Page 224 - Geology and Geochemistry of Oil and Gas

CLASSIFICATION OF OIL AND GAS RESERVOIRS BASED ON DRIVE MECHANISM 193

Fig. 10.10. Performance of an open combination-drive pool, Redwater D-3 reef pool, Redwater Field,
Alberta, Canada. (Data from Alberta Oil and Gas Conservation Commission.).

wells close to the watertable and controlling the rates of the wells situated higher on
the structure. On the other hand, to take full advantage of the water influx, wells
producing at high water cuts should be shut in and rates on other wells carefully
regulated to reduce problems of non-uniform water influx and water coning.
The combined effect of water drive and an expanding gas cap serve as a very
effective oil displacement mechanism. For example, the Leduc D-3 Pool was under
both water and gas-cap injection, and flushing efficiencies in the gas cap and water
zone were about 80%.
When a pool with an initial free gas cap is subject to an active water drive, steps
must be taken to prevent oil movement into the primary gas cap. Oil encroaching
into the gas cap, even though subsequently displaced, will leave the residual oil
saturation, causing a substantial loss in ultimate recovery. Bruce (1944) cited the case
of the Mount Holly Pool in Arkansas, producing from the Smackover Oolite Lime-
stone. An oil recovery was 35% because of oil lost in the shrinking primary
gas cap. The recovery rate of the other Smackover pools in the area should be at
least 60%.

10.3.7. Closed combination-drive reservoirs
There is no significant water encroachment in closed combination-drive reservoirs
owing to the faulting and/or facies changes so common in carbonate rocks. A gas-
expansion stage is usually followed by gravity drainage. Reservoir pressure is low,
gas/oil ratios are falling, and updip wells produce very little fluid of any kind during
the secondary stage. The role that gravity drainage plays depends, of course, on the
net effect of the dip and permeability of the zone and the oil gravity, in addition to
the individual well rates. The control of production rates to allow for effective