Material Balance for Gas Reservoir

For gas reservoirs, the material balance concept can be applied to determine gas in place and expected gas reservoir reserve.

Gas Production = Expansion of Free Gas In Reservoir

equation 1 Gp


  • Dry gas reservoir
  • No external energy support like water drive.


Gp = gas production (std cu-ft)

Bg = gas formation volume factor (res cu-ft/std cu-ft)

G = gas in place (std cu-ft)

Bgi = initial formation volume factor (res cu-ft/std cu-ft)

Gas formation volume factor (Bg)

equation2 bg


Ps = pressure at standard condition (14.7 psia)

P = pressure at a certain condition, R

z = compressibility factor at a certain condition

T = temperature at a certain condition, R

T= standard temperature, 520 R (60F)

With Bg, the first equation can be written like this

equation 3

Rearrange the equation like this:

equation 4

The rearranged equation can be plotted between P/z and Gp (Figure 1).

equation 5

Figure 1 - P/z plot

Figure 1 – P/z plot

Figure 1 is called” P/z” plot and the interception at x-axis is gas in place (G).

Note: The P/z plot is valid for dry gas reservoir. If a reservoir has pressure support from an aquifer, the p/z plot will give an overestimated gas in place (Figure 2).

Figure 2 - Over estimated gas in place

Figure 2 – Over estimated gas in place

P/z plot is a good tool, but if you use the plot with few data points, it will give you a big error on gas in place (G) and reserves.


Abhijit Y. Dandekar, 2013. Petroleum Reservoir Rock and Fluid Properties, Second Edition. 2 Edition. CRC Press.

L.P. Dake, 1983. Fundamentals of Reservoir Engineering, Volume 8 (Developments in Petroleum Science). New impression Edition. Elsevier Science.

Tarek Ahmed PhD PE, 2011. Advanced Reservoir Management and Engineering, Second Edition. 2 Edition. Gulf Professional Publishing.

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