The tank material balance (MB) equation for gas reservoirs has been written taking into account the effective compressibility of matrix and fractures.

The method has direct application on stress-sensitive naturally fractured reservoirs (nfr's). Under some conditions ignoring the effect of fracture compressibility (cf) can lead to over-estimating the volume of original gas in place using a cross-plot of p/z vs. Gp. The equation presented in this paper has been developed to overcome this weakness. The use of this MB is illustrated with an example.

It is concluded that fracture compressibility can play an important role in the calculation of gas in place in naturally fractured reservoirs.

The subject matter is significant because historically formation and water compressibilities have been neglected when carrying out MB calculations of conventional gas reservoir. This assumes that these compressibilities are negligible compared to gas. The assumption implies that the reservoir strata are static. When water influx is ignored, the assumption leads to a straight line in a cross-plot of p/z vs. cumulative gas production (Gp). However, this study shows that in those instances where fracture compressibility is large, the assumptions can lead to significant error.

Keywords:
compressibility, gas reservoir, Reservoir Characterization, matrix compressibility, fracture compressibility, porosity, fracture, fracture permeability, equation, Upstream Oil & Gas
Subjects:
Reservoir Characterization, Unconventional and Complex Reservoirs, Naturally-fractured reservoirs
Copyright 2006, Society of Petroleum Engineers
You can access this article if you purchase or spend a download.