this is a reservoir engineering paper

1. Eric Hagenimana
A00019505
Reservoir Engineering (PET 201)
American University of Nigeria
October 13, 2014
Oil production from well discovery to the last stage of production
Oil and gas are now the most reliable source of energy. These crucial sources of
energy are all generated from fossil fuels. They are extracted from underground reservoir
during production, however, before production numerous geologic researches are taken in
place. The early stage in oil and gas circle is exploration where geophysical technics like
gravity and seismic survey are made use of to evaluate the potential of oil and gas
availability in the basin being studied. After exploration where it is confirmed that the
amount of oil and gas available are economically recoverable production facilities are put
in place to start production. Both oil and gas exploration and production and technics that
are used during both processes are discussed briefly below in this paper.
Oil and gas exploration this is the early stage which involves the geologic research
for the availability of oil and gas in basin being considered. Exploration starts first by
observation of terrain and location of faults. These analyses are very crucial in oil and gas
exploration because they indicated where potential structural traps may lie beneath the
surface in reservoir rocks. There are three type of methods used to find oil and gas in the
subsurface namely geophysical survey, remote sensing, and exploration wells.
Geophysical surveys these are geophysical technics used for oil and gas
exploration that use equipment to measure things like electrical currents, gravitational
and magnetic anomalies, heat flow, geochemical relationships, and density variation deep
within the earth. Each tool used records different properties which can help to locate oil
and gas underground the surface of the earth. Geophysical surveys also use Seismic
surveys which are techniques that use vibration to show the picture of subterranean rock
formations at depth, often as deep as 30,000 feet below the ground level. Seismic surveys

2. generate sound waves downwar d into the earth’s crust which then reflects off various
boundaries between different rock strata. On land, the sound waves are generated by
small explosive charges embedded in the ground. This type of survey helps to know how
deep the reflecting layers are based on time it takes the sound waves to reflect. The
greater the time the deeper the rock layers are. Seismic surveys also can determine what
type of rock is present based on how the rock transmits sound waves. The most
sophisticated seismic surveys are three dimensional (3-D). The recorded data using 3-D
seismic survey are processed using a computer and the results are detailed 3-D picture of
the formations and structures below the surface. There are other geophysical techniques
like magnetometers and gravimeters. A magnetometer is a device that is pulled behind an
airplane that detects variations in the earth’s magnetic field. Sedimentary rocks generally
have low magnetic properties compared to other rock types. A gravimeter measures
minute differences in pull of gravity at the earth’s surface. Therefore, mapping the
differences reveals large masses of dense subsurface rock which allows geologists to
have a better ides of the structure below the ground. In addition, geochemical prospecting
uses sensitive instrument to detect minute quantities of gases that seep upward from
petroleum deposit. Geochemical prospecting is a new technique, but is one that is gaining
wider acceptance.
Remote sensing this method uses aerial photograph to locate and map surface
features. It uses satellites which help to take images of large areas on the surface of the
earth. Satellites images reveal subtle variations in soil moisture, mineral and vegetation
distribution, soil type, all of which play an important role in exploration processes. Once
the field is selected and satellites images are obtained, the exploration geologists uses
their mapping techniques to produce geologic map which is a map that shows geologic
structures by using conventional symbols. Remote sensing method helps exploration
geologist to map the field being studied and from the features available it is for them to
know if oil and gas may be found.
Exploration wells after conducting the above mentioned surveys it is not yet
confirmed that the oil and gas expected in the reservoir are commercially recoverable;
hence, exploration wells are drilled to help to know how much amount of oil or gas is
available in the reservoir. It is after drilling exploration wells that a company will know if
it is going to develop the field or it is going to abandon it because the amount available is
commercially recoverable.
After exploration, now the company has all information needed about the field.
When the data obtained from exploration process show that the amount of oil or gas
available is commercially the company goes on develop the field. The first thing the

3. company does here is wells development. There already some wells drilled during
exploration, but they don’t suffice for them to be used in production. Therefore,
additional wells are required. At this stage drilling intensely takes place; hence, let have a
look at how drilling is done and techniques used during drilling. Two most methods used
in drilling are air rotary drilling and mud rotary drilling.
Air rotary drilling is a method used to drill deep boreholes in rock formations.
Most water wells and environmental monitoring wells drilled into bedrock are
constructed using air rotary. It is used for the drilling in all rock types, igneous,
metamorphic, and sedimentary rock. The mining industry uses rotary drilling to drill ore
body test boreholes and pilot boreholes for guiding larger shaft borings. Deep drilling for
the recovery of crude oil and natural gas are almost exclusively carried out in
sedimentary rock. The rotary drilling method requires the use of rock cutting or crushing
drill bit, typically a mill tooth tri-cone roller bit. A mill tri-cone roller cone bit uses more
of crushing action to advance the bit in the rock. Impact energy is supplied to the drill bit
from either an above ground impact or down hole impact hammer. This impact force aids
in the drilling. is a method used to drill deep boreholes in rock formations. Most water
wells and environmental monitoring wells drilled into bedrock are constructed using air
rotary. It is used for the drilling in all rock types, igneous, metamorphic, and sedimentary
rock. The mining industry uses rotary drilling to drill ore body test boreholes and pilot
boreholes for guiding larger shaft borings. Deep drilling for the recovery of crude oil and
natural gas are almost exclusively carried out in sedimentary rock. There are variations in
air rotary drilling method such as Direct Air rotary Casing Hammer, Reverse Circulation,
Down Hole Hammer, and Under Reaming. In Direct Air Rotary known top drive both
rotational down force and impact force are provided by an above hole impact hammer
and a rotating drill head. Drilling cuttings are taken out from the borehole by means of
injecting high compressed air down the middle of the drill rod exiting out of the annular
space between the drill rod and bore hole. In Reverse Circulation Air Drilling, rotational
and impact energy are delivered in the same way as Direct Air Rotary the difference lies
in that the samples are obtained when high pressure compressed air is injected down the
annular space between the drill rod and bore hole and exits up the middle of the drill rod.
Air Rotary. Under Reaming is a variation of the Top Drive, casing advancer method that
uses a rotating cutting bit in front of the advancing casing that cuts a slightly larger
diameter hole than the outside diameter of the casing. The larger diameter hole allows the
casing to be more easily advanced in tight formations and in deeper drilling applications
where the longer drill stem encounters significant side wall friction. Mud Rotary Drilling
like Air Rotary Drilling Mud Rotary also uses tri-cone roller. Mud Rotary Drilling is
often used in soft sediments that may or not be saturated with groundwater.

4. They are types of drilling such as Horizontal Drilling, Multilateral Drilling,
Extended Reach Drilling, and Complex Path Drilling. Horizontal drilling starts with a
vertical that turns horizontal within the reservoir rock in order to expose more open hole
to the reservoir. Horizontal wells are very important because they are used where
conventional drilling is impossible or costly. They reduce surface disturbance by
requiring less wells to reach the reservoir, and they can also produce much oil and gas
(15 to 20 times) compared to a vertical well. Multilateral Drilling, there exist situations
whereby oil and gas reserves are located in separate layers underground. Therefore,
multilateral drilling helps to branch the well to the reserves. It increases production and
reduces the number of wells drilled on the surface. Extended reach drilling helps
producers to reach deposits that are at a great distance away from drilling rig. Extended
reach drilling helps producers to produce oil and natural in areas where vertical well
cannot be used. The Complex Path Drilling has multiple twists and turns which helps to
hit multiple accumulations from single well location. They are cost effective and produce
less waste and surface impacts than drilling multiple wells.
Furthermore, there are exploration wells which were drilled during exploration to
help to estimate the amount of hydrocarbons available in the underground reservoir.
Those wells may be used during production when the economically recoverable amount
of hydrocarbons were found, or may be cemented and abandoned when the amount of
hydrocarbons found in reservoir are not economically recoverable. On the other hand,
production wells are wells that are drilled due to production purposes. They are used to
extract hydrocarbons throughout the lifelong of the reservoir. When the reservoir matured
i.e. hydrocarbons are being recovered in relatively small amount, this the time of the field
abandonment; therefore, the production wells are cemented and ceased to be used.
After drilling well the next stage is well completion. The drilled wells must be
completed before oil and gas production begins. The first process of this stage is casing
pipe well. Oil and gas wells require four concentric strings of pipe: conductor pipe,
surface casing, intermediate casing, and production casing. The production casing known
as oil string is the final casing for most wells. The production casing completely covers
the producing formation from water aquifers. It runs to the bottom of the hole or stops
above the production zone. Casing and cement run to the bottom of the hole and seal off
the reservoir and prevent fluids from leaving. The casing is then perforated to allow
fluids to follow into the well. After the completion of well it is now the time of starting
the flow. Before oil production takes place the drilling mud must be removed from inside
the casing. This is done by pumping salt water into the tubing. The flow is started by
forcing high-pressure gas into the tubing. In cases when the well does not produce at fast

5. rate, in such situation, flow from the reservoir is increased by stimulation. Stimulation is
among the processes that enlarge channels in the reservoir rock so that the oil gas can
move through it and into the well.
The flow has been started, but what drives the oil or gas to flow from underground
reservoir to surface. Drive mechanisms are mechanisms that give the oil or gas the
required energy for it to move from the reservoir to the surface. The first stage of
reservoir’s producing life the drive mechanism of hydrocarbon flow is the pressure drop.
This stage of production is termed ‘primary recovery.” There are four principal primary
recovery drive mechanism namely water drive, solution gas drive, gas cap drive, and
compaction drive. Depending on nature of reservoir all the mentioned drive mechanisms
play an important role in primary recovery of hydrocarbons.
The reservoir continues to produce under natural drive mechanisms; however,
there come time at which the natural drive mechanisms are no longer able to drive the
production. In this case, other means are used to increase the production from the
reservoir. Artificial lifting are methods used when the pressure of reservoir decreases to a
point at which the wells are not able to produce. Artificial lifting uses Pumps and gas
injection. The most known method of pumping oil in land based wells is beam pumping.
When the use of artificial lifting is no longer effective i.e. when the injected gas cannot
increase the pressure of the reservoir, and the pump cannot pump the oil, based on the
statistics of the reservoir.
If the amount of oil remaining in the reservoir is economically recoverable,
advanced technics are used. At this level “Enhanced Recovery Techniques” are used to
boost the production. The major methods of enhanced recovery techniques are water
flooding, gas injection, chemical flooding, and thermal recovery. Water flooding is
technique where water is injected into the formation using the wells that are no longer
producing. The injected water enters the reservoir and displaces the oil toward the
producing wells. Water flooding is the least expensive and most widely used secondary
recovery method. Chemical flooding injects chemicals dissolved in water to push the
remaining oil toward the producing wells. All of those methods are used to push the
remained oil toward the producing wells where it is pumped out to the surface. However,
these methods are costly; they are not used every time whenever the amount of oil
remaining in the reservoir is not economically recovered the field is abandoned instead of
using enhanced recovery techniques.

6. References
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1, 2014).
Oil and Gas BMP Project. n.d. http://www.oilandgasbmps.org/resources/development.php (accessed 10
1, 2014).
Petroleum Online: Petroleum Geology and The exploration Process. n.d.
PetroStrategies, Inc. . n.d. http://www.petrostrategies.org/Learning_Center/production.htm (accessed
October 13, 2014).