A new approach to gas material balance in tight gas reservoirs

Gas material balance in conventional, volumetric reservoirs is described by a linear relationship between pressure/z-factor (p/z) and cumulative production. Unfortunately, tight gas reservoirs do not exhibit this type of behavior, but instead develop a nonlinear trend, which is not amenable to conventional analysis. The nonlinearity is a function of two items: the testing method (time) and the reservoir characteristics. For these type of reservoirs the testing time is insufficient to reach average reservoir pressure and from a practical viewpoint, it is not possible to shutin for extended periods of time. Therefore, a method has been developed to use key intersection points and slopes from a tight gas material balance plot to better understand the reservoir behavior. This work begins by explaining the nature of the nonlinear trend in terms of flow regimes. The primary objective is to improve the estimate of gas-in-place and recovery in a tight gas reservoir. Typically, gas-in-place is underestimated using conventional techniques. It is demonstrated that by using the appropriate slope with the initial pressure an improved (increased) estimate of gas-in-place is achieved. Furthermore, it is possible to distinguish the effect of infill wells and subsequently determine the incremental recovery. Included are field examples from the San Juan Basin and southeast New Mexico, which demonstrate the technique.