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:
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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
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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).
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