Page 139 - Fundamentals of Reservoir Engineering

MATERIAL BALANCE APPLIED TO OIL RESERVOIRS 78

- determining the main producing characteristics, the producing gas oil ratio
and watercut

- determining the pressure decline in the reservoir
- estimating the primary recovery factor

- investigating the possibilities of increasing the primary recovery.

3.5 SOLUTION GAS DRIVE

A solution gas drive reservoir is one in which the principal drive mechanism is the
expansion of the oil and its originally dissolved gas. The increase in fluid volumes
during the process is equivalent to the production.

Two phases can be distinguished, as shown in fig. 3.2 (a) when the reservoir oil is
undersaturated and (b) when the pressure has fallen below the bubble point and a free
gas phase exists in the reservoir.

a) Above bubble point pressure (undersaturated oil)

For a solution gas drive reservoir it is assumed that there is no initial gascap, thus
m = 0, and that the aquifer is relatively small in volume and the water influx is
negligible. Furthermore, above the bubble point, R s = R si = R p, since all the gas
produced at the surface must have been dissolved in the oil in the reservoir.

Under these assumptions, the material balance equation, (3.7), can be reduced to

(B − B ) (c S + c )
NB = NB oi o oi + w wc f ∆ p (3.15)

p
o
−
B oi 1 S wc

Sealing
fault

OWC OWC

(a) (b)

Fig. 3.2 Solution gas drive reservoir; (a) above the bubble point pressure; liquid oil,
(b) below bubble point; oil plus liberated solution gas

The component describing the reduction in the hydrocarbon pore volume, due to the
expansion of the connate water and reduction in pore volume, cannot be neglected for
an undersaturated oil reservoir since the compressibilities c w and c f are generally of the
same order of magnitude as the compressibility of the oil. The latter may be expressed
as described in Chapter 2, sec. 6, as