Page 269 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

238 Reservoir Engineering

production is caused only by liquid expansion, the material balance equation
obtained from Figure 5-144 for pressures above the bubble point is:

(5-156)

Gas Liberation
When reservoir pressure declines below the bubblepoint pressure, the original
gas in solution has either been produced as Gp, is still in solution in the oil,
(N - Np)Rs, or exists as free gas. For this condition as shown in Figure 5-144,
the material balance is:

N,Bo + B,(G, - N,R,)
N=
Bo - B, + (R" - R,)B, (5-1 57)

Predicting Primary Recovery in Solution-Gas Drive Reservoirs

Several methods for predicting performance of solution-gas behavior have
appeared in the literature. These methods relate pressure dedine to gas-oil ratio
and oil recovery. Because neither water influx nor gravity segregation is
considered, time is not a factor with solution-gas reservoirs, and time must be
inferred from the oil in place and production rate [17]. The following assump-
tions are generally made: uniformity of the reservoir at all times regarding
porosity, f hid saturations, and relative permeabilities; uniform pressure through-
out the reservoir in both the gas and oil zones (which means the gas and oil
volume factors, the gas and oil viscosities, and the solution gas will be the same
throughout the reservoir); negligible gravity segregation forces; equilibrium at
all times between the gas and the oil phases; a gas liberation mechanism which
is the same as that used to determine the fluid properties, and no water
encroachment and negligible water production.

The Schllthuis Method [249]
For solution-gas drive reservoirs where the reservoir pressure is about equal
to the saturation pressure and for gas cap drive reservoirs, Equation 5155 can
be written [197]:

N,Bo +(G,-N,R,)Bg-G(Bg-Bgl)
N= (5158)
Bo - B, +(Re - R,)B,
If this equation is rearranged and solved for cumulative oil produced:

NIB. - B, -t- (R, - R,)Bg] + G(B, - Bd)- BEG
N, = (5159)
Bo - BgR,

In order to predict the cumulative oil production at any stage of depletion, the
original oil and gas in place and the initial reservoir pressure must be known.