The material balance can be simplified depending on what type of reservoir we are interested in analyzing. The single-phase gas reservoir is the most simple. We’ll cover calculating gas in place using the material balance. The material balance equation can be simplified, as all the terms for oil production, liquid expansion and rock expansion can be neglected. See Chapter 3 of Applied Petroleum Reservoir Engineering Third Edition for the derivation. See Material Balance for Nomenclature.

Single-phase gas reservoirs have two possible drive mechanisms: segregation (gas cap) drive and water drive. Segregation drive will occur in all reservoirs, while water drive will only occur in select reservoirs. We’ll cover segregation drive.

##### Segregation Drive

Under segregation drive, the equation can be simplified even further, first by neglecting water influx and water production and then by substituting in expressions for the gas formation volume factor.

As pi, zi and G are constants for a given reservoir, a plot of early production data for p/z versus Gp gives us a straight line. The initial gas in place (G) is the intersection with the x axis. Please note that neglecting to correct the pressure term by the gas compressibility factor results in an incorrect extrapolation. In the same manner, a reservoir experiencing water drive will have a slower decline, as the water influx assists in stabilizing the pressure. Using this method for reservoirs with a water drive will also result in an incorrect extrapolation.

Terry, Ronald E., J. Brandon. Rogers, and B. C. Craft. *Applied Petroleum Reservoir Engineering*. Third ed. Massachusetts: Prentice Hall, 2014. Print.