Reservoir Engineering
Introduction:
→The volume of
hydrocarbons contained in a reservoir may be calculated either directly by
volumetric methods, or indirectly by material balance. Accuracy of the
volumetric method depend s primarily on accuracy of data for porosity, net
thickness ,hydrocarbon saturation, and areal extent of the reservoir. Accuracy
of the material balance method is primarily dependent on reliability of
production data and PVT relationships for the reservoir hydrocarbons. Since
produced water has no economic value, production records for water are
frequently less reliable than for oil or gas. Casing leaks and poorly cemented
casing are other possible sources of error in determining the volume of water
produced from a reservoir.
Accuracy of material balance calculations
increases as more hydrocarbons are produced from the reservoir. Unfortunately,
this means that the calculations are least reliable when accurate information
on reservoir volume would be most useful: early in the life of the reservoir.
Satisfactory accuracy from material balance calculations can usually be
achieved after roughly five to ten percent of the hydrocarbons originally in
place have been produced.
Volumetric
analysis:
The volumetric method for estimating hydrocarbon
volume is based on the use of geologic maps, usually derived from log and core
data.
Material
balance analysis:
The
term “material balance” is well accepted in reservoir engineering that it can’t
be changed, however the subject could more accurately be called “volumetric
balance”.
When a volume of oil is produced from a
reservoir the space once occupied by this oil must be filled by something else.
Applications
of material balance:
Material balance equation has been in
general used for:
1) Determining the initial oil in place.
2) Calculating water influx.
3) Predicting reservoir pressure.
General
difficulties in applying material balance:
1) Lack of
PVT data for specific reservoirs.
2) The
assumption of constant liberated gas composition.
3) Accuracy of production data.
4) Accuracy of reservoir pressure data.
Limitations
of material balance:
1) Thicker formations of high permeabilities and low oil
viscosities where the average reservoir
pressures are easily obtained.
2) Producing formations composed of homogenous strata of nearly
the same permeability.
3) In case of no very active water drives and no gas caps which
are large compared with oil zone because of the very small pressure decline in
case of very active water drive and large gas cap formations.
Sources
of reservoir energy and primary production:
1) Water drive:
A water drive reservoir has a hydraulic
connection between the reservoir and a porous, water saturated rock called an
aquifer.
The water in an aquifer is compressed. As reservoir
pressure is reduced by oil production, the water expands, creating a natural
water flood at the reservoir /aquifer boundary
2) Solution –gas drive:
This type of reservoir the principle sources of
energy is a result of gas liberation from the crude oil and the subsequent
expansion of the solution gas as the reservoir pressure is reduced.
3) Rock and liquid expansion:
When an oil reservoir initially exists at a
pressure higher than its bubble point pressure, the reservoir is called under
saturated reservoir. As the reservoir pressure declines, the rock and fluids
expand due to their individual compressibility so the expansion of the fluid
and reduction in the pore volume, force the crude oil and water out of the pore
space to the well bore.
4) Gas cap drive:
When a
reservoir has a large gas cap, there maybe a large amount of energy stored in
the form of compressed gas, the gas cap expands as fluids are withdrawn from
the reservoir displacing the oil by a gas drive assisted by gravity drainage.
5)Gravity Drainage:
Gas bubbles that are evolved from solution as
pressure declines near a producing well will migrate toward the well and be
produced. Gas bubbles that are evolved at a greater distance from the well will
migrate up dip displacing oil downward toward the well.
Ø Pressure maintenance:
Ultimate
recovery from oil reservoir can often be increased by augmenting the natural reservoir energy.
This increased
recovery is due to one or both of the following factor:
1.
Decreasing the depletion
drive index by maintaining reservoir pressure the maximum possible
2.
Replacing the natural
displacing force,
as for example :
replacing the gas cap drive with an artificial water drive
Returning gas to the reservoir to
maintain the reservoir pressure and displace the oil from the reservoir by an
expanding artificial gas cap (secondary gas cap) , could be classified in both
of the above categories, since the
depletion drive index will be reduced and
expanding external gas drive is certain to be more efficient than the
dissolved gas drive.
Pressure maintenance operations can
be divided into four distinct categories:
1.
Gas injection
2.
Water injection
3.
Miscible fluid injection
4.
Combinations of the aforementioned fluid
The installation
of pressure maintenance facilities often requires the expenditure of large sums
of money, and although addition oil recovery must be more than the pay cost of
the installing and operating the pressure maintenance facilities.
Maintaining
reservoir pressure at a high level offers several advantages:
1.
Oil viscosity is reduced
because of the larger amount of gas retained in solution
2.
Effective permeability
to oil is increased as a direct result of the decreased liberation of gas from
the oil
3.
The flowing life of the
reservoir is extended
Pressure maintenance by the gas injection:
Gas is the widely
used fluid for Pressure maintenance operation for the following reasons :
1.
Gas is readily available
in many areas, either from the reservoir being produced or from extraneous
sources.
2.
So, it have low costs.
3.
The gas is nonreactive
with the reservoir rock
4.
It may be desirable to
conserve the produced gas for a future gas injection processes where it will
not only stored in the reservoir ,but will also displace oil.
The problemsof the gas injection:
(especially for heavy oil and/or high viscous oil):
1.
Lower efficiency of
displacing Gas
2.
Gas fingering(fingering
effect)
3.
Trapping oil in the gas
zone
Pressure maintenance by the water
injection:
Commonly used
where sutible water is available,(as near the shore or supply water wells)
Pressure maintenance by the water injection has allthe
inherent advantages Pressure maintenance by the gas injection and also
has additional
advantages of:
1.
Amore efficient
displacing fluid
2.
The displacing water
travels more uniformly through the reservoir with less oil-bypassing.
Disadvantages
of water injection (the problem):
The
principle problems
1.
being the reaction of
water injection with reservoir rock
2.
the corrosion of both
surface and subsurface mechanical equipments by corrosion materials in water
3.
sometimes be very costly
(for treatment to be compatible with the reservoir conditions
