INTRODUCTORY PARAGRAPH
Collection and preparation of well products for further transportation is the
final stage of gas and condensate production.
Modern gas enterprises (GE) collect and prepare well products for further
transportation at complex gas treatment plants (CGTP) and gas treatment points
(GTP), which include: well product collection system, booster compressor stations
(BCS), air-cooler exchangers (ACE) and other supplemental processing facilities
(heat exchangers, furnaces, flare stacks, etc.), as well as operating procedures for
field processing of natural gas and gas condensate for interfield and long-distance
transportation.
Collection of gas and gas condensate well products is a process of in-field
transportation of crude gas from wells or well clusters to facilities for its preparation
for long-distance transportation via main gas pipelines (МGP).
Preparation of well products from gas and gas condensate fields for further
transportation is carried out, depending on its composition, using such main
technological processes as: separation (removing of impurities), absorption and
adsorption drying and gas purification, stabilization of hydrocarbon condensate, lowtemperature separation and absorption and adsorption drying and gas purification
processes, separation of gases, rectification of water-methanol solution, methanol
regeneration, etc.
Quality parameters of gas industry products are established by international
standards (ISO), relevant national standards and technical regulations.
GENERAL DESCRIPTION OF FIELD COLLECTION SYSTEMS
AND PROCESSING OF WELL PRODUCTS OF GAS AND GAS
CONDENSATE FIELDS. BASIC CONCEPTS ON FIELD GAS AND
CONDENSATE COLLECTION
Purpose of the gas collection system
The collection system for gas and gas condensate fields products is designed
for pipeline transportation of natural gas and gas condensate to field processing units
for their subsequent processing in accordance with the current regulatory documents.
Collection of gas and gas condensate well products (i.e., crude gas, unstable
gas condensate, reservoir and condensation water, which condense in the well bore,
flow lines and collecting pipes, also maybe with oil admixtures, inhibitors of
corrosion, salt and hydrate deposition, injected if necessary to the well bore or to the
flow line or collecting pipe) is a process for the in-field transportation of crude
products.
When transporting crude gas from wells or well clusters to facilities for natural
gas and gas condensate preparation for long-distance transportation, a productcollecting system or gas-collecting pattern is created.
The elements of the gas collection system are common for different fields and
usually consist of Christmas trees (bradenheads), gas outlet lines (manifolds, flow
lines), cutting off valves, gas collecting pipes, condensate pipelines, and field gas
collection points (FGCP).
The gas and gas condensate well production collection system in general case
is understood as:
1. An extensive network of in-field pipelines connecting wells and clusters
(wells,
mainly incline or directional (deviating), the well heads of which are grouped
at a close distance from each other at a common limited area (base) with field natural
gas and gas condensate treatment plants;
2. Systems and devices that ensure the reliable functioning of this pipelines:
- systems for distribution and injection of inhibitors of corrosion, salt
deposition and hydrate formation;
- systems for periodic cleaning of pipeline interior from liquid and solid
phases;
- well head and track heaters;
- gas pre-separation units located at the wells and clusters;
- instrumentation systems (control-gauging instruments), including measuring
of the temperature, pressure, well rate, temperature along the flow lines, etc.
Concepts and definitions of gas collection system
In-field gas pipelines are usually divided into flow lines and gas collecting
pipes, differing in pipe diameter. It is generally accepted that flow lines − gas
pipelines starting from the well head and ending with a tie-in to the gas collecting
pipes or at the inlet to the CGTP at the point of pressure control and flow distribution
(such a system is called a "comb" or point, switching valve building, etc.).
Flow lines usually have a smaller diameter than their collecting pipes. Flow
lines from individual wells have internal diameter of 102, 125 or 150 mm, and from
well clusters 219, 279, 325 mm, less often 426 and 500 mm.
Product flows from gas and gas condensate fields from several flow lines can
unite into a gas pipeline, which is called a gas collecting pipe (or gas collection
header). Typically, the gas collection headers leading to the facilities of field crude
gas treatment at the CGTP have the diameters of 325 mm, 426 mm, and 500 mm.
However, it should be noted that this terminology is not always maintained and
often the well cluster header is called a flow line, and the well cluster flow lines of a
sufficiently large diameter are often called gas collection headers.
Characteristics of collection systems of gas and gas condensate fields well
products
Currently, the following systems for collecting of well products from gas and
gas condensate fields have got the greatest practical application: individual, group,
centralized, decentralized.
The choice of a particular collection system during design or during its
modernization is due to a number of objective and subjective factors, the paramount
importance of which depends both on technical and economic indicators and on aims
facing the industry.
The current state of the collection system is directly due to the historical stages
of the gas industry development.
The classification of the products collection systems for gas and gas
condensate fields can be seen in Fig. 1.
Systems for collection and in-field transportation of gas and gas condensate
well products
Individual collection systems
(wells, gas collecting pipes, gas
pipelines, main gas pipelines)
linear
radial
circular
Scope of application
At the small and
medium fields
At the initial stage of
field development (gas
for own needs)
Group collection systems
Centralized wells (well
clusters), flow lines,
GPTP (GE), MGP
Decentralized well clusters,
flow lines (gas collecting
pipes), GCTP, MGP
Scope of application
At medium and large
fields; at several
nearby small fields
At large fields
Figure 1 – The general classification of the products collection systems for gas and gas condensate
fields, importance of which depends both on technical and economic indicators and on aims facing
the industry.
So, during the formation of the gas industry, only small and medium-sized gas
fields were put into development and operation.
During this period, individual gas collection and preparation schemes were
developed and put into practice. (Fig. 2). In such collection schemes for each or
several nearby wells were designed and operated their own compact complex of
process equipment for gas preparation (separators, condensate collectors, etc.). This
complex provided primary separation of well products from mechanical impurities
and reservoir water (hydrocarbon condensate released in a small amount was
practically not used and was mainly burned in flare stacks).
Subsequently, in the 60th of the twentieth century, the gas industry faced a new
scientific and practical task of introducing relatively large gas fields into development
and operation, such as, for example, Shebelinske in Ukraine and a number of others.
Figure 2 – Individual gas collection schemes:
а - linear; b – radial (like beams); c – circular (like loops);
1 - well (well cluster); 2 - flow line (in case of well cluster - gas collecting pipe);
3 - gas collecting pipe; 4 - gas pool outline
The following deficiencies in the individual gas collection and preparation
schemes were identified during operation of the well products collection and
preparation systems for gas and gas condensate fields:
- the difficulty of reliable operation of remote control systems for the process
mode of wells and field equipment for the collection and preparation of well
products;
- field equipment is distributed over a large area, since it is tied to almost every
well, which leads to high metal consumption of pipelines and process equipment in
general, significant lengths of field roads, etc.;
- a larger number of qualified personnel are needed to operate wells and well
products collection and preparation system.
Therefore, in the future, technologically more efficient and economically
feasible group gas collection and preparation schemes began to be used (Fig. 3).
Figure 3 – Group well products collection and preparation schemes for gas and gas
condensate fields:
а - centralized; b - decentralized;
1 - well (well cluster); 2 - flow line (in case of well cluster - gas collecting pipe);
3 - gas collecting pipe; 4 - gas pool outline
In these technological schemes the well products flow via flow line to
collection points (CP) where measurements and primary gas separation are
performed. Further, the gas is ducted to the system of gas collection headers
(sometimes looped to improve reliability), through which it flows to the group or
central gas collection point (GCP) - now CGTP. At the GCP, the gas is subjected to
final drying and cleaning and flows to the main gas pipeline (MGP).
The group centralized collection scheme for gas and gas condensate field is
currently the main typical collection scheme for Ukrainian fields. According to the
same scheme, a number of relatively small gas and gas condensate fields were mainly
engineered.
In the 70th and 80th of the twentieth century, the general direction of
development of the domestic gas industry was the development of unique gas and gas
condensate fields in Ukraine.
During this period, a group decentralized collection scheme was developed,
which significantly reduced capital and operational costs for the development of large
fields.
In accordance with this scheme, gas from well clusters flow via flow lines
(and/or gas collecting pipes) to high-productiveness GCTP or gas processing plant
(GPP), where in accordance with the requirements of regulatory documents, it
undergoes full field processing for subsequent transport. At large gas and gas
condensate fields may be several GCTP.
The choice of a particular collection system is due to the characteristic of the
field, a number of technological factors, the socio-production structure of the region,
etc., and their current state is directly related to the historical stages of the gas
industry development.
Current trends in the design of well products collection and preparation
systems of gas and gas condensate fields
Current trends in the design of natural gas and gas condensate collection and
preparation systems are the development of high-power and extra-power GE and
GCTP.
When enlarging well clusters and increasing the productivity of the GTP, an
urgent task is to optimize the quantity and productivity of the GCTP, which would
ensure, with minimal capital and operational costs, the production of the required
quality products, reliability of the equipment, environmental and industrial safety for
the long-term operation of field facilities.
Therefore, it is high-potential to design group systems for well products
collection, which should optimally combine the advantages of both centralized and
decentralized schemes.
Such optimal well products collection schemes are presented in Fig. 4.
Figure 4 – High-potential flowsheets of high-power and extra-power gas treatment complexes:
а - the GCTP has two process modules for field processing of well products - from the near clusters
α and the far clusters β; also in this scheme, one module can process the products of wells of a gas
productive formation, and the second one - a gas-condensate productive formation or an oil rim;
b – the well products of the far clusters are pre-processed at the GCTP -1 and GCTP -2, and the
well products from the near clusters are processed at the GCTP at once;
1 - well cluster; 2 - flow line, gas collecting pipe; 3 - gas collecting pipe
These flowsheets increase the reliability of the collection system operation and
make it possible to significantly reduce the consumption of inhibitors of hydrate
formation;.
The well products collection scheme from enlarged well clusters and fluid flow
through individual flow lines to the GСTP may be promising. This will reduce
operating costs associated with the consumption of inhibitors of hydrate formation,
corrosion, salt deposition, etc., in gas collection networks.
Current trends are associated with the potential of group gas collection systems
designing that optimally combine the advantages of both centralized and
decentralized schemes (Fig. 4) and the design of high-power GCTPs (and thus, with a
decrease of their number at the field). This will be associated with the need to
increase the number of wells connected to the GCTP or their flow rates, and
significantly increase the distances from the well clusters to the GCTP.
The main feature of the well product collection and processing schemes
presented in Fig. 4 is the division of well clusters in accordance with the length of
their flow lines into three groups: near, middle and far.
In this case, field treatment of crude gas coming from each cluster group can be
carried out separately on the GCTP process lines. Also, the well products from the far
clusters can undergo primary separation at the GPTP, from where it will flow through
the header to another GCTP for deeper gas drying and purification and field
processing of gas condensate to transport.
After the development and choice of a gas collection scheme, choice of the
general structure and length of gas collection networks for the designed GCTP, it is
necessary to solve the following tasks: to develop methods for laying flow lines and
headers and optimize their diameters and thicknesses.
The internal diameter of the gas pipeline (flow line, header) is selected in such
a way as to ensure the design pipeline-transmission capacity with the minimal
hydraulic losses and sufficiently high hydrodynamic efficiency of the flow of well
products, as well as to minimize the consumption of hydrate formation inhibitors if
possible. It is assumed that gas pressure losses during the flow of well products in the
gas pipeline should not exceed 0.05-0.1 MPa per 1 km of the flow line or header, and
the gas flow rate should be such that the liquid phase is completely removed from the
pipeline.
Determination of the optimal diameter and length of flow lines and headers
should be achieved by multifactorial and detailed analysis of many variants for
forecast thermal, hydrodynamic calculations of the operation modes of pipelines
when their pipeline-transmission capacity and composition of the well product of the
gas and gas condensate field change.
Prediction of the thermobaric mode of fluid flow in flow lines and headers also
makes it possible to substantiate the viability of including wellhead gas heaters into
the well product collection system at well clusters operating in automatic mode, and
during operation at cluster areas of fields of two groups of wells producing gas and
gas condensate from different production facilities (pay formations), also substantiate
the technology of heating colder gas by heat exchange with warmer gas coming from
the underlying pay horizon.
MAIN PHYSICAL AND CHEMICAL CHARACTERISTICS
OF GAS FIELD PRODUCTS
Natural gases
Natural gases are characterized by high methane content (67.0-99.0% vol.), and
heavy hydrocarbons content (C5+) is low (from complete absence up to 6.0% vol.).
Most gases contain 1-5% vol. non-hydrocarbon components: nitrogen, carbon
dioxide, hydrogen sulfide, carbonyl sulfide, carbon disulfide, etc.
The main physical and chemical characteristics of natural gases of some gas
and gas condensate fields are given in Table 1.
Таble 1
Content of components, % vol.
Field
Shebelynske,
Ukraine
Karachaganakske,
Republic of
Kazakhstan
Gazlynskе,
Conden- Content
Densate of С3Н8+
sity,
efficien- in gas,
3
g/m
cy, g/m3 g/m3
СН4
С2Н6
С3Н8
∑С4Н10
∑С5Н12+
N2
H2S
92,95
3,85
1,05
0,10
0,21
1,50
-
75,30
5,50
2,60
1,40
6,00
0,70
4,0
2,5
94,60
2,06
0,27
0,32
0,21
2,30
0,07
0,16 0,755
CO2
0,09 0,782
-
10-15
30
-
-
15-20
18
Uzbekistan
Shatlykske,
Turkmenistan
95,05
1,63
0,20
0,07
0,07
1,75
-
1,20 0,759
20-25
7
Quality requirements to final products
Sales gas
Sales gas quality parameters are based on the following requirements:
- gas during transportation should not cause corrosion of pipelines, valves,
instruments, etc. (the content of mechanical impurities, hydrogen sulfide, thiol
(mercaptan) sulphur and oxygen should be within the permissible limits);
- gas quality shall ensure its transportation in a single-phase gaseous state (the
parameter "gas dew point by moisture and hydrocarbons" indicates this);
- sales gas should not cause problems for the consumer when using it.
The presence of moisture, liquid hydrocarbons, aggressive (especially
sulphuric) and mechanical impurities in the gas reduces the throughput capacity of
gas pipelines, increases the consumption of inhibitors, increases corrosion, increases
the required power of compressor units, causes the clogging of monitoring and
metering instruments and regulation apparatuses lines.
All of this reduces the reliability of process systems, increases the possibility of
emergencies at compressor stations and gas pipelines.
In addition, dust and mechanical impurities contribute to metal abrasion and
sediment accumulation on the surfaces of heat exchangers, thereby impairing their
thermal characteristics.
Due to the fact that natural gas is transported over long distances from
production areas to the consumer through main gas pipelines crossing various
climatic zones, the issue of its high-quality treatment and drying to the dew point,
which excludes condensation of water from gas, is of particular importance.
The dew point in this case is the temperature at which the components of the
gas mixture reach the saturation state and condense. Thus, the dew point by moisture
is the water condensation temperature and the dew point by hydrocarbon is the
hydrocarbon condensation temperature from the gas mixture.
If during gas tanportation its temperature decreases, then at constant pressure
the equilibrium concentration of components of such gas decreases: the gas becomes
oversaturated. In this case, a portion of the moisture or hydrocarbons condenses and
precipitates in the pipe.
An important parameter of the sales gas quality is the oxygen content in it. The
value of this parameter is not more than 1%. With a higher oxygen content, the gas
becomes explosive. In addition, oxygen causes the increasing of corrosion in the
system.
Currently, sales gases supplied to main gas pipelines shall meet the
requirements given in Table 2.
Table 2
Climate region
Parameter
temperate
frigid
s
w
s
w
– by moisture
0
-5
-10
-20
– by hydrocarbon
0
0
-5
-10
1,0
1,0
1,0
1,0
0,02
0,02
0,02
0,02
0,036
0,036
0,036
0,036
0,003
0,003
0,003
0,003
Dew point,оС, not higher:
Oxygen content, % vol. not more
Hydrogen sulfide content, g/mм3,
not more
Thiol (mercaptan) sulphur content,
g/m3, not more
Mechanical impurities content, g/m3,
not more
Note: s – summer period (May 1 to September 30); w - winter period (October 1 to April
30). This classification is done for northern hemisphere, and for the southern hemisphere
- on the contrary.
Gas condensates
Some gas fields with high formation pressure (up to 25-30 MPa) differ in that
the gases are saturated with liquid petroleum hydrocarbons from С5Н12 to С20Н42 in
amounts of 5-400 g/m3 of gas. During the development of these fields, the pressure
decreases, liquid hydrocarbons condense and can be separated from the gas in the
form of liquid condensate. After condensate separation, the gas approaches dry gases
in composition, and the condensate contains gasoline and kerosene fractions. Gas
condensates are a significant resource of hydrocarbon raw materials.
Gas condensate (gas condensate) is a mixture of liquid hydrocarbons (С5Н12
and higher) released from natural gases when pressure decreases during their
production, preparation and transportation. In appearance, it is a colorless or slightly
colored liquid with a density of 600-800 g/cm3.
Gas condensates boil off in the temperature range of 25 to 360 °C and above,
some ones have a higher initial boiling point (103-210 °C) and some ones have a
lower final boiling point (197-234 °C). Condensates from different fields differ
greatly each from other in group chemical composition and total sulfur content.
The composition of the gas condensate roughly corresponds to the gasoline or
kerosene fraction of the oil or mixture thereof. During primary separation in the
fields, the mass content of dissolved gas in the gas condensate is up to 10%. This is
the so-called unstable condensate. After removal of volatile propane-butane fractions
at gas processing plants, stable condensate is obtained, which is a raw material for the
production of fuels and other oil products.
The technological classification of gas condensates is based on: saturated vapor
pressure, sulfur content in gas condensates and their fractions, content of aromatic
hydrocarbons in the gasoline fraction with a final boiling point of 200 °C, content of
n-alkane hydrocarbons in the diesel fuel fraction (200-320 °C), a fractional
composition (final boiling point) and the possibility of obtaining jet engine fuels and
diesel ones with and without dewaxing (deparaffination), the combination of values
of which offers a glimpse into the physics and chemical nature of a gas condensate
with the determination of the effective direction of its processing and qualified use.
Depending on the saturated vapors pressure, gas condensates are divided into
two kinds: unstable (P1) and stable (P2).
Unstable (deethanized) gas condensates are ones with saturated vapor pressure
above 93,325 Pa (700 mm. Hg.) containing hydrocarbons С3, С4, С5+ and partially С2.
Stable (debutanized) gas condensates are ones with a saturated vapor pressure of not
more than 93325 Pa, consisting of hydrocarbons С5+.
In order to eliminate the loss of light hydrocarbons, all gas condensates with a
saturated vapor pressure above 93325 Pa shall be stabilized. The obtained broad
fraction of light hydrocarbons contains propane, butane and partially pentane (i- and
n-structures), which are raw materials of the petrochemical industry.
Depending on the sulphur content, stable gas condensates are divided into three
classes:
I - low-sulphur or sulphur-free;
II – sulphur;
III – high-sulphur.
Sulfur content in condensates and their distillation products for I, II and III
classes shall meet requirements given in Table 3.
Table 3
Class
in gas
condensate
Sulfur mass %
in distillation fractions
gasoline
kerosene
diesel fuel
(final boiling
(jet engine fuel)
fraction
point is not
о
(135 - 200 С) (200 - 320 оС)
higher 200 °C)
І
not more than
0,05
ІІ
0,051 – 0,08
ІІІ
more than 0,08
not more than
0,03
not more than
0,1
more than 0,1
not more than
0,01
not more than
0,1
more than 0,1
not more than
0,03
not more than
0,5
more than 0,5
If fractions obtained from low-sulphur gas condensate contain sulfur more than
the limits specified for class I, these gas condensate shall be classified as class II gas
condensate.
If the fractions separated from sulphur gas condensate contain sulphur not
more than the limits specified for products obtained from low-sulphur gas
condensate, it should be classified as class I gas condensates.
If one or all distillation fractions obtained from sulphur gas condensate contain
sulphur more than the limits specified for this class, it should be classified as class III
gas condensates.
Class III gas condensates can be classified as class II only when all distillate
fuels obtained from them contain sulfur not more than the limits specified for sulphur
gas condensate products.
Thus, the class of gas condensate is determined not only by the content of
sulphur in the gas condensate, but also by the amount of sulfur in its fractions.
Depending on the content of aromatic hydrocarbons in the gasoline fraction (up
to 200 °C), gas condensates are divided into three sorts: A1, A2, A3.
Sort A1 includes gas condensates with an aromatic hydrocarbon content in the
gasoline fraction of more than 20 %. Using these gas condensates, it is cost effective
preliminary aromatic hydrocarbons extacting using raffinate as a catalytic reforming
raw materials to produce aromatic hydrocarbons and high-octane components.
Sort A2 includes gas condensates with an aromatic hydrocarbon content in the
gasoline fraction of 10–20 %. If the content of naphthenic hydrocarbons is not lower
than 38 %, it is reasonable to use the gasoline fraction of these gas condensates as a
catalytic reforming raw materials.
Gas condensates of sort A3 are characterized by content of aromatic
hydrocarbons not more than 10%. This sort of raw materials is used for pyrolysis, but
can also be used for catalytic reforming if they have a high content of naphthenic
hydrocarbons.
Depending on the content of n-alkane hydrocarbons in the fraction of 200-320
°C, which make it possible to obtain fuel for jet engines, winter diesel fuels without
dewaxing or using it and liquid paraffins for the microbiological and chemical
industries, gas condensates are divided into four types: N1, N2, N3, N4. Another
characteristic of the gas condensate type is its pour point.
N1 - high-paraffin gas condensates, in the fraction of 200-320 °C of which the
content of n-alkane hydrocarbons is more than 25 % by weight. The pour point of
these condensates is not lower than minus 15 °C. From these gas condensates, jet
engine and winter diesel fuels can be obtained with dewaxing. These gas condensates
can be used to produce liquid n-alkanes for the synthesis of protein-vitamin
concentrates as raw materials.
N2 - paraffin gas condensates, in the fraction of 200-320 °C of which the
content of n-alkane hydrocarbons is from 18 to 25 % by weight. The pour point of
these condensates is from minus 10 °С to minus 25 °С. From these gas condensates,
jet engine and winter diesel fuels can be obtained without dewaxing. Gas condensates
of this type are also used for recovering liquid n-alkanes with dewaxing.
N3 - paraffin gas condensates, in the fraction of 200-320 °C of which the
content of n-alkane hydrocarbons is from 14 to 18 % by weight. The pour point of
these condensates is from minus 40 °С to minus 60 °С. From these gas condensates,
jet engine and winter diesel fuels can be obtained without dewaxing and are not used
for separation of liquid n-alkanes. The diesel fractions of these gas condensates can
be used for recovering n-alkanes mixing them with a high-paraffin raw materials.
N4 - paraffin-free gas condensates, in the fraction of 200-320 °C of which the
content of n-alkane hydrocarbons is less than 14 % by weight. The pour point of
these condensates is below minus 60 °C. These include gas condensates of light
fractional composition that do not contain diesel fuel fractions or are distilled at a
temperature not exceeding 250 °C, and gas condensates of depleted (marginal) fields
with a formation pressure less than 98∙105 - 147∙105 Pa (100 – 150 kgf/cm2). From
these gas condensates, jet engine and winter diesel fuels can be obtained without
dewaxing and they are not used for recovering liquid n-alkanes.
Depending on the fractional composition (final boiling point), gas condensates
are divided into three groups:
F1 – high-boiling gas condensates with final boiling point higher than 320 °С.
F2 – gas condensates with intermediate fractional composition which have final
boiling point from 250 to 320 °С.
F3 - gas condensates with light fractional composition which have final boiling
point lower than 250 °С.
When indexing each gas condensate, the following shall be indicated:
kind – P1, P2;
class - I, II, III;
sort - А1, А2, А3;
type - N1, N2, N3, N4;
group - F1, F2, F3.
The combination of designations of kind, class, sort, type and group represents
the code of technological characteristics of gas condensates.