Page 220 - Geology and Geochemistry of Oil and Gas

CLASSIFICATION OF OIL AND GAS RESERVOIRS BASED ON DRIVE MECHANISM 189

Fig. 10.6. Pressure-production performance of some water-drive pools (after Elliott, 1946; courtesy of
AIME).

Hobbs and Yates pools produce from the Permian limestone. The remaining pools
are sandstone reservoirs. All of the pools, except the solution gas-drive Schuler Jones
Sand Pool, which is included for comparison purposes, are subject to at least sub-
stantial, if not complete, water drive (Elliot, 1946).
To summarize water-drive performance, the producing zone first of all is in
contact with broad aquifer, normally of high permeability. Decrease in production
capacity is minor until water begins to be produced. The produced gas/oil ratio
is substantially constant. Figures 10.7 and 10.8 illustrate this type of perform-
ance. Recovery factor depends on reservoir–rock characteristics such as pore size
and fracture distribution, values for mobility ratio (k w m =k o m ), and pool geometry.
o
w
The rate at which the pool is produced may also affect recovery, particularly if
the pool is subject to only partial water drive. Reservoir withdrawal rates greatly
in excess of the rate of water influx can lead to performance similar to that of
solution gas-drive pools. Free gas saturation in the reservoir can develop in the
more permeable section to the extent that incoming water will bypass tighter sections
to move preferentially into areas of high gas saturation, with a resulting loss in
recovery.