Abstract
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 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. In this paper, overview will presented on 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 compressibility’s 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.
This Paper will represent another two method of MBE for unconventional reserve.one is appropriate for estimating OGIP and second is appropriate for making Reservoir prediction. These techniques are differing from MBE of conventional gas reservoir in which effect of absorbed gas are included. For calculating OGIP, the assumption of equilibrium between free gas and absorbed gas phase is required. No additional assumptions are required for reservoir prediction.
