Reservoir Engineering

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