Page 152 - Fundamentals of Reservoir Engineering

MATERIAL BALANCE APPLIED TO OIL RESERVOIRS 91

Both methods tend to confirm that the volumetric estimate of the oil in place is probably
correct, within about 6%, and the gascap size is between m = 0.5 and 0.54. With the
slight scatter in the production data it is not meaningful to try and state these figures
with any greater accuracy.

This estimate is made after the production of 17.7 million stb of oil or 15% recovery. As
more production data become available the estimates of N and m can be revised.

The pressure and production history of a typical gascap drive reservoir, under primary
recovery conditions, are shown in fig. 3.8.
p i

producing GOR

pressure
R = R si

watercut

time

Fig. 3.8 Schematic of the production history of a typical gascap drive reservoir

Because of the gascap expansion, the pressure decline is less severe than for a
solution gas drive reservoir and generally the oil recovery is greater, typically being in
the range of 25−35 %, dependent on the size of the gascap. The peaks in the
producing gas oil ratio curve are due to gas oil ratio (GOR) control being exercised. As
the gascap expands the time will come when the updip wells start to produce gascap
gas and the uppermost row of wells may have to be closed, both for the beneficial
effect of keeping the gas in the reservoir and also to avoid gas disposal problems.

Just as described in sec. 3.5 for a solution gas drive reservoir, if the economics are
favourable water and/or gas injection will enhance the ultimate recovery.

3.7 NATURAL WATER DRIVE

Natural water drive, as distinct from water injection, has already been qualitatively
described, in Chapter 1, sec. 7, in connection with the gas material balance equation.
The same principles apply when including the water influx in the general hydrocarbon
reservoir material balance, equ. (3.7). A drop in the reservoir pressure, due to the
production of fluids, causes the aquifer water to expand and flow into the reservoir.

Applying the compressibility definition to the aquifer, then

Water = Aquifer Initial volume Pressure
Influx Compressibility × of water × Drop