EXPLORATORY DRILLING
The data collected from the geologic and geophysical surveys are used to formulate probable
definitions and realizations of the geologic structure that may contain oil and/or gas.
However, we still have to determine whether petroleum exists in these geologic traps, and if it
does exist, would it be available in such a quantity that makes the development of the oil/ gas
field economical?
The only way to provide a definite answers is to drill and test exploratory well(s).
The exploratory well, known as the wildcat well, is drilled in a location determined by the
geologists and geophysicists. The well is drilled with insufficient data available about the
nature of the various rock layers that will be drilled or the fluids and pressures that may exist
in the various formations. Therefore, the well completion and the drilling program are usually
overdesigned to assure safety of the operation. This first well, therefore, does not represent
the optimum design and would probably cost much more than the rest of the wells that will be
drilled in the field.
As this exploratory well is drilled, samples of the rock cuttings are collected and examined for
their composition and fluid content. The data are used to identify the type of formation versus
depth and to check on the presence of hydrocarbon materials within the rock. Cores of the
formations are also obtained, preserved, and sent to specialized laboratories for analysis.
Whenever a petroleum-bearing formation is drilled, the well is tested while placed on
controlled production. After the well has been drilled, and sometimes at various intervals
during drilling, various logs are taken. There are several logging tools, or techniques, (electric
logs, radioactivity logs, and acoustic logs) that are used to gather information about the drilled
formations. These tools are lowered into the well on a wireline (electric cable) and, as they are
lowered, the measured signals are transmitted to the surface and recorded on computers. The
signals collected are interpreted and produced in the form of rock and fluid properties versus
depth.
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The exploratory well will provide important data on rock and fluid properties, type and
saturation of fluids, initial reservoir pressure, reservoir productivity, and so forth. These are
essential and important data and information, which are needed for the development of the
field. In most situations, however, the data provided by the exploratory well will not be
sufficient. Additional wells may need to be drilled to provide a better definition of the size
and characteristics of the new reservoir. Of course, not every exploratory well will result in a
discovery. Exploratory wells may result in hitting dry holes or they may prove the reservoir to
be an uneconomical development.
DEVELOPMENT OF OIL AND GAS FIELDS
The very large volume of information and data collected from the various geologic and
geophysical surveys and the exploratory wells are used to construct various types of map.
Contour maps are lines drawn at regular intervals of depth to show the geologic structure
relative to reference points called the correlation markers. Isopach maps illustrate the
variations in thickness between the correlation markers. Other important maps such as
porosity maps, permeability maps, and maps showing variations in rock characteristics and
structural arrangements are also produced. With all data and formation maps available,
conceptual models describing the details of the structure and the location of the oil and gas
within the structure are prepared.
The data available at this stage will be sufficient to estimate the petroleum reserves and decide
and plan for the development of the field for commercial operation.
The development of petroleum fields involves the collective and integrated efforts and
experience of many disciplines. Geologists and geophysicists are needed, as described earlier,
to define, describe, and characterize the reservoir. Reservoir engineers set the strategy for
producing the petroleum reserves and managing the reservoir for the life of the field.
Production and completion engineers design the well completions and production facilities to
handle the varying production methods and conditions, and drilling engineers design the welldrilling programs based on well-completion design. In the past, each group used to work
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separately and deliver its product to the next group. That is, when geologists and
geophysicists finish their work, they deliver the product to the reservoir engineering group.
Then, reservoir engineering would deliver the results of their work to production engineering,
and so on. In almost all cases, it was necessary for each group to go back to the previous
group for discussion, clarification, or requesting additional work. This has been realized as a
very inefficient operation. In recent years, most major companies have adopted what is known
as the multidisciplinary team approach for field developments. In this approach, a team
consisting of engineers and scientists covering all needed disciplines is formed. The team
members work together as one group throughout the field development stage. Of course, other
specialists such as computer scientists, planners, cost engineers, economists, and so forth
work closely with the team or may become an integral part of the team. Experience has shown
that this field development approach is very efficient; more and more companies are
moving in this direction.
The following section provides brief descriptions of the roles and functions of drilling.
DRILLING ENGINEERING AND OPERATIONS
Following the preparation stage of field development (i.e., setting the production strategy,
determining the locations of the wells in the field, and designing the well completions), the
drilling-related activities begin. The drilling program is first designed. Then, plans are
prepared and executed to acquire the required equipment and materials. The drilling sites in
the field are then prepared for the equipment and materials to be moved in, and the drilling
operations begin. Depending on the organization of activities within the oil company, drilling
engineers may only be responsible for drilling and casing of the well, and production
engineers will be responsible for completion of the well. Alternatively, drilling engineers may
be responsible for drilling and completion of the wells.
The drilling program consists of three main stages:
(1) drilling the hole to the target depth,
(2) setting the various casings, and
(3) cementing the casing.
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DRILLING THE WELL
Well drilling has gone through major developments of drilling methods to reach the modern
method of rotary drilling. In this method, a drilling bit is attached to the bottom end of a string
of pipe joints known as the drilling string. The drilling string is rotated at the surface, causing
rotation of the drilling bit. The rotation of the bit and the weight applied on it through the
drilling string causes the crushing and cutting of the rock into small pieces (cuttings). To
remove the cuttings from the hole, a special fluid, called the drilling fluid or the drilling mud,
is pumped down through the drilling string, where it exists through nozzles in the bit as jets of
fluid. This fluid cleans the bit from the cuttings and carries the cuttings to the surface through
the annular space between the drilling string and the wall of the hole. At the surface, the mud
is screened to remove the cuttings and is circulated back into the drilling string.
The drilling operation is performed using huge and complex equipment known as the drilling
rig. This is briefly described next.
THE DRILLING RIG
Figure 1 shows a schematic of a rotary drilling rig. It consists of two main sections, the
substructure (bottom section) and the derrick (top section). The substructure, which ranges
from 15 to 30 ft in height, is basically a rigid platform that supports the derrick. The rig is
composed of several systems and components; the major systems are as follows:
1. Power System:
It consists of diesel-engine-driven electric generators to supply electric power to the various
systems and components of the rig. About 85% of the power generated is consumed by the
drilling-mud circulating system.
2. Hoisting System:
The function of this system is to lower and raise the drilling string into and out of the well.
The main components of the system are the crown block, the drawworks, the traveling block,
the hook, the swivel, the elevator, the drilling line, and the dead-line anchor.
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3. Rotating System:
The system consists mainly of a motor-driven rotating table that is used to rotate a pipe with a
square or hexagonal cross section, called the kelly, while allowing it to slide through. The
kelly is suspended by the hoisting system and is connected at its bottom to the drill pipe and
the bit.
4. Circulating System:
The system consists mainly of drilling-mud storage tanks, high-pressure pumps, circulating
hoses and pipes, and a shale shaker. Its function is to circulate the mud through the well to
bring the cuttings to the surface.
5. Drilling Bit:
The drilling bit is the device that does the actual drilling by crushing and cutting the rock as it
rotates with some force applied by the drilling string. There are different types and shapes
used for different types of rock.
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Fig. 1 A schematic of a rotary drilling rig.
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DRILLING FLUID (MUD)
The drilling fluid is a very important element of the drilling operation. Its importance stems
from the many essential functions it serves. Some of these functions are as follows:
1. Transporting the cuttings from the bottom of the hole to the surface
2. Cooling of the bit and lubrication of the drill string
3. Exerting hydrostatic pressure to overbalance the pressure of the formation and thus prevent
flow of formation fluids into the well
4. Supporting the walls of the hole to prevent it from caving in
5. Enhancing drilling by its jetting action through the bit nozzles
The drilling fluid can be prepared in different formulations to provide the desired properties
(density, viscosity, and filtration) under the bottom hole conditions. The basic drilling fluid
consists of water and clay (water-base mud). Other materials and chemicals are also added to
control the properties of the fluid. Other fluids such as foam and air have also been used in
drilling operations.
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CASING THE WELL
The casing is a steel pipe that is placed in the drilled hole (well) to support the wall of the hole
and prevent it from collapsing. When cemented to the wall, it seals the subsurface formation
layers and prevents communications between the various layers.
Normally, four strings of casing of different diameters are installed in the well at various
depths that are specified by the geologist. These are the conductor, the surface casing, the
intermediate casing and the production casing. The conductor has the largest diameter and
shortest length of the four casing strings; the production casing has the smallest diameter and
longest casing.
Casings of various outside diameters are available in different grades and weights. The grade
refers to the type of casing steel alloy and its minimum yield strength. Commonly available
grades are H-40, J-55, N-80, C-75, L-80, and P-105. The letter (H, J, etc.) identifies the type
of alloy and heat treatment; the number (40, 55, etc.) refers to the minimum yield strength in
thousands of pounds per square inch (psi). For a given outside diameter and grade, casings are
available in different weights (i.e., various inside diameters) expressed in pounds per linear
foot of casing. The weight and grade of the casing specify its resistance to various loads such
as burst, collapse, and tension loads. In designing casing strings, weight and grade must be
selected such that the casing string will not fail under all loads to which it will be subjected
during drilling, setting casing, and production.
To set a casing string, the drilling operation is stopped when the desired depth is reached and
the drill string and the pit are pulled out of the hole. The casing string is then lowered into the
hole, joint by joint, using the hoisting system of the rig, until the total length of casing is in
the hole. A round, smooth object called the guide shoe is attached to the bottom of the first
joint of casing to ease and guide the movement of the casing into the hole. The casing is then
cemented to the wall and drilling operation is resumed until the target depth for the next
casing string is reached. Normally, before setting the production casing, the petroleum
formation is logged and evaluated. The casing will be set only if the logging results indicate
the presence of a productive formation. Otherwise, the well will be abandoned.
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CEMENTING THE CASING
To cement the casing, the annulus between the casing and the wall of the hole must be filled
with cement. To achieve this, the required volume of the cement slurry (prepared on location)
is pumped through the casing.
A special rubber plug is normally inserted ahead of the cement to separate it from the mud
and prevent any contamination of the cement with mud. Another plug is inserted after
pumping the specified volume of cement. This is followed by pumping a fluid (normally
mud) to displace the cement. When the first plug reaches the bottom, pumping pressure is
increased to rupture the plug and allow the flow of cement behind the casing. When the top
plug reach the bottom, the cement must have filled the annulus to the surface. The cement is
then left undisturbed until it sets and acquires enough compressive strength before resuming
the drilling operation for the next casing string.
Once the casing is cemented, it becomes permanently fixed into the hole. It is very important
to have a good cement bond between the casing and the wall of the hole. For this purpose, a
special log (cement bond log) is conducted to check the integrity of the cementing operation.
Failure to have a good cementing job will necessitate expensive remedial cementing
operations.
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