocating beam, connected to a
polished rod passing into the tubing via a stuffing box. The motor speed and torque is controlled
for efficiency and minimal wear with a pump off controller (PoC). Flows up to about 40 liters
(10 gal) per stroke.
Down-hole Pumps
Down-hole pump insert the whole pumping mechanism into the well. An electrical submerged
pump (esp) is inserted into the well. Consisting of a long narrow motor and a multi-phase pump,
such as a pcp (progressive cavity pump) or centrifugal pump. Hangs by an electrical cable with
tension members down the tubing. Down to 3.7 km with power up to 750 kw
Gas Lift
Gas lift injects gas into the well flow. The downhole reservoir pressure falls off to the wellhead
due to the counter pressure from weight of the oil column in the tubing. By injecting gas into this
oil, the specific gravity is lowered and the well will start to flow. Typically gas in injected
between casing and tubing. A release valve on a gas lift mandrel is inserted in the tubing above
the packer.
Plunger Lift
Plunger lift is normally used on low pressure gas wells with some condensate, oil or water, or
high gas ratio oil wells. Liquid starts to collect downhole and eventually blocks gas so that the
well production stops. A plunger with an open/close valve can be inserted in the tubing. Gas,
condensate and oil can pass though the plunger until it reaches bottom.
 Well work over, intervention and stimulation.
This the process of performing major maintenance on an oil or gas well. Include replacement of
the tubing, cleanup. Or new completions, new perforation. And various other maintenance works
such as installation of gas lift mandrels, new packing etc. Well maintenance without killing the
well and performing full workover is time saving and is often called well intervention.
Various operations that are performed by lowering instruments or tools on a wire into the well
are called wireline operations. Work on the reservoir such as chemical injection, acid treatment,
heating etc, is referred to as reservoir stimulation. Acids are used open up calcareous reservoirs
(Marble, limestone, dolomite, etc. are some of the calcium predominant rocks) and to treat
accumulation of calcium carbonates in the reservoir structure around the well.
When the pressure is high enough to open fractures, the process is called fracture acidizing. If the
pressure is lower, it is called matrix acidizing. Well workover, intervention and stimulation.
Hydraulic fracturing is an operation in which a specially blended liquid is pumped down a well
and into a formation. Under pressure high enough to cause the formation to crack open, forming
passages through which oil can flow into the well bore. Sand grains, aluminum pellets, walnut
shells, glass beads, or similar materials (propping agents) are carried in suspension by the fluid
into the fractures. Hydraulic fracturing, or fracking, is a method used to extract natural gas and
oil from deep rock formations known as shale. Using this method, drilling operators force water,
sand, and a mix of chemicals into horizontally drilled wells, causing the shale to crack and
release natural gas or oil.
MANIFOLDS AND GATHERING
Manifolds are used extensively throughout the oil and gas industry for the distribution of gases
and fluids. They are designed to converge multiple junctions into a single channel or diverge a
single channel into multiple junctions. Simple manifold systems typically are used to divide one
supply input to multiple outputs, while more complex systems incorporate integral valves or an
electronic network interface. The specific features of a manifold will largely be determined by
the specific application it is to be used in.
In the oil and gas sector, manifold systems are used within exploration, development and
production phases, particularly in wells using surface testing equipment. They can perform any
number of functions, including:
 Divert oil or gas to surge or gauge tanks for storage or measurement
 Direct flow to a production line, or from the separator to crude oil burner for disposal
 Maintain flow when testing requires that certain equipment be pulled out of service.
Pipelines and risers
This facility uses Subsea production wells. The typical High Pressure (HP) wellhead at the
bottom right, with its Christmas tree and choke, is located on the sea bottom. This line may
include several check valves. Slugging may be controlled manually by adjusting the choke, or
with automatic slug controls. If production is shut down or with long offsets. This may be
prevented by injecting ethylene glycol.
Production, test and injection manifolds
Check valves allow each well to be routed into one or more of several Manifold Lines. There
will be at least one for each process train plus additional Manifolds for test and balancing
purposes. Test, Low Pressure and High Pressure Manifolds. The test manifold allows one or
more wells to be routed to the test separator. The chokes are set to reduce the wellhead flow and
pressure to the desired HP and LP pressures respectively.
Figure 4: Production manifold
SEPARATION
Oil and gas separation is a common technique in upstream oil and gas wells, where crude oil and
natural gas are often found in the same well. Oil and natural gas have different density levels, so
when they mix together it can cause problems down the road. Oil and gas separation allows for
these two products to be distinct from one another.
Oil is less dense than water, so it can float to the top. Generally, natural gas is much less dense
than crude oil. Oil has a specific gravity of 0.85 g/cm3, while natural gas has a specific density of
about 0.55 g/cm3. Gravity separation uses these differences in material to separate out different
products found in one single stream into different streams that are distinct from each other, using
mainly gravitational forces. A separator is a vessel in which a mixture of fluids that are not
soluble in each other are segregated from one another.
In the oilfield, separators are used to segregate gas from liquid; or one liquid, such as crude oil,
from another liquid, such as water.
The oil/gas separators can also classified into;
• Test Separators and Well test
• Production separators
• Second stage separator
• Third stage separator
• Coalescer
• Electrostatic Desalter
• Water treatment
Test Separators and Well test
Used to separate the well flow from one or more wells for analysis and detailed flow
measurement. The behavior of each well under different pressure flow conditions can be
determined. Typically 1-2 months and will measure the total and component flow rates under
different production conditions. The separated components are also analyzed in the laboratory to
determine hydrocarbon composition of the gas oil and condensate. The test separator can also be
used to produce fuel gas for power generation when the main process is not running.
Production separators
The main separators are gravity type. The production choke reduces the pressure to the HP
manifold and First stage separator to about 3-5 MPa (30-50 times atmospheric pressure). Inlet
temperature is often in the range of 100-150 degrees C. The pressure is often reduced in several
stages. Purpose to achieve maximum liquid recovery and stabilized oil and gas, and separate
water. Emergency Valves (EV) are sectioning valves that will separate the process components
and blow-down valves that will allow excess hydrocarbons to be burned off in the flare.
In the first stage separator, the water content is typically reduced to less than 5%. Slug catcher
reduce the effect of slugs. Vortex breakers reduce disturbance on the liquid table inside. The gas
outlets are equipped with demisters.
Second stage separator
The second stage separator is quite similar to the first stage (High pressure) HP separator. It will
also receive production from wells connected to the Low Pressure manifold. The pressure is now
around 1 MPa (10 atmospheres) and temperature below 100 degrees C. The water content will be
reduced to below 2%. An oil heater could be located between the first and second stage separator
to reheat the oil/water/gas mixture. The heat exchanger is normally a tube/shell type where oil
passes though tubes in a cooling medium placed inside an outer shell.
Third stage separator
The final separator here is a two phase separator, also called a flash-drum. The pressure is now
reduced to about atmospheric pressure (100 kPa) so that the last heavy gas components will boil
out. In some processes where the initial temperature is low, heat the liquid before the flash drum
to achieve good separation of the heavy components. There are level and pressure control loops.
Coalescer
The oil go to a coalescer for final removal of water. In this unit the water content can be reduced
to below 0.1%. Inside electrodes form an electric field to break surface bonds between
conductive water and isolating oil in an oil water emulsion. The critical field strength in oil is in
the range 0.2 to 2 kV/cm.
Electrostatic Desalter
If the separated oil contains unacceptable amounts of salts, it can be removed. The salts, which
may be Sodium, Calcium or Magnesium chlorides comes from the reservoir water and is also
dissolved in the oil. The desalters will be placed after the first or second stage separator
depending on Gas Oil Ratio (GOR) and water cut.
Water treatment
A water cut of 40% gives a water production of about 4000 cubic meters per day (4 million
liters) that must be cleaned before discharge to sea. Often this water contains sand particles
bound to the oil/water emulsion.
Sand cyclone
This removes most of the sand. The sand is further washed before it is discharged.
Hydro cyclone
A centrifugal separator that will remove oil. The water is collected in the water de-gassing drum.
Dispersed gas will slowly rise to the surface and pull remaining oil droplets to the surface by
flotation.
GAS TREATMENT AND COMPRESSION
Incoming gas (on the right) is first cooled in a heat exchanger. It then passes through the
scrubber to remove liquids and goes into the compressor. The anti-surge loop (thin orange line)
and the surge valve allows the gas to recirculate.
The components of Gas treatment and Compression
• Heat exchangers
• Scrubbers and reboilers
• Compressor anti surge and performance
• Gas treatment
Heat exchangers
For the compressor operate in an efficient way, the temperature of the gas should be low. When
gas is compressed, it must remain in thermodynamic balance. Plate heat exchangers consist of a
number of plates where the gas and cooling medium pass between alternating plates in opposing
directions. Tube and shell exchangers place tubes inside a shell filled with of cooling fluid.
Scrubbers and reboilers
The separated gas may contain mist and other liquid droplets. A scrubber is designed to remove
small fractions of liquid from the gas. A scrubber is based on dehydration by absorption in Tri
Ethylene Glycol (TEG). The glycol is recycled by removing the absorbed liquid, this is done in
the reboiler. For higher capacity there are often two reboilers which alternate between heating
rich glycol and draining recycled lean glycol.
Compressor
Compressor equipment is used in oilfield facilities to maintain or boost the pressure in
transported gas as it moves along the pipelines to the supplier and final consumer. These are
classified into:
• Reciprocating Compressor
• Screw compressors
• Axial blade and fin type
• Centrifugal
Gas treatment
Remove unwanted components such as hydrogen sulfide and carbon dioxide. These gases are
called acids and sweetening /acid removal is the process of taking them out. Natural gas
sweetening methods include absorption processes, cryogenic processes; adsorption processes and
membranes. Gas treatment could also include calibration. Calibration gas, also known as a
calibration gas mixture, is generally a compressed mixture of gases or gaseous components.
Calibration gases are used as comparison standards in the calibration of many instruments. They
ensure that instruments such as gas analyzers or gas detectors read correctly.
OIL AND GAS STORAGE, METERING AND EXPORT
The final stage before the oil and gas leaves the platform consists of storage, pumps and pipeline
terminal equipment.
Oil and Gas Storage, Metering and Export
• Fiscal Metering
• Storage
• Marine Loading
• Pipeline terminal
Fiscal Metering
Some small installations are still operated with dipstick and manual records, larger installations
have analysis and metering equipment. Analyzers will measure hydrocarbon content and energy
value (MJ/scm or BTU, Kcal/scf) as well as pressure and temperature. The meters are normally
orifice meters or ultrasonic meters. Larger new installations therefore choose ultrasonic gas
meters that work by sending multiple ultrasonic beams across the path and measure the Doppler
Effect. LNG is often metered with mass flow meters that can operate at the required low
temperature. Fiscal Metering
Storage
Occasionally underground mines, caverns or salt deposits can be used to store gas. For onshore
fixed roof tanks are used for crude, floating roof for condensate. Also rock caverns are used.
Pressure or Float are used to measure the level in storage tanks, cells and caverns. A tank farm
consists of 10-100 tanks of varying volume for a total capacity typically in the area of 1 - 50
million barrels.
Marine Loading
A marine loading arm, also known as a mechanical loading arm, loading arm, or MLA is a
mechanical arm consisting of articulated steel pipes that connect a tankship such as an oil tanker
or chemical tanker to a cargo terminal. Loading systems consist of one or more loading arms /
jetties, pumps, valves and a metering system. More complexes both because of the volume
involved, and because several loading arms. The tanks must be filled in a certain sequence;
otherwise the tanker's structure might be damaged due to uneven stresses.
Figure 5: LNG marine loading arm
Pipeline terminal
Pipeline terminal means a large storage facility which receives product via pipeline. The gas
pipeline is fed from the High Pressure compressors and Oil pipelines are driven by separate
booster pumps. For longer pipelines, intermediate compressor stations or pump stations will be
required. The pipeline terminal includes termination systems for the pipeline. Pigging device that
is used to clean or inspect the pipeline on the inside.
Figure 6: Pipeline terminal
Note: The shareholders in EACOP are affiliates of the three upstream joint venture partners (the
Uganda National Oil Company, Total Energies E&P Uganda and CNOOC Uganda) together
with the Tanzania Petroleum Development Corporation. Shareholdings are Total Energies 62%,
UNOC and TPDC 15% each and CNOOC 8%.
FOSSIL FUELS
Fossil fuels are natural fuels formed in the geological past from the remains of
living organisms. Fossil fuels are those fuels that can be mined from underground
(or undersea) deposits and include high percentages of carbon. The main types of
fossil fuels are coal, petroleum, and natural gas
The world of today is dependent on the use of fossil fuels as a primary energy
source, and this is especially obvious in all industrialized countries. Some major
users of fossil fuels are the utility, industrial, residential, and transportation
sectors.
PETROLEUM
Liquid fuels are mainly in the form of petroleum. Petroleum is an oily, flammable,
thick dark brown or greenish liquid that occurs naturally in deposits, usually
beneath the surface of the earth; it is also called crude oil. Because of its high
demand in our daily life, it is also called “black gold.”
It is mostly used to produce fuel oil, which is the primary energy source today.
Petroleum is also the raw material for many chemical products, including solvents,
fertilizers, pesticides, and plastics. Because of its high energy content and ease
of use, petroleum remains the primary energy source.
ORIGIN OF PETROLEUM
1. Biogenic Theory
Most geologists view crude oil, like coal and natural gas, as the product of
compression and heating of ancient vegetation over geological time scales.
According to this theory, it is formed from the decayed remains of prehistoric
marine animals and terrestrial plants.
Over many centuries, this organic matter, mixed with mud, is buried under thick
sedimentary layers of material. The resulting high levels of heat and pressure
cause the remains to metamorphose, first into a waxy material known as kerogen
and then into liquid and gaseous hydrocarbons in a process known as catagenesis.
These then migrate through adjacent rock layers until they become trapped
underground in porous rocks called reservoirs, forming an oil field from which the
liquid can be extracted by drilling and pumping.
One hundred fifty meters is generally considered the “oil window.” Although this
corresponds to different depths for different locations around the world, a “typical”
depth for an oil window might be 4-5 km.
Three situations must be present for oil reservoirs to form: - a rich source rock, a
migration conduit, and a trap (seal) that forms the reservoir.
2. Abiogenic Theory
In 1866, Berthelot proposed that carbides are formed by the action of alkali metal
on carbonates. These carbides react with water to give rise to large quantities of
acetylene, which in turn is converted to petroleum at elevated temperatures and
pressures.
For example, one can write the sequence as follows:
Mendelejeff proposed another reaction sequence involving acetylene in the
formation of petroleum. He proposed that dilute acids or hot water react with the
carbides of iron and manganese to produce a mixture of hydrocarbons from which
petroleum could have evolved. The reaction sequence according to the proposal
of Mendelejeff is:
These postulates based on inorganic chemicals, although interesting, may not be
completely accepted because:
1. One often finds optical activity in petroleum constituents that could not have
been present if the source of petroleum were only these inorganic chemicals.
2. The presence of thermo-labile organic constituents (biomarkers) in petroleum
cannot be accounted for in terms of their origin from these inorganic chemicals.
3. It is known that oil is exclusively found in sedimentary rocks, which would not
have been the case if the origin of oil could be attributed to processes involving
only these inorganic chemicals.
COMPOSITION OF PETROLEUM
Petroleum is a combination of gases, liquid, and solid mixtures of many alkanes.
It principally consists of a mixture of hydrocarbons, with traces of various
nitrogenous and sulphurous compounds.
Gaseous petroleum consists of lighter hydrocarbons with an abundant methane
content and is termed “natural gas.” Liquid petroleum not only consists of liquid
hydrocarbons but includes dissolved gases, waxes (solid hydrocarbons), and
bituminous material. Solid petroleum consists of heavier hydrocarbons, and this
bituminous material is usually referred to as bitumen or asphalt. Along with these,
petroleum also contains smaller amounts of nickel, vanadium, and other elements.
The carbon: hydrogen ratio of crudes on a weight basis usually lies in the range
6:1 to 8:1.
Petroleum Hydrocarbons
A typical crude oil will contain many thousands of different and distinct
hydrocarbon compounds. Obviously it would be an enormous undertaking to
analyse completely such a complex mixture, and in practice this is not attempted,
since it has been found that with few exceptions the hydrocarbons present in crude
oil and natural gas belong to just five clearly defined families. These families are
known as homologous series, referring to the structural characteristics which
define each family (or series) and which are associated with definite physical and
chemical properties. This approach to the analysis of petroleum fluids is
sometimes called PNA analysis or PONA analysis, the initials referring to the
paraffin (P), olefin (O), naphthene (N) and aromatic (A) families of hydrocarbons
respectively. The five homologous series occurring in petroleum are as follows:
1) n-paraffins (or n-alkanes)
2) iso-paraffins (or iso-alkanes)
3) naphthenes (or cyclo-alkanes or cycloparaffins)
4) aromatics (or arenes)
5) mixed naphtheno-aromatics (or cycloalkanoaromatics)
In the petroleum industry the older names - paraffins, naphthenes, aromatics are still widely used in preference to the modern chemical names - alkanes,
arenes, etc.
1) n-paraffins
These are straight-chain saturated hydrocarbons, i.e. hydrocarbons with a carbon
skeleton having all its carbon atoms in an unbranched straight chain; this is
indicated by the n- (standing for normal). (Strictly speaking, the carbon chain in
these compounds is staggered and not straight, since the four valencies of the
carbon atom are equally distributed in space, and the angles between the adjacent
bonds are all therefore 109.5°: however, for convenience the chain is usually
represented in diagrams as straight).
A saturated hydrocarbon is one which has no double or triple bonds - i.e. all the
four valencies of each carbon atom are fully utilised in single bonds linking it with
other carbon atoms or with hydrogen atoms. It is therefore not possible to add
any more hydrogen atoms to the molecule.
The first (or lowest) member of the n-alkane series is taken to be methane, CH4,
although it has only one carbon atom in the molecule and therefore has no chain
as such. Methane is the main constituent of natural gas. The next higher member
of the n-alkane series is ethane, C2H6, the third member is propane, C3H8, the
fourth member is butane, n-C4H10 and so on.
It can easily be shown that the general formula covering all the n-alkanes is
CnH2n+2 where n is the number of carbon atoms in the molecule. If n has the
value 4 or greater, the n- (for normal) must be inserted in the chemical formula
for an n- alkane, because as will be seen below alternative alkane structures exist
with the same chemical formula but with the carbon atoms arranged in a branched
chain layout.
At room temperature the n-alkanes with between 4 and 18 carbon atoms in the
molecule are liquids, and those with more than 18 carbon atoms in the molecule
are waxy solids. N-alkanes up to C42H86 have been identified in some crudes.
2) iso-paraffins
These are saturated hydrocarbons with the same general formula as n-paraffins,
but having a branched rather than an unbranched carbon chain. The first member
of the series is iso-butane, written i-C4H10.
Compounds with a low degree of branching (only 1 side-chain) are usually present
in crudes in larger amounts than those with 2 or 3 side-chains. The simplest sidechain is a methyl group (CH3-); others may be ethyl (C2H5-), propyl (C3H7-),
etc. These groups (methyl, ethyl, etc) are collectively referred to as alkyl groups.
Where only one side-chain is present, it is most often attached to the second
carbon atom in the main carbon chain; if this side-chain is a methyl group, the
resulting compound would be referred to as a 2-methyl derivative (numbering the
carbon atoms in the main chain from the end).
Compounds such as n-butane and iso-butane which have the same empirical
formula but different molecular structures are called isomers, and it can easily be
seen that butane has only two isomers, since there are two possible structures
both having the chemical formula C4H10. As the number of carbon atoms in the
molecule, n, is further increased, the number of possible isomers increases
rapidly. Pentane (C5H12) has 3 isomers, hexane (C6H14) has 5 isomers and
heptane (C7H16) has 9 isomers. The alkane C15H32 has 4,347 isomers. (In
petroleum industry practice, any hydrocarbon with a branched chain carbon
skeleton tends to be identified by the iso- prefix, although organic chemists use
this term in a more restricted manner).
Different isomers of a given formula (e.g. C4H10) have different physical
properties (density, boiling point, etc.) and show some differences in chemical
properties. In the formal system of chemical nomenclature each isomer will have
its own name reflecting its own distinct chemical structure. In practice, however,
these differences are usually quite small, and the practice in the petroleum
industry is often to ignore them and group all the isomers of a given chemical
formula together; e.g. the heptanes as C7, the octanes as C8 and so on. In
carrying out analyses of reservoir fluids, the common practice is to give a detailed
analysis from methane (usually denoted by C1) up to the hexanes, and then to
group together the heptanes and all the higher hydrocarbons (C8, C9, C10, etc)
as ‘heptanes plus’, indicated by the symbol C7+.
3) Naphthenes, cyclo-paraffins.
These have molecules based on one or more saturated carbon rings, with or
without side-chains attached. The commonest types occurring in crudes are
cyclopentanes (having five carbon atoms in the ring) and cyclohexanes (having
six carbon atoms in the ring) and their derivatives:
The examples given above are monocyclic, i.e. they contain only 1 ring in the
molecule. Naphthenes may also have 2 (bicyclic) or more rings in the molecule,
linked together in various ways. Bicyclic naphthenes are important constituents in
many crudes and the heavier fractions distilled from them such as kerosene and
gas-oil. It is estimated that many crudes contain from 30% to 60% of naphthenes.
The general formula for naphthenes is CnH2n+2-2R where R is the number of
rings in the molecule. In the above examples, only the carbon atoms are shown,
for simplicity, and it is assumed that the remaining (single) bonds of each carbon
atom are linked to hydrogen atoms.
4) Aromatics
These are compounds which contain one or more benzene rings in the molecule,
with or without attached alkyl side-chains. The lowest member of the arene family
is benzene, C6H6, discovered by Faraday in 1825 in the liquid recovered from the
mains transporting coal-gas. The next higher member of the series is toluene,
C6H5CH3, followed by xylene, C6H5(CH3)2. Xylene has three isomers, m- (for
meta), o- (for ortho), and p- (for para).
Aromatics are unsaturated compounds having distinctive properties; for example,
the distinctive sweetish smell which many of them possess and which is
responsible for the name. Aromatics have higher specific gravities than alkanes or
cycloalkanes of similar molecular weight. Since the aromatics are unsaturated, it
is possible to add hydrogen to them; in the case of benzene, for example, it may
be converted to cyclohexane, C6H12, by reacting it with hydrogen in the presence
of a suitable catalyst. Aromatics with two or more benzene rings in the molecule
(polynuclear or polycyclic aromatics) form a significant proportion of the heavier
molecules in many crudes.
5) Mixed naphtheno-aromatics
These compounds have molecules which contain at least 1 naphthenic ring and 1
benzene ring with alkyl side-chains. These will be large molecules, hence the
compounds will have fairly high boiling points and will be present in the heavier
fractions of the crude and the products distilled from it.
Other homologous series of hydrocarbons apart from those mentioned above are
important in petroleum processing, in particular the olefins (or alkenes) which
contain a double carbon-carbon bond, and the acetylenes (or alkynes), which
contain a triple carbon-carbon bond. These are both unsaturated series, and are
chemically much more reactive than the paraffins. Olefins and acetylenes are not
found to any significant extent in naturally occurring petroleum crudes, although
it has been reported that some Pennsylvania crudes contain small amounts of
higher olefins. Olefins play an important role in some refinery processes,
particularly those involving cracking, and the first member of the olefin series,
ethylene (C2H4), is the basic building block of the modern petrochemical industry.
It is found that although crude oils contain many different hydrocarbon
compounds, each of these hydrocarbons belongs to one of the five families (or
series) just described, with many members of each family being present in each
crude. Within each of these families the lower members are gases or volatile
liquids at ordinary temperatures, with quite small molecules and low boiling points,
while the higher members are solids with very large molecules and
correspondingly high boiling points. Although the molecular size and physical
properties of the family members vary enormously, each member has the basic
chemical structure associated with its own family. Hence although differences in
the hydrogen to carbon ratios of crudes are small, the physical properties of these
crudes can vary very widely depending on the relative proportions of the lower
and higher members of each family present.
Non-Hydrocarbons in Petroleum
The elements oxygen, nitrogen and sulphur, together with some metals, are found
in all crude oils and have an important effect on their properties, although the
amounts present are relatively small. Oxygen is present in the form of metal salts
of organic acids (such as carboxylic acids) and phenols. Nitrogen occurs in a
variety of complex organic compounds. Sulphur is present in the form of organic
compounds such as mercaptans or disulphides, or in inorganic form as hydrogen
sulphide, carbonyl sulphide (COS) or occasionally as free elemental sulphur.
Some of the metals are present in complexes such as porphyrins, which have
structures related to such biological materials as chlorophyll and haemoglobin.
Some crudes (e.g. from Venezuela and California) contain up to 400 ppm of
vanadium. Natural gas reservoirs may contain high concentrations of nitrogen,
carbon dioxide or hydrogen sulphide. Small amounts of mercury are quite
commonly found in natural gases, and it has been recently shown that traces of
arsenic compounds are present in some natural gases.
Asphaltenes and resins are non-hydrocarbon materials of very high boiling point
and complex chemical composition which are found in crude oils and which are
major constituents of bitumen and similar materials. Bitumen is a solid or semisolid form of naturally-occurring petroleum. The terms bitumen and asphalt are
often used interchangeably, although recent practice has been to use the term
asphalt for the manufactured materials produced during petroleum processing
(and used, e.g. for road making and roofing purposes), and to use the term
bitumen for the various semi-solid materials which occur naturally in certain
locations (and have been used from very ancient times for waterproofing and
building as mentioned in the book of Genesis in the Old Testament). Bitumen is
also sometimes called ‘native asphalt’.
When crude oils are heated and the more volatile components distilled off, the
liquid residue left behind (which becomes semi-solid on cooling) consists of resins
and asphaltenes together with waxes and some other hydrocarbon compounds of
very high molecular weight. In refinery operations this material is referred to as
residuum (or residue or bottoms). Most crudes when distilled in refinery
processing are found to give between 10% and 30% by weight of residuum, while
some heavy crudes give more. Waxes are alkanes (including some isoalkanes and
cycloalkanes) and have already been discussed above.
Asphaltenes are more readily soluble in carbon disulphide and aromatic solvents,
and when obtained in pure form are brittle powdery solids, while resins are viscous
liquids. In most crudes, it is found that resins are present in larger amounts than
asphaltenes. During transport of crude oils, asphaltenes and waxes may
precipitate out and obstruct flow in pipelines. This can be a particularly difficult
problem in subsea pipelines, where the cooling caused by low sea temperatures
encourages such deposition. Various inhibitors can be added to the crude to
control the problem.
CRUDE OIL PROPERTIES
The physical and chemical properties of crude oils vary considerably and are
dependent on the concentration of the various types of hydrocarbons and minor
constituents present.
A. Specific gravity
The specific gravity of crude oils is normally defined based on a standard
temperature of 60°F and is given the symbol γ° or d: -
It is assumed that both the crude and the water are at atmospheric pressure
(although the value obtained will be relatively insensitive to pressure). The density
of the water is approximately 62.4 lb/ft3
SG Less than 1.0, oil floats on water. Greater than 1.0, oil sinks in water. Majority
of oils “float” In the oil industry it has long been common practice to use the API
gravity (API= American Petroleum Institute) to specify the densities of crudes and
of liquid petroleum fractions.
B. API (American Petroleum Institute) Gravity
This is defined by:
There is thus an inverse relationship between the API gravity (quoted in degrees
API) and the specific gravity, and in order to avoid confusion it is essential to make
clear which gravity is being referred to. There is an inverse relationship between
API gravity and density; the higher the density the lower the API gravity.
Light oils, containing a high proportion of low molecular weight hydrocarbons, will
have low specific gravities and API gravities up to 50º or more. Heavy oils have
API gravities between 20° and 10°; very heavy oils have API gravities less than
10° usually have very high viscosities (up to 10,000 cp) and will consist mostly of
high-molecular weight compounds, giving 40% or more of residuum on distillation.
Very heavy oils of less than 10° API are often referred to as bitumen, asphalt or
tar. Pure Water has arbitrary API Gravity of 10.
C. Surface tension
Surface Tension is the force of attraction between the surface molecules of a liquid.
Surface Tension together with viscosity affects the rate of spread over water or
ground. The lower the surface tension, the greater its potential spreading rate.
Low surface tensions characteristic of low specific gravity oils. As temperature
increases, surface tension decreases, and the rate of spread increases.
Interfacial or Surface tension exists when two phases are present. These phases
can be gas/oil, oil/water, or gas/water. Interfacial tension is the force that holds
the surface of a particular phase together and is normally measured in dynes/cm.
The surface tension between gas and crude oil ranges from near zero to
approximately 34 dynes/cm. It is a function of pressure, temperature, and the
composition of each phase.
D. Viscosity
The viscosity of an oil is a measure of the oil’s resistance to shear. Viscosity is
more commonly known as resistance to flow. Viscosity is determined by the
amount of light ends. High viscosity implies a high resistance to flow while a low
viscosity indicates a low resistance to flow. The principal factors affecting viscosity
are: Oil composition, Temperature, Dissolved gas and Pressure
E. Pour point
Temperature at which the oil becomes “plastic” and will not flow. Lighter oils with
low viscosities have lower pour points.
F. Flash point
The flashpoint is the lowest temperature at which a sample of liquid gives off
sufficient vapour to flash momentarily on application of a small flame. The
flashpoint is an important indication of the degree of hazard presented by a
flammable liquid. Fuel oil, which has had almost all its lower hydrocarbons
removed during refining, has a high flashpoint, and is therefore much less
hazardous to store and handle than crude oil, which typically has a low flashpoint.
The density of hydrocarbon gases and vapours is a vital factor in considering safety
issues in processing and storage facilities. Most natural gases at atmospheric
temperature are much less dense than air; if released to atmosphere they will
therefore rise, and tend to be diluted and dispersed in the surrounding air quite
rapidly once the source of the release is closed off. However, fuel gases such as
propane, LPG, butane and some streams of heavier hydrocarbons occurring in
processing operations have densities greater than air, and if released will sink and
can accumulate in hollows, pits, low-lying spaces, the hulls of vessels, etc. after
displacing the air originally present.
These dense gases can remain in such locations for long periods if undisturbed,
presenting very serious dangers long after the original release has been closed
off. Such accumulations have been the cause of numerous injuries and fatalities
due to fire, explosion (and asphyxiation) in places such as basements, cellars,
bunded areas surrounding storage tanks, boats and floating vessels, etc.
It should also be remembered that gases released from pressurised vessels,
pipelines,
etc. will undergo Joule-Thomson cooling, causing their densities to increase.
Methane
gas boiled off from stored LNG will have a density greater than air owing to its
very low
temperature, and can therefore give rise to the same type of hazard as just
described, although on a more short-term basis
G. Solubility in water
Solubility of oil in water is generally very low ~ 5 ppm or one grain of sugar in a
cup of water. Despite the low solubility, crude oil can have important
consequences for the potential toxicity of hydrocarbons to aquatic organisms.
H. Bubble point
In their original condition, reservoir oils include some natural gas in solution. The
point at which this natural gas begins to come out of solution and form bubbles is
known as the bubble point. The corresponding temperatures and pressure are
bubble point temperature and bubble point pressure.
Tankers and barges are also employed for long-distance transportation, often for
international transport. Rail and truck can also be used for longer distances but
are most cost-effective for shorter routes.
CLASSIFICATION AND CHARACTERISATION OF PETROLEUM
In the petroleum industry, crude oil can generally be classified by the geographic
location it is produced in (eg, West Texas Intermediate, Brent, or Oman), its
gravity, sulphur content, etc.
Different types of petroleum contain different chemical compositions and
properties such as density, viscosity, colour, boiling point, pour point, etc., and
can vary widely among different crude oils. There are various methods of
categorising petroleum fluids. These include: 1. Classification according to sulphur content
Sulphur is an undesirable impurity in crude oil and natural gas, and when it is
burned, forms sulphur dioxide, a gas that pollutes the air and forms acid rains. On
the basis of sulphur content, petroleum is classified as sour or sweet. Crude oils
with less than 1wt% sulphur content and more than 1wt% sulphur content are
called sweet and sour crude oils, respectively.
2. Classification according to petroleum base.
This is a very simple qualitative method which has been in use for many years.
The base is determined by observing the behaviour of the liquid residue remaining
after the lighter fractions (of lower boiling-point) have been distilled off from the
crude sample.
If the solid separating as the residue cools as a wax, the crude is described as
paraffinic base. If no wax separates but the cooled residue consists of asphaltic
material (bitumen) the crude is described as asphaltic base (or naphthenic base).
Crudes yielding cooled residues containing both wax and asphaltic material are
described as mixed base. It is found that the great majority of crudes are mixed
base.
This simple scheme is fairly primitive, and can be misleading, although it has long
been widely used. The term ‘naphthenic’ for asphaltic base crudes originated
through a misconception, and does not necessarily reflect the chemical
composition of the crude, as it has been found that some crudes classified as
asphaltic base contain only relatively small amounts of naphthenes while having
high aromatic contents.
3. Classification by characterisation factor.
This method is a quantitative one, and is based on the work of Watson, Nelson
and Murphy of Universal Oil Products (UOP). These workers defined a
characterisation factor (CF) based on the behaviour of the crude in a standard
IP/ASTM distillation procedure. (IP = Institute of Petroleum, ASTM = American
Society for Testing Materials).
The characterisation factor, given the symbol K or KW, is defined by:
Where
TB = average boiling point of crude (°R) at atmospheric pressure
d = specific gravity of crude (at 60°/60°F)
The average boiling point, TB, is obtained from the IP/ASTM distillation test data
by a prescribed averaging procedure.
It is found that:
Paraffins have K values above 12.0
Naphthenes have K values between 11.0 and 12.0
Aromatics have K values less than 10
K values have proved valuable for correlating properties of crudes and petroleum
fractions.
4. Classification according to API gravity
API gravity or density is defined by:
The API gravity is used to classify oils as light, medium, heavy, or extra heavy. As
the weight of oil is the largest determinant of its market value, API gravity is
exceptionally important. The ⁰API values for each weight are as follows:
• Light crude oil: ⁰API > 31.1
• Medium crude oil: ⁰API between 22.3 and 31.1
• Heavy crude oil: ⁰API < 22.3
• Extra heavy crude oil: ⁰API < 10
It is important to note that not all parties use the same grading.
5. Classification according to Solution/ dissolved Gas Oil Ratio
The solution gas-oil ratio (GOR) is a general term for the amount of gas dissolved
in the oil. Heavy oils (lower API gravity) have lower capacity to contain dissolved
gas than lighter oils therefore, they have low GOR.
PETROLEUM PRODUCTS AND UTILISATION
Petroleum is recovered mostly through oil drilling after studies of structural
geology, sedimentary basin analysis, and reservoir characterization (mainly in
terms of the porosity and permeability of geologic reservoir structures) are
completed.
Petroleum is refined and separated, most easily by distillation, into a large number
of consumer products, from gasoline (petrol) and kerosene to asphalt and
chemical reagents used to make plastics, pharmaceuticals, solvents, fertilizers,
pesticides, dyes, and textiles.
In fact, petroleum is used as fuel (primary source of energy) and in the
manufacture of a wide variety of materials, and it is estimated that the world will
need it until scientists can find and develop alternative materials and technologies.
Petroleum products are materials derived from crude oil (petroleum) as it is
processed in surface facilities and oil refineries. The majority of petroleum is
converted to petroleum fuels, which includes several classes of fuels.
According to the composition of the crude oil and depending on the demands of
the market, refineries can produce different shares of petroleum products.
Refineries also produce other chemicals, some of which are used in chemical
processes to produce plastics and other useful materials. Since petroleum often
contains sulphur-containing molecules, elemental sulphur is also often produced
as a petroleum product. Carbon, in the form of petroleum coke,
and hydrogen may also be produced as petroleum products.
Typical refinery products include: 





Gaseous fuels such as propane.
Liquid fuels such as automotive and aviation grades of gasoline, kerosene,
various aviation turbine fuels, and diesel fuels.
Lubricants such as light machine oils, motor oils, and greases.
Paraffin wax and slack wax.
Sulphur
Bulk tar and asphalt.


Petroleum coke.
Petrochemicals and petrochemical feedstocks. Petrochemicals are organic
compounds that are the ingredients for the chemical industry, ranging from
polymers and pharmaceuticals.
Numerous products are made from petroleum waste by-products including:
fertilizers, linoleum, perfume, insecticide, petroleum jelly, soap, vitamin capsules.
GASEOUS FUELS
Gaseous fuels are fuels that under ordinary conditions are gases. These fuels are the most
convenient requiring the least amount of handling and simplest and most maintenance
free burner systems. They play a vital part in modern energy demand and dominate all
markets at present because of its abundance and quality.
The importance of gaseous fuel







Generally, very clean burning. Little soot.
Easy to burn. No grinding. Excellent mixing
No problems with erosion or corrosion
No ash problems
The gas is easy to clean. E.g. if sulphur is present, it may be easily removed prior
to combustion.
Simplest combustion plant of all (burners)
Can be started up and shut down very easily and quickly.
Problems associated with gaseous fuels are distribution and storage, explosion risk, high
volatility, relatively costly to produce.
Classification of Gaseous Fuels
The following is a list of the types of gaseous fuel:
(A) Fuels naturally found in nature:


Natural gas
Methane from coal mines
(B) Fuel gases made from solid fuel



Gases derived from Coal
Gases derived from waste and Biomass
From other industrial processes (Blast furnace gas)
(C) Gases made from petroleum



Liquefied Petroleum gas (LPG)
Refinery gases
Gases from oil gasification
(D) Gases from some fermentation process
NB: Classifications B, C and D are together termed as synthetic gas (gases which are
chemically made by some process).
WOBBE INDEX
When deciding whether an alternative gas can be used in an appliance, three factors must
be considered:

For the same pressure drop is the heat release roughly the same


For the same air and fuel flows is the flame shape the same
For the same heat release conditions are pollutants within a specified tolerance
The first criteria is best summarised by consideration of the Wobbe Index. The Wobbe
Index (WI) or Wobbe number is an indicator of the interchangeability of fuel gases. It
is used to compare the combustion energy output of different composition fuel gases in
an appliance (fire, cooker etc.). If two fuels have identical Wobbe Indices, then for given
pressure and valve settings the energy output will also be identical. Typically, variations
of up to 5% are allowed as these would not be noticeable to the consumer.
𝑊𝐼 =
𝐶𝑉
𝑆𝐺 0.5
Where:
CV = the higher or gross calorific value in MJ/m3 or BTU/ft3
SG = the specific gravity
There are three ranges or "families" of fuel gases that have been internationally agreed
based on Wobbe index and wobbe numbers. Family 1 covers manufactured gases, family
2 covers natural gases and family 3 covers liquefied petroleum gas (LPG). Combustion
equipment is typically designed to burn a fuel gas within a particular family: hydrogenrich town gas, natural gas or LPG.
The international gas union assign the following gas families:
Family 1 - Wo = 17.8 - 35.8MJ/Sm3 (Coke Oven gas, Low CV gas, Town gas)
Family 2 – Wo = 35.8 - 71.5 (Natural gases)
Family 3 – Wo = 71.5 - 87.2 (Liquefied Petroleum Gas)
NATURAL GAS
Natural gas is a naturally gaseous hydrocarbon mixture that is formed under the earth’s
surface. It is a gaseous fossil fuel that is found in oil fields, natural gas fields, and coal
beds. The primary constituent of NG is methane, it also contains (C2+ hydrocarbons, N2,
CO2, He, H2S and noble gases) according to its origin.
Throughout the 19th century, NG was used locally as a source of light due to lack of safe
structure for long distance gas transport. After World war II NG was extensively used after
construction of pipelines for transportation. Today, NG is widely used for: space heating,
generating electricity, as an industrial fuel source, as a feedstock for petrochemicals, as a
feed stock to make gasoline and other transportation fuels.
In its pure state, NG is odourless, colourless, and shapeless. It is highly combustible and
gives off a significant amount of energy when burned. It is considered to be the cleanest
fossil fuel and a safe source of energy. During NG combustion, the emission of SO2, nitrous
oxide and CO2 are negligible
According to the BP Statistical review of World Energy (2015) the total worldwide proved
reserves of NG were 187.1 trillion cubic metres (tcm) at the end of 2014. NG accounts for
23.7% of primary energy consumption
Dry gas
A mixture of gases that does not contain hydrocarbon species that become condensate or
liquid. Also note that “dry gas” does not mean that there is no liquid or water, it means
that there are no marketable liquid hydrocarbons, there can be a considerable quantity of
water in a “dry gas” field.
Crude oil and natural gas compared
The NG industry differs from the oil industry in a number of important ways. The physical
and chemical differences between oil and gas mean that the storage, handling and
transportation of gas are much more challenging compared to oil. The consequences of
this are a less flexible supply chain and significantly higher infrastructure costs.
Crude oil and NG share many characteristics. Both are found in subsurface and originate
from the same organic rocks. Both have variable compositions of hydrocarbons and nonhydrocarbon components. The physical processes of forming, accumulating and storing
NG and crude oil in sub-surface reservoirs are virtually identical. It is very rare to produce
crude oil without associated gas.
Despite these similarities, the characteristics of oil and gas industries are fundamentally
different with consequent impacts on projects in terms of capital costs, marketing, prices
and revenues, technical and reserves risks and supply chain. The differences between oil
and gas industries are entirely due to the challenges in handling gaseous material
compared to crude oil in a liquid state. Transportation and storage of NG is more difficult
and expensive compared to crude oil.
In comparison to crude oil however, NG contains a greater proportion of lighter
hydrocarbons, mainly from methane to butane. Various terms are used to describe the
different components of NG as seen in table below.
Terms used to describe different components of NG
NG production and reservoirs
A gas reservoir is a naturally occurring storage area; it consists of permeable and porous
rocks surrounded by impermeable materials.
Natural gas origin
NG originates from three major processes: thermogenic, biogenic and abiogenic
processes.
A) Thermogenic process
This process involves relatively slow decomposition of organic material that occurs in
sedimentary basins under the influence of temperature and pressure in association with
increased depth. As a result of this decomposition reaction, NG (also called thermogenic
methane) and petroleum are presumed to be formed.
B) Biogenic process
In this process, methane is formed by the action of living organisms (methanogenic
bacteria) on organic materials during the decomposition of sediments in the early part of
their burial. This anaerobic decomposition is endothermic, slow and smelly. It can take
hundreds or thousands of years for this anaerobic process to come to completion. The
time required is largely a function of energy input into the process.
Aerobic decomposition of organic matter is exothermic and rapid. Aerobic decomposition
converts waste material to CO2 and H2O.
The methanogens are abundant in habitats where electron acceptors such as O 2, NO3- ,
Fe3+ and SO42- are limiting. These habitats include anaerobic digesters, anoxic sediments,
flooded soils and gastrointestinal tracts. Examples: - Cow's stomach. Action of certain
bacteria on biomass in the absence of oxygen breaks down the H/Carbons in organic
compounds at temperatures of around 37C.
For petroleum exploration, the distinction between thermal and bacterial gas is necessary.
In a thermal gas basin, oil must form first, but this is not the case in bacterial gas basin.
Therefore, the exploration techniques must be planned differently.
C) Abiogenic process
In this process, the starting material is the volcanic gas (not the organic matter) Methane
is formed by the reduction of carbon dioxide during magma cooling, commonly in
hydrothermal systems during water-rock interaction.
CLASSIFICATION OF NG
NG can be classified according to its origin and chemical composition.
1. Classification according to origin
i.
Conventional gas: It occurs in deep reservoirs that are either associated with
crude oil (associated gas) or contain little or no crude oil (non-associated gas).
Associated gas (wet gas) coexists in the reservoir rock with an oil reservoir. It may be
present in different forms as solution gas in the oil (dissolved gas) or as gas cap gas, lying
above the oil reservoir (casing head gas). Associated gas is usually leaner in methane and
richer in higher molecular weight paraffinic constituents.
The non-associated gas (dry gas) is produced from a geological formation that typically
does not contain much, if any, crude oil. This gas is usually richer in methane and leaner
in higher molecular weight hydrocarbons
ii.
Unconventional gas. Several types of unconventional gas are found such as
shale, coal bed methane, deep aquifer gas and gas hydrates.
Historically, we have thought about hydrocarbons being stored in some sort of void space.
Unconventional gas requires that we rethink this concept.
In coal bed methane (coal mine methane), the preponderance of the gas is adsorbed to
the surface of coal. CBM is formed by the coalification process. Thermogenic CBM is formed
by the action of increasing temperature and pressure in the buried organic matter that is
slowly transformed into methane. Bacterial processes form another type known as
biogenic CBM in thermally immature coals.
Shale gas is found in low permeability shale, impermeable sandstones, siltstones, sandy
carbonates, limestones, dolomite and chalk reservoirs. The methane produced from these
reservoirs is not associated with crude oil.
Deep aquifers gas is formed during gas migration through the aquifers to the reservoir
rocks; the aquifers are then largely saturated with methane. The solubility of methane in
water is low, so the aquifer gas content is influenced greatly by the pressure, salinity and
temperature.
Gas hydrates are crystalline cell of hydrogen bonded water molecules (ice-like water
lattice) in which gas molecules are trapped in their cages. At different conditions of
temperature and pressure, all gases can form hydrates. The hydrate crystals are formed
through the nucleation step followed by crystal growth from the nuclei.
Methane hydrate, which is known as the ice that burns is commonly found in marine
environments (deep sea) and sometimes in deep lake sediments. According to BP
Technology Outlook (2015), methane hydrate recovery may account for 5% or more of
world gas production by 2050.
2. Classification according to chemical composition
 Hydrocarbon content
Natural gas may be classified according to the hydrocarbon content of the produced gas.
Dry gas (unassociated gas) consists of methane as the major constituent with little or no
C2+ component, whereas wet gas (associated gas) contains C2+ constituents higher than
10 vol%.
 Sulfur content
Natural gas can also be classified according to the amount of sulfur content (generally
H2S) in the produced gas. In this classification, the natural gas could be sweet or sour.
Sweet gas contains no or a negligible amount of H2S, whereas sour gas contains
unacceptable quantities of H2S (more than 5 (mg/Nm3))
Natural gas composition
According to the reservoir from which the natural gas is extracted, its composition will
vary. Natural gas may contain different hydrocarbon and non-hydrocarbon constituents;
consequently, the gas composition is never constant. Table 2, above, illustrates the typical
natural gas composition.
A. Hydrocarbon constituents
Methane may associate with paraffinic hydrocarbons such as ethane, propane, the butanes
and a small proportion of C5+ hydrocarbons. It also may contain some aromatics such as
benzene, toluene, and xylenes.
B. Non-hydrocarbon constituents
The non-hydrocarbon constituents of natural gas can be classified as diluents,
contaminants and solid matter. The diluents are non-combustible gases (such as CO2, N2,
He) that reduce the heating value of the gas. They could be used as fillers when it is
necessary to reduce the heat content of the gas.
The contaminants are gases that are detrimental to production and transportation
equipment in addition to being obnoxious pollutants. These contaminants include:
i.
Sulfur species: hydrogen sulfide (H2S) can be formed during the reduction of
sulfate ions in sulfates dissolved in water by sulfur reducing bacteria in recent
sediment by a biochemical mechanism.
It can also be formed by thermal degradation of sulfur-rich kerogen at depth.
Different sulfur species may be present in the produced gas, such as carbonyl
sulfide (COS), carbon disulphide (CS2), elemental sulfur, and mercaptans (RSH,
where R represents an alkane group e.g. mehyl mercaptan CH3SH, ethyl mercaptan
C2H5SH, etc.) if the H2S concentration is greater than 2-3%.
ii.
Mercury is thought to be formed through the reduction or thermal decomposition
of mercury sulfide (cinnabar) in contact with hydrocarbons. It was mentioned that
mercury has a high affinity for carbon as well as sulfides. Mercury may be present
in different forms as elemental, organometallic compounds, such as dimethyl
mercury, methyl ethyl mercury, and dimethyl mercury, or inorganic mercury such
as HgCl or HgCl2
iii.
Arsenic can be found in oil and gas operations, and it is believed to be as a result
of the formation of geological gas. It has been found in different forms in natural
gas and gas condensate. It not only exists as arsine (H 3As) but also as
trialkylarsines, such as trimethylarsine (Me3As), dimethyl ethyl arsine (Me2EtAs),
methyl diethyl arsine (MeEt2As), triethyl arsine (Et3As), and triphenylarsine
(Ph3As). In sour natural gas systems, arsenic sulfide minerals could be found.
It is reported that arsenic levels can range from parts per billion levels up to high
parts per million. The trialkylarsines are a less reactive group than arsine itself and
are more difficult to remove from natural gas.
iv.
Naturally Occurring Radioactive Material (NORM)
In the produced gas, most of the radioactivity in NORM is derived from the decay
of uranium (U-238) and thorium (Th-232) present in the subsurface formations.
Deposition of thin, active lead (Pb-210) and polonium (Po-210) films on the internal
surfaces of production equipment and transport facilities may be present as a result
of Rn-222 decay. In the gas and oil industry, accumulation of NORM may be present
in the form of scale at the wellheads, in the form of sludge at Gas/Oil Separation
Plants, or in the form thin films as the result of radon gas decay in gas plants.
Solid matter may be present in the form of fine silica (sand) and black powder resulting
from the scaling in the pipe. The black powder mainly consists of iron oxides and iron
sulfides. However, it may also contain metallic lead, lead oxides, lead sulfide, zinc sulfide,
barium sulfate, calcium carbonate, as well as polonium (Po-210) deposits.
COAL BED METHANE (CBM)
Coal is a heterogeneous, combustible, sedimentary rock consisting of organic compounds
formed by a complex process of conversion of fossilized biomass into solid material. It is
a black or brownish-black sedimentary rock usually occurring in rock strata in layers or
veins called coal beds.
Uses of coal
From the beginning of the eighteenth to the mid-twentieth century, coal was the dominant
fuel in industrialized societies. Coal can be used as fuel for heating, steam generation, or
power generation (in coal fired power plants) and as feedstock for various industries.
Gas in coal
For as long as coal has been mined in the subsurface, the presence of gas in coal seams
has been known. This gas created problems in the mines when it would accumulate
threatening both asphyxia and explosive ignition. Eventually, boreholes were drilled into
the coal seams to vent off the gas ahead of the mining operations to reduce the chance of
this happening. In time, this gas was being collected and used locally instead of being
vented, giving rise to the concept of CBM.
CBM is still a significant source of natural gas in the United States, as well as Australia,
Canada, and China and is being developed in other coal-rich countries. CBM is considered
a clean fuel because its combustion releases no toxins, produces no ash and emits less
carbon dioxide per unit of energy than the combustion of coal, oil or even wood. Extraction
of CBM, in addition to providing economic value, also reduces the hazard of gas explosions
in coal mines.
Properties and origin of CBM
CBM is actually a misnomer. The gas recovered from coal seams is essentially natural gas.
While it does consist mainly of methane, it typically contains other hydrocarbon gases, like
ethane and propane and the nonhydrocarbon gases nitrogen and carbon dioxide). CBM is
also known as coalbed gas (CBG), coalbed natural gas, and coal seam gas.
Coal beds for the most part are self-sourcing reservoirs.
The coal generates gas and heavier hydrocarbons in place from the organic material
present in coal as it matures in a similar fashion to source rocks and hold these
hydrocarbons mainly by adsorption. Because of the high adsorptive capacity of coals, they
can also trap biogenic methane early in their history or capture migrating thermogenic
hydrocarbons from other sources. If the coal is uplifted to a shallow enough depth and
meteoric water percolates down to it, microbial activity biodegrading the coal’s bitumen
can contribute secondary biogenic gas to the CBG.
With regard to origin, CBM can be either biogenic, thermogenic or mixed.
Biogenic coal bed gas is generated by the breakdown of coal organic matter by
methanogenic consortia of microorganisms at low temperature usually <56°C (or 150°F);
low-rank coals are especially favoured as the starting material for microbial generation. In
contrast to microbial gas,
thermogenic coal bed gas is produced from the organic matter in coal by chemical
degradation and thermal cracking, mainly at temperatures higher than 100°C, where
microbial methanogenic activity becomes biochemically improbable. Higher rank coal is
expected to generate more thermogenic CBM than relatively lower rank coal, which would
translate into a higher CBM content if the gas has not escaped.
Note: According to the so-called coal rank there are four categories of coals: lignite,
subbituminous, bituminous, and anthracite. Coal rank expresses the geological maturity
of coal as a mineral. In underground deposits, coal transforms over time, starting with
peat, then proceeding to lignite and up to anthracite. Peat, the precursor of coals—
represents partially decayed vegetation of hemic, fibric, or sapric type that accumulates
in wet soils under the absence of oxygen.
With respect to being a reservoir, the porosity of coals is typically less than 10%, making
it less than desirable as a conventional reservoir. Instead, the vast majority of the gas is
stored adsorbed on the surface of the organic matter in the coal matrix. Only small
amounts of gas are stored as free gas in porosity and natural fractures (cleats) or as
solution gas in water in the cleats and pores. The adsorption capacity of coal increases
with its rank.
Extraction of CBM
Two basic drilling and completion technologies are used in CBM basins:
1.Open hole with cavity
2.Cased hole with hydraulic fracture stimulation.
In the open-hole cavity technique, a hole typically 22.2 cm in diameter is drilled to the top
of the targeted coal using a conventional drilling rig, drilling fluid and standard drilling
operations. Subsequently, the well is cased and cemented, and the conventional drilling
rig is removed from the well. A custom-designed drilling/completion rig is then put into
the well to drill through the coal and create a cavity. This rig is equipped with air
compressors that are able to inject large volumes of air into the wellbore at high pressure
up to 10.3 MPa (1500 pounds per square inch). After the hole in the coal has been created
and the casing is pulled back, the open-hole interval is ready for cavitation. The purpose
of this completion technique is to create a large cavity in the coal seam and remove any
drilling-caused formation damage so that the final cavity is stable. The final wellbore
diameter in the coal can reach up to 3 m. The wellbore is then completed with uncemented
pre-perforated liner or left-open hole. This open-hole drilling/completion technique is
especially successful in high permeability coal or over pressured areas.
In the technique using cased holes with hydraulic fracture stimulation, typically a hole of
20 cm in diameter is drilled with the drilling fluid through the coal and located some
distance (≈30 to 60 m) below the coal to provide space for coal fines and a sump for a
dewatering pump.
Removing the water from the formation is necessary to reduce the pressure, transform
adsorbed gas into free gas and allow free gas to flow to the wellbore. The well is cased
with the casing cemented across the coal seam interval. The coal seam is then selectively
perforated and fracture stimulated. Various fracture stimulation techniques have been
developed for coals, including various fluid types, pressures, etc.
Both the open-hole cavity and the fracture stimulation techniques have been successfully
used in US basins, such as the San Juan, Black Warrior and Powder River Basins. In most
cases, CBM wells require some stimulation because gas and water flows from coal naturally
are very low.
Environmental Issues of CBM Extraction
As is the case with any new reservoir development, there are some environmental issues
and challenges that CBM operators must face. The main environmental issues of CBM
extraction are related to: 1.disposal of co-produced water;
2.underground water table drawdown
3.methane contamination.
Other concerns are related to noise (caused by compressors and pumps), air pollution
(with some gases related to drilling and extraction operations) and surface disturbances.
PRODUCER GAS
Producer gas is mixture of flammable gases (principally carbon monoxide and hydrogen)
and non-flammable gases (mainly nitrogen and carbon dioxide) made by the partial
combustion of carbonaceous substances, usually coal, in an atmosphere of air and steam.
Producer gas has lower heating value than other gaseous fuels (4.5 to 6 MJ/m 3 depending
upon the quantity of its constituents), but it can be manufactured with relatively simple
equipment. Not suitable for distribution. Can be used on site.
It is used mainly as a fuel in large industrial furnaces (iron and steel manufacturing, such
as firing coke ovens and blast furnaces, cement and ceramic kilns, or for mechanical power
through gas engines.
Air is blown through a burning bed of coal. It should be enough air to maintain the bed
temperature and not enough air to complete the combustion reaction. The reaction
is exothermic and proceeds as follows:
2C + O2 + 3.73 N2 → 2CO+ 3.73 N2
WATER GAS
Water gas is a gaseous fuel generated in the similar way as producer gas, by gasifying a
burning bed of solid fuel with superheated steam. The gas is mainly composed of an equal
proportion mixture of carbon monoxide and hydrogen. The gas emits blue flame when
ignited due to high content of carbon monoxide, hence, it is named as blue water gas.
The reactions involved when steam is passed through the burning bed of coal or coke
above 10000C are,
This reaction is strongly endothermic and temperature falls below 1000 0C gradually with
the progress of the reaction. At lower temperature steam starts to react with carbon to
form carbon dioxide, according to the following reaction,
As carbon dioxide does not contribute to the calorific value of water gas, the above reaction
is undesirable. To prevent the formation of carbon dioxide and to lower the cost of the
process by not supplying heat from outside to the fuel bed, the whole process of water
gas generation is divided into two stages.
a) Blasting period, when fuel bed is heated up by passing air through it by the
exothermic reaction,
Carbon dioxide formed in this reaction is converted into carbon monoxide by the reaction
with the carbon of the solid fuel as the temperature of the fuel bed rises.
b) Run or gas-making period, when blue water gas is produced by passing steam
through the incandescent bed of coal.
At this period, along with the reactions 1) and 2), other reactions such as water gas shift
reaction and methanation reaction also occur. The objective of the blast period is to
accomplish only reaction 1) to produce maximum possible heat by this exothermic reaction
and the objective of run period is to produce carbon monoxide as much quantity as possible
by utilizing maximum amount of steam.
Both these objectives keeping in consideration, a fuel bed with moderate thickness is used,
with enough contact time of steam with the bed. In practice, the rate of decomposition of
steam and the formation of water gas are controlled by alternate blast and run period.
As soon as temperature falls below 10000C, steam blast i.e, run period is stopped and air
blast period is started to raise the temperature of the fuel up to 1400 0C. When this
temperature is attained, the air blast is stopped and steam blast is again resumed. In this
way, the process alternated. After the air blast period is over, the bed is purged with steam
for 2 seconds to remove the carbon dioxide formed in the blast period and this way CO 2 is
prevented to mix with the blue gas. This steam is called purge steam.
On the other hand, after the run period, a second air purging is done to remove last trace
of water gas present in the generator, before the blast period is started. The period for
which air is passed through the fuel bed is called hot blow. The run period is carried out
for 3 min and this period is also called cold blow. The hot blow is done for about 1 min. In
the modern period, it is aimed to decrease hot blow period and to increase the run period.
CARBURETED WATER GAS (CWG)
Water gas is a low calorific value fuel gas and is still not of sufficient quality for distribution.
To increase its calorific value, water gas is carbureted by adding gaseous hydrocarbons
obtained by cracking petroleum oils. In addition to the gas generator, a carburetor, a super
heater with purifiers, scrubbers and condensers are used to produce carbureted water gas.
Carburetor and super heaters are the two big steel vessels, lined with refractory checker
bricks to provide intensive heat transfer surface.
The gas produced at the air blast period contains large amount of heat which is passed
through these vessels to heat the top and side walls and then escape through a chimney
to waste heat boiler for raising steam. Water gas produced during the run period in the
gas generator is passed to the carburetor at the top and hydrocarbon oil is sprayed through
atomizer into the red hot fire brick.
The sprayed oil in the carburetor is cracked. The products of cracking and the water gas
pass down the carburetor and enter the super heater at its bottom. The gas mixture rises
up through the super heater and the cracking process is completed at this stage. Some
tar produced during cracking is separated in a separator.
The gas then passed through a series of pipes cooled by water spray. After cooling, the
gases are led to the purifiers and finally to the gas holders. In purifiers, hydrated ferric
oxide and lime impregnated wood sharing are placed which remove hydrogen sulfide as
iron sulfide.
This is now suitable for distribution and is known as town gas. It is of higher heating value
than water gas. The cost of production is relatively high because oil must be used as well
as coal.
The composition of carbureted water gas is as follows: H2: 34-38%, CO: 23-28%,
saturated hydrocarbons: 17-21%, unsaturated hydrocarbons: 13-16%, CO2: 0.2-2.2%
and N2: 2.5-5%. Water gas is used as a source of industrial hydrogen in the manufacture
of methyl alcohol and carbureted water gas is used for welding purpose, and illuminating
purposes.
REFINERY GAS
When crude oil undergoes the refining process in an oil refinery in which the higher chain
hydrocarbons are broken down into simpler hydrocarbon chains (various petroleum
products), there are flue gases (exhaust gases) that are generated in every process. These
gases are collected by a refinery and used as feedstock fuel for operating all the refinery
processes. These gases are known as refinery gases.
Using all these flue gases is an act of recycling. Refinery gases have common components
such as hydrogen, methane, ethane, butane, propane and ethylene which when burnt
produce enough energy to run all the refinery processes. Many a times, refineries along
with petroleum products, package the leftover refinery gases that can be sold to other
refineries that would like to indulge in the refinery gas trade.
The flue gases can be treated for pollutants such as sour gases. These acid gas removal
processes used in the refinery are required either to purify a gas stream for further use
in a process or for environmental reasons.
LIQUEFIED PETROLEUM GAS
Liquefied petroleum gas (LPG or LP gas), also referred to as simply propane or butane,
are flammable mixtures of hydrocarbon gases used as fuel in heating appliances, cooking
equipment, and vehicles. It is increasingly used as an aerosol propellant and a refrigerant.
LPG is produced during oil refining or is extracted during the natural gas production
process. If you release LPG, gas is emitted. In order to transport it, LPG needs to be placed
under modest pressure to form a liquid. It can then be stored and transported in LPG
cylinders or by pipeline and by specially built seagoing tankers. Transportation by truck,
rail, and barge has also developed.
Propane and butane products separated from the fractionation plant contain some
impurities as residual water, H2S, carbon disulfide, and sulfur compounds. These impurities
should be removed in order to meet the desired product specifications.