Page 223 - Hydrocarbon

210 Gas Reservoirs

Build Plateau Decline
P i up

Reservoir
Pressure

Oil Rate
Water Cut

Produced GOR
R si

Time (Years)
First Oil RF = 30 - 70%
Figure 9.8 Characteristic production pro¢le: water drive.

abandonment. Water cut may exceed 90% in the final part of the field life. This is
important to the process engineers, who need to design a facility capable of
handling large water throughputs in later field life. As water cut increases, so oil
production typically declines; a constant gross liquids (oil plus water) production
may be maintained.
The RF is in the range 30–70%, depending on the strength of the natural
aquifer, or the efficiency with which the injected water sweeps the oil. The high
RF is an incentive for water injection into reservoirs which lack natural water drive.

9.2.4. Combination drive
It is possible that more than one of these drive mechanisms occur simultaneously;
the most common combination being gas cap drive and natural aquifer drive.
Material balance techniques are applied to historic production data to estimate the
contribution from each drive mechanism.

9.3. Gas Reservoirs

Gas reservoirs are produced by expansion of the gas contained in the reservoir.
The high compressibility of the gas relative to the water in the reservoir (either
connate water or underlying aquifer) make the gas expansion the dominant drive
mechanism. Relative to oil reservoirs, the material balance calculation for gas
reservoirs is rather simple. A major challenge in gas field development is to ensure a