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.
Introduction
The material balance equation (MBE) is a very import tool used by reservoir engineers in the oil and gas industry. MBE can provide an estimate of initial hydrocarbon in place independent of geological interpretation and can also serve the purpose of verifying volumetric estimates. It can also help determines the degree of aquifer influence, understanding the applicable "drive mechanism" and in some cases estimate recovery factor and recoverable reserves.
Conventionally, MBE is applied by considering different time intervals in the production history of the reservoir and maintaining that there exists a volumetric balance in the reservoir at these different time intervals. Several methods have been developed and published on applying the MBE to various types of reservoirs and solving the equation to obtain the initial oil-in-place (N) or initial gas-in-place (G) and the ratio of the initial free gas to oil (m) in the reservoir. One of such methods is the straight-line method popularized by Havlena and Odeh2,3 which instead of considering each time interval and corresponding production data as being separate from other time interval, combines all time intervals and obtain a solution that satisfies all the intervals together.
In applying the straight-line method however, it is usually required that an independent source of determining the value of m exist. Most application uses an m that is derived from geological data on relative ratio of gas cap to oil column volumes. Another important requirement is the need to accurately estimate the average reservoir pressure at the various time intervals. The standard practice is to estimate the average reservoir pressure from well test conducted on individual wells producing from the reservoir. In thick formations with high permeability and low viscosity hydrocarbons, average pressures obtained from the individual well tests are good estimates of the average reservoir pressures in the drainage area of the well. But for thinner formations of lower permeability and higher viscosity hydrocarbons, there are often large variations in reservoir pressure throughout the reservoir and obtaining an average drainage area reservoir pressure usually require longer testing times and obtained values are often inaccurate. Accurately determining this average reservoir pressure is critical to the accuracy of the reserves estimate obtained from the MBE.
