Page 262 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

Material Balance and Volumetric Analysis 231

Materlal Balance for Gas Reservoirs

Material Balance Equatlona

Reservoirs that contain only free gas are called gas reservoirs. These reservoirs
contain a mixture of gaseous hydrocarbons, which may be dry, wet, or con-
densate gas. Gas reservoirs may be volumetric with no water influx. For such
reservoirs, the material balance is:

GB, = (G - Gp)Bg (5-145)
where all terms have been defined earlier in the notation. In the case of a gas
reservoir with water encroachment and water production, the material balance
equation is:
GB, (G - Gp)Bg + (We - Wp) (5-146)

In either equation, the gas formation volume factor can be obtained, as a
function of pressure and temperature, as outlined earlier. A schematic repre-
sentation [197] of the material balance equations for dry-gas reservoirs is
depicted in Figure 5-141.
P=Pi P=P

water
drive

---------
drive
Water Water

- -
attached
oil zone

Figure 5-141. Schematic of material balance equations for a dry-gas
reservoir [197].