Effect of Fracture Compressibility on Gas-in-Place Calculations of Stress-SensitiveNaturally Fractured Reservoirs

摘要

The tank material balance (MB) equation for gasreservoirs has been written taking into account theeffective compressibility of matrix and fractures.The method has direct application on stress-sensitivenaturally fractured reservoirs (nfr’s). Under someconditions ignoring the effect of fracturecompressibility (cf ) can lead to over-estimating thevolume of original gas in place using a cross-plot ofp/z vs. Gp . The equation presented in this paper hasbeen developed to overcome this weakness. The useof this MB is illustrated with an example.It is concluded that fracture compressibility can playan important role in the calculation of gas in place innaturally fractured reservoirs.The subject matter is significant because historicallyformation and water compressibilities have beenneglected when carrying out MB calculations ofconventional gas reservoir. This assumes that thesecompressibilities are negligible compared to gas. Theassumption implies that the reservoir strata are static.When water influx is ignored, the assumption leads toa straight line in a cross-plot of p/z vs. cumulative gasproduction (Gp ). However, this study shows that inthose instances where fracture compressibility islarge, the assumptions can lead to significant error.