1. p = average reservoir pressure
Z = Z-factor evaluated at p
ce = cumulative effective compressibility, accounting for rock and water volumes in the HC-bearing reservoir*
Gp = cumulative surface gas produced
G = initial surface gas in place
Bg = gas FVF at p
We = cumulative water encroachment from aquifer (e.g. simplified Fetkovich, Hurst-van Everdingen type aquifers).
*Note:
For a pot aquifer the We term can be incorporated in the ce term as outlined by Fetkovich, Reese, and Whitson (SPE 22921), in which case the material balance is non-iterative in solution.