Page 268 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

Material Balance and Volumetric Analysis 237

energies, Slider [ 1971 suggests beginning all material balance applications with
Equation 5-155. With the use of Equation 5-155, it is not necessary to list
separate equations for the various drive mechanisms and conditions since this
one equation can be reduced to any of the individual cases. For reservoirs above
the bubble point, some of the terms in Equation 5-155 become zero or else
cancel out: G = 0 since there is no free gas, Gp, and N R, cancel out since the
gas production term is equal to the total gas produce& Rd - q is zero since
the gas in solution at any pressure is equal to the gas originally in solution.
For a reservoir with no initial free gas saturation or no initial gas cap, G = 0. If
there is no water encroachment, We = 0; however, the water production term
should remain, even if there is no water drive, because connate water may be
produced when the reservoir pressure declines.
When a reservoir contains free gas, the pore volume expansion or com-
pressibilities of the formation and water are insignificant compared to the free
gas terms. Since the gas compressibility is about 100 times the compressibility
of the water and formation, a gas saturation of only 1% may provide as much
energy as the water and formation compressibility terms. Thus, when the gas
saturation is substantial the change in pore volume is insignificant.
Material Balance for Soiution-Gas Drive Reservoirs

A schematic representation of material balance equations for solution-gas
reservoirs, when the change in pore volume is negligible, is shown in Figure 5144.
When these reservoirs are producing above the bubble point or saturation
pressure, no gas is liberated and production occurs by expansion of liquids in
the reservoir. When reservoir pressure drops below the bubble point, gas is
liberated in the reservoir and will be produced with the oil.
Liquld Expansion

For some very large reservoirs (often with limited permeability), production
may occur for extended periods by expansion of liquids in the reservoir. If

Free Gas

for P<Ps

Pi P < Ps

Flgure 5-1 44. Schematic of material balance equations for a solution-gas-
drive reservoir [ 1971.