Reservoir Drive Mechanisms

 The production of hydrocarbon from a reservoir into the wellbore involves several

stages of recovery. The available drive mechanisms determine the performance of

the hydrocarbon reservoir. When the hydrocarbon fluids are produced by the natural

energy of the reservoir, it is termed primary recovery; which is further classified

based on the dominant energy responsible for primary production. There are six

primary drive mechanisms, they are:

• Solution Gas (Depletion) Drive

• Water Drive

• Gas Cap Expansion (segregation) Drive

• Rock Compressibility and Connate Water Expansion Drive

• Gravity Drainage

• Combination Drive

5.7.1 Basic Data Required to Determine Reservoir Drive

Mechanism

• Reservoir pressure and rate of decline of reservoir pressure over a period of time.

• The character of the reservoir fluids.

• The production rate.

• Gas-Oil ratio.

• Water-oil ratio.

• The cumulative production of oil, gas and water.

5.7.2 Solution Gas (Depletion) Drive

A solution gas or depletion drive reservoir is a recovery mechanism where the gas

liberating out of the solution (oil) provides the major source of energy. We simply

define it as the oil recovery mechanism that occurs when the original quantity of oil

plus all its original dissolved gas expansion as a result of fluid production from its

reservoir rock (Fig. 5.7).

This drive mechanism is represented mathematically as:

Production Characteristics (Prof Onyekonwu MO, Lecture Note

on Reservoir Engineering)

• Pressure

– declines rapidly and steadily

– decline rate is dependent on production rate

• Oil Rate

– declines rapidly at first as oil mobility decreases

– steady decline thereafter

• Producing GOR

– Increases rapidly as free gas saturation increases.

– Thereafter, decreases rapidly as the remaining oil contains less solution gas.

• Water Production

– Mostly negligible as depletion type reservoirs are volumetric (closed) systems.

• Ultimate Oil Recovery

– It may vary from less than 5% to about 30%. Thus, according to Cole (1969)

these characteristics can be use to identify a depletion drive reservoir.

5.7.3 Gas Cap Expansion (Segregation) Drive

Segregation drive (gas-cap drive) is the mechanism wherein the displacement of oil

from the formation is accomplished by the expansion of the original free gas cap as

shown in Fig. 5.8.

The following are some of the points to note in a gas cap expansion drive

mechanism:

• A gas cap, existing above an oil zone in the structurally higher parts of a reservoir,

provides a major source of energy. The pressure at the original GOC (Fig. 5.8) is

the bubble point pressure since the underlain oil is saturated.

• As pressure declines in the oil column, two things happen:

– Some dissolved gas comes out of oil

– Gas cap expands to replace the voidage

Formation of free gas in the oil column should be minimized as much as possible.

This is achieved if:

– Gas is re-injected in the gas cap, and

– Gas is allowed to migrate upstructure (Gravitational Segregation) (Fig. 5.9).

 Production Characteristics (Prof Onyekonwu MO, Lecture Note

on Reservoir Engineering). The characteristics trend for gas cap

reservoir listed below were comprehensively summarized by

Clark (1969)

• Pressure

– The reservoir pressure falls slowly and continuously

• Oil Rate

Increase in gas saturation leading to increase in the flow of gas and a drop in

the effective permeability of oil.

• Producing GOR

– The gas-oil ratio rises continuously in up-structure wells. As the expanding gas

cap reaches the producing intervals of upstructure wells, the gas-oil ratio from

the affected wells will increase to high values.

• Water Production

– Absent or negligible water production

• Ultimate Oil Recovery

– The expected oil recovery ranges from 20% to 40%