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
Assumption:
- Dry gas reservoir
- No external energy support like water drive.
Where;
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)
Where;
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
Rearrange the equation like this:
The rearranged equation can be plotted between P/z and Gp (Figure 1).
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
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.
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