Material-Balance Calculations in Tight-Gas Reservoirs: The Pitfalls of p/z Plots and a More Accurate Technique
- David A. Payne (Shell Canada Ltd.)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Engineering
- Publication Date
- November 1996
- Document Type
- Journal Paper
- 260 - 267
- 1996. Society of Petroleum Engineers
- 5.5.8 History Matching, 5.1 Reservoir Characterisation, 4.1.5 Processing Equipment, 5.5 Reservoir Simulation, 4.6 Natural Gas, 5.1.5 Geologic Modeling, 4.1.2 Separation and Treating, 5.7 Reserves Evaluation, 1.6 Drilling Operations, 5.2.1 Phase Behavior and PVT Measurements, 5.8.1 Tight Gas
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Through the use of theoretical and field examples, the potentially large errors associated with using straight line P/z decline (tank) analysis to assess gas in place and reserves in tight gas reservoirs are illustrated. The basic tank assumption that reservoir pressure gradients are small can be violated, even with long shut-in times. It is also demonstrated that a reasonably straight line P/z decline does not necessarily indicate that the reservoir behaves as a tank. The Communicating Reservoir (CR) model is presented as a simple, yet much more accurate, method of performing material balance calculations in tight reservoirs. The results of the application of the CR model to the Waterton Gas Field are used to illustrate the success and large impact that can be obtained by examining the pressure behavior more closely than simply plotting P/z.
The use of pressure decline versus gas production (Gp) material balance plots (P/z plots) to estimate gas initially in place (GIIP) is a very common technique within gas reservoir engineering. The technique is very simple to apply, since it is not dependent on production rates, well details or reservoir properties. Essentially, the method consists of using the rate of pressure decline to estimate the size of a reservoir. If the reservoir is behaving in a tank-like manner, then pressure, corrected for non-ideal gas behavior, will decline linearly with production. Extrapolation of this line to abandonment pressure and zero pressure estimates ultimate recovery and GIIP respectively.
The simplicity of the technique and success within relatively permeable gas reservoirs have resulted in its almost universal application. Acquisition and divestment evaluations, particularly for smaller assets that don't justify simulation, are often based solely on the method. The use of P/z plots has become so entrenched as an industry standard that the fundamental assumptions behind the method are often forgotten or ignored.
The key assumption involved in the straight line P/z plot material balance technique is that the reservoir behaves as a tank. That is, there is little or no pressure variation within the reservoir and there is no additional pressure support to the system. The lack of pressure variations within the reservoir is required to ensure that pressure measurements taken at well locations represent true average reservoir pressures, and that the entire reservoir can be described with one pressure value. In high permeability reservoirs, a generally low gas viscosity ensures small pressure gradients exist away from the wellbore and the average reservoir pressure estimates can be readily made using short-term build-ups or static pressure surveys.
In many cases, however, reservoir characteristics can cause the P/z plot to be non-linear. For example, condensation below the dew point will cause the overall compressibility of the system to change. In such cases the use of a "two-phase z", based on constant volume depletion experiments, will correct the curvature. In other cases, curvature caused by an aquifer, an oil leg, or rock compressibility may require more complex algorithms to "straighten" the P/z decline.
This paper demonstrates that the concept of the straight line P/z plot material balance also fails in tight, lower permeability reservoirs. In these cases, the tank assumption does not apply by definition - tight reservoirs lead to substantial pressure gradients. These gradients manifest themselves in terms of scattered, generally curved, and rate dependent P/z plot behavior. This behavior is readily illustrated through the use of both reservoir simulation and field examples, which shows that the P/z plot methodology can incur greater than 100% error in estimating GIIP and reserves.
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