Summary
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 (Cf ) can lead to overestimating the volume
of original gas in place with a crossplot of p/z vs. cumulative gas
production (Gp ). 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.
Gp . However, this study shows that in those instances in
which fracture compressibility is large, such assumptions can lead to
significant error.
Introduction
Forecasting the performance of NFRs is a major challenge. Various authors
have tackled the problem throughout the years with material-balance
calculations. To the best of my knowledge, the effect of fracture
compressibility usually has been ignored in material-balance equations for gas
reservoirs. The work presented in this paper is not meant to replace a detailed
reservoir simulation, which, in my opinion, is the best way to try to solve the
problem, provided that the reservoir characterization and the quality of the
pressure and production data is good. The idea is to have a tool that can
provide a quick idea with respect to potential gas in place and recovery from
stress-sensitive NFRs.
The conventional material balance for gas reservoirs leads to a straight
line in a Cartesian crossplot of p/z vs. Gp , provided
that (1) water influx is equal to zero, (2) the reservoir strata are static,
and (3) the water and formation compressibilities are negligible compared to
gas compressibility. Although these assumptions are reasonable in many
instances, there are cases in which the fractures are quite compressible. In
these cases, the conventional approach can lead to significant errors in the
estimation of original gas in place. Similar problems have been observed in the
past in geopressured reservoirs (Roach 1981; Ramagost and Farshad 1981) and
stress-sensitive naturally fractured oil reservoirs. Possible solutions have
been proposed by Aguilera (2006, 2007).
This paper presents a material-balance equation that takes into account the
effective compressibility of matrix and fractures. Stress-sensitive properties
such as fracture porosity, fracture permeability, and the portions of gas
stored in matrix and fractures are taken into account.
© 2008. Society of Petroleum Engineers
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History
- Original manuscript received:
27 February 2006
- Meeting paper published:
15 May 2006
- Revised manuscript received:
3 December 2007
- Manuscript approved:
15 December 2007
- Version of record:
25 April 2008
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