Page 217 - Hydrocarbon Exploration and Production Second Edition

204 Reservoir Drive Mechanisms

stored in the system. Three sets of the fluid initial conditions can be distinguished
for an oil, and reservoir and production behaviour may be characterised in each
case

Drive Mechanism Fluid Initial Condition

Solution gas drive (or depletion drive) Undersaturated oil (no gas cap)
Gas cap drive Saturated oil with a gas cap
Water drive via injection or with a large Saturated or undersaturated oil
underlying aquifer

9.2.1. Solution gas drive (or depletion drive)
Solution gas drive occurs in a reservoir which contains no initial gas cap or
underlying active aquifer to support the pressure and therefore oil is produced by
the driving force due to the expansion of oil and connate water, plus any
compaction drive. The contribution to drive energy from compaction and connate
water is small, so the oil compressibility initially dominates the drive energy.
Because the oil compressibility itself is low, pressure drops rapidly as production
takes place, until the pressure reaches the bubble point.
The material balance equation relating produced volume of oil (N p stb) to the
pressure drop in the reservoir (DP)isgivenby

N p B o ¼ NB oi C e DP
where B o is the oil formation volume factor at the reduced reservoir pressure (rb/stb);
B oi the oil formation volume factor at the original reservoir pressure (rb/stb); C e
1
the volume averaged compressibility of oil, connate water and rock (psi ); N the
STOIIP (stb).
Once the bubble point is reached, solution gas starts to become liberated from
the oil, and since the liberated gas has a high compressibility, the rate of decline of
pressure per unit of production slows down.
Once the liberated gas has overcome a critical gas saturation in the pores, below
which it is immobile in the reservoir, it can either migrate to the crest of the
reservoir under the influence of buoyancy forces, or move toward the producing
wells under the influence of the hydrodynamic forces caused by the low pressure
created at the producing well. In order to make use of the high compressibility of
the gas, it is preferable that the gas forms a secondary gas cap and contributes to the
drive energy. This can be encouraged by reducing the pressure sink at the producing
wells (which means less production per well) and by locating the producing wells
away from the crest of the field. In a steeply dipping field, wells would be located
down-dip. However, in a field with low dip, the wells must be perforated as low as
possible to keep away from a secondary gas cap (Figure 9.2). The problem of water
coning, discussed in Section 10.2, Chapter 10 is a constraint on just how low down
the perforation can be placed without producing excessive amounts of water.
The characteristic production profile for a reservoir developed by solution gas
drive is shown in Figure 9.3.