Page 217 - Geology and Geochemistry of Oil and Gas

186 CLASSIFICATIONS OF OIL AND GAS ACCUMULATIONS

finite gas saturation quickly develops and continues to increase as depletion pro-
ceeds. At the time the gas saturation reaches the equilibrium value of 5–10%, the gas
phase has sufficient mobility and free gas is flowing to the wellbore along with the
oil. This results in a rather abrupt increase in the producing gas/oil ratio. The gas/oil
ratio continues to rise with increased gas saturation, reflecting the rapid increase in
gas flow rate and the attendant decrease in oil production rate. At a gas saturation of
20–30%, the flow of oil becomes negligible, and the gas/oil ratio reaches a peak and
then declines as the reservoir reaches the latter stages of depletion.
In undersaturated reservoirs, where the initial reservoir pressure is substantially
above the saturation pressure, the production mechanism is oil expansion. Under
these conditions, the producing gas/oil ratio will remain at a low level during the
time that the reservoir pressure is above the bubble-point pressure. It will approx-
imate the solution gas/oil ratio and ideally should actually decrease slightly as the
pressure falls, even though this is rarely observed in the field. The peak gas/oil ratio
before it begins to decline, reflecting ultimate reservoir depletion, will normally be
5–10 times as great as the solution gas/oil ratio.
In pure solution gas-drive pools with intergranular porosity, reservoir pressure
depends primarily on cumulative oil recovery. Neither reservoir pressure nor ulti-
mate oil recovery is sensitive to oil production rate, unless the production rate affects
the producing gas/oil ratio. A rapidly increasing gas/oil ratio, after equilibrium gas
saturation is reached, is characteristic of solution gas-drive pools in general. Re-
ducing the production rate will not serve to increase the ultimate oil recovery ap-
preciably. An exception to this rule is found when excessive drawdowns at individual
wells lead to excessive transient effects on the reservoir. Any tendency for the res-
ervoir to exhibit significant gravity drainage or water influx, or to form a secondary
gas cap, may make ultimate recovery sensitive to production rate.
Solution gas-drive performance is closely related to a number of physical pa-
rameters. The ratio of reservoir oil viscosity to reservoir gas viscosity (m =m ), so-
o
g
lution gas/oil ratio, formation volume factor, interstitial water saturation, and oil
and gas permeability relationships largely control performance. The close interre-
lationship among these parameters is indicated by the fact that a change in one
factor results in a change in one or more of the others. Some general and meaningful
observations can be made, however, regarding the effect of altering the value of a
single factor. As oil viscosity increases, there is corresponding rise in the instanta-
neous producing gas/oil ratio because of greater gas bypassing, which results in
lower solution gas-drive efficiency and lower oil recovery. Also, as the amount of gas
available to be in solution decreases, the oil recovery declines. Muskat (1944), how-
ever, found that doubling the solution gas/oil ratio resulted in only 10% increase in
ultimate recovery. The greater oil shrinkage at higher solution gas/oil ratios serves to
dampen somewhat the effect of increased oil solubility on oil recovery, but the
shrinkage effect is of only minor importance. An increase in the crude oil gravity
(1API) as an overall characteristic of the fluid system likewise results in an increase in
ultimate recovery. Again the effect is dampened at the higher gravity ranges owing to
the greater oil shrinkage, and the ultimate recovery will actually decrease with an
increase in oil gravity in the 40–501API range.