Page 222 - Geology and Geochemistry of Oil and Gas

CLASSIFICATION OF OIL AND GAS RESERVOIRS BASED ON DRIVE MECHANISM 191
10.3.4. Gravity drainage

In oil pool subject to gravity drainage, the gravity segregation of fluids during the
primary production process is clearly evident in the production history. Oil migrates
down-dip to maintain down-structure oil saturation at a high level, and free gas
accumulates high in the structure. If a primary gas cap exists, it will expand as a result
of the segregation process. A pool without a primary gas cap will soon form a
secondary gas cap. Early in the life of the pool, the gas/oil ratios of the structurally
high wells will increase rapidly. A program of shutting-in wells with high gas/oil ratios
and of overall discreet control of individual well rates will provide for maximum
benefit from gravitational fluid movement. Fig. 10.9 indicates the two cases of the
generalized performance of a gravity-drainage pool without (Fig. 10.9A) and with
(Fig. 10.9B) such control. The oil gravity, permeability of the zone, and formation dip
dictate the magnitude of the gravity drainage. The combination of low viscosity and
low specific gravity (high API gravity), along with the high zone permeability and
steeply dipping beds, accentuates the down-structure oil movement. Typically, in
gravity-drainage pools the water influx is minor and the down-dip wells produce at
the lowest gas/oil ratios and have the highest oil recovery. In cases of strict gravity
drainage, a major portion of the recovery occurs after complete pressure depletion,
when gravity is the primary dynamic force moving the oil to the wellbore.

10.3.5. Combination-drive reservoirs

Some reservoirs may produce under the influence of one or more of the drives
discussed above. Throughout the life of a pool a sequence such as solution-gas
drive–gravity drainage-water drive may develop, or two different drives may be
competing simultaneously (e.g., water-drive reservoir with a primary gas cap).
Combination-drive reservoir may be further classified as open or closed, as proposed
by Babson (1965).

10.3.6. Open combination-drive reservoirs
The performance of open combination-drive reservoirs is affected by the method
of operation and by individual well and pool rates. This type of reservoir is in open
communication with the aquifer. Sources of energy include water drive, gravity
drainage, and gas expansion. In uncontrolled operations, a gas-expansion stage may
occur first, accompanied by a rising gas/oil ratio and declining pressure. A gravity-
drainage stage may come next at a reduced reservoir pressure. The effect of a mod-
erate water influx will frequently be evident in the latter stages of the life of the pool.
Curtailing production rate will cause a buildup in reservoir pressure as water influx
exceeds fluid withdrawals. The performance of the D-3 Pool, Redwater Field in
Alberta, Canada, which is a typical open combination-drive pool, demonstrates this
effect in Fig. 10.10.
In order to facilitate gravity drainage, the rate of water influx into the reservoir can
be reduced by maintaining high water-withdrawal rates from the high-water/oil-ratio