Material Balance Revisited

Abstract The material balance is a very important part of the reservoir engineer’s toolbox that is being relegated to the background in today’s reservoir evaluation workflow. This paper examines some issues that normally preclude its regular use especially as a pre-step before moving into full reservoir simulation and the use of a new method of analyzing the material balance equation called the dynamic material balance method for solving some of these issues. The dynamic material balance method allows the simultaneous determination of the initial oil-in-place (N) or initial gas-in-place (G), ratio of initial gas to oil (m), reservoir permeability (K) or skin factor (S) and average pressure history of a reservoir from the combination of solution to the material balance equation and pressure transient analysis theory. Cumulative production history and PVT data of the reservoir are used with limited or no pressure data. By introducing a time variable into the classical material balance equation (MBE) and combining the solutions of the resulting equations with the theory of pressure transient analysis, the cumulative production history of the reservoir and readily available PVT data of the reservoir fluids, we can estimate not only the original reserves in place, but also determine the average reservoir pressure decline history as indicated by the net fluid withdrawal from the reservoir. The reservoir permeability and skin factor as seen within the drainage area of each producing well can then be estimated from the already determined average pressure decline history. This method is expected to improve the use of material balance by expanding the list of problems that can be tackled using material balance especially to reservoirs in marginal fields and reservoirs in which limited pressure data is available.