Material balance has long been used in reservoir engineering practice as a simple yet powerful tool to determine the Original-Gas-In-Place (G). The conventional format of the gas material balance equation is the simple straight line plot of p/Z versus cumulative gas production (Gp) which can be extrapolated to zero p/Z to obtain G. The graphical simplicity of this method makes it very popular. The method was developed for a "volumetric? gas reservoir. It assumes a constant pore volume of gas and accounts for the energy of gas expansion, but it ignores other sources of energy such as the effects of formation compressibility, residual fluids expansion and aquifer support. It also does not include other sources of gas storage such as connected reservoirs or adsorption in coal/shale. In the past, researchers have introduced modified gas material balance equations to account for these other sources of energy. However, the simplicity of the p/Z straight line is lost in the resulting complexity of these equations.
In this paper, a new format of the gas material balance equation is presented which recaptures the simplicity of the straight line while accounting for all the drive mechanisms. It uses a p/Z ** instead of p/Z. The effect of each of the mentioned drive mechanisms appears as an effective compressibility term in the new gas material balance equation. Also, the physical meaning of the effective compressibilities are explained and compared with the concept of drive indices. Furthermore, the gas material balance is used to derive a generalized rigorous total compressibility in the presence of all the above-mentioned drive mechanisms, which is very important in calculating the pseudo-time used in rate transient analysis of production data.
It has been of great interest to find the original-gas-in-place by using material balance. The conventional gas material balance equation was developed for a "volumetric? gas reservoir. Therefore, the p/Z versus cumulative gas production plot may give misleading results in some situations e.g. when the formation compressibility is of the same order of magnitude as gas compressibility (overpressured reservoirs) or where desorption plays a role (CBM/shale). Figure 1 shows p/Z versus Gp for several scenarios with the same original-gas-in-place (G). It can be seen from this figure that except for the volumetric reservoir, the plot is not a straight line, because gas expansion is not the only drive mechanism. In fact, water encroachment in water-drive reservoirs, formation and residual fluid expansion in overpressured reservoirs and gas desorption in coalbed methane (CBM) or shale reservoirs can have a significant role as a driving force in these cases. In these situations, where the gas expansion is not the dominant driving force, modified material balance equations have been developed by several researchers. Among them, Ramagost and Farshad  modified the conventional material balance equation to account for pore volume shrinkage due to formation and residual fluid expansion and introduced a new plotting function that keeps the material balance as a straight line. So that the modified material balance equation can be used for overpressured reservoirs.