Effect of fracture compressibility on gas-in-place calculations of stress-sensitive naturally fractured reservoirs

A tank material-balance equation for gas reservoirs has been written, taking into account the effective compressibility of matrix and fractures. The method has direct application to stress-sensitive naturally fractured reservoirs (NFRs). Under some conditions, ignoring the effect of fracture compressibility (C-f) can lead to overestimating the volume of original gas in place with a crossplot of p/z vs. cumulative gas production (G(p)). The equation presented in this paper has been developed to overcome that weakness. The use of the tank material balance is illustrated with an example. It is concluded that fracture compressibility can play an important role in the calculation of gas in place in stress-sensitive NFRs. The subject matter is significant because, historically, formation and water compressibilities have been neglected when carrying out material-balance calculations for conventional gas reservoirs. This assumes that these compressibilities are negligible compared to that of gas. The assumption implies that the reservoir strata are static. When water influx is ignored, the assumption leads to a straight line in a crossplot of p/z vs. G(p). However, this study shows that in those instances in which fracture compressibility is large, such assumptions can lead to significant error.