Introduction
According to Lakshminarayanan (2006), the relative permeability values for coal bed methane reservoirs are difficult to obtain. Lakshminarayanan states that they cannot be accurately measured in the laboratory due to difficulties in obtaining a representative coal sample from the reservoir, and that the only practical method to obtain the realistic values of relative permeability values is by history matching the production history.
Towler et al (2016) discusses that there are several methods to determine relative permeability including unsteady-state, steady-state, capillary pressure and numerical inversion methods; however, upscaling of these results to real reservoir conditions is subect to significant uncertainty.
Academic work by Cox [2001] suggests that experience modelling 1000’s of CSG wells in North America has demonstrated that the water relatively permeability curves are very steep, and that the gas curves are more conventional in nature. Similar results were observed by Gash and Conway et al (1995) where the water relative permeability initially declined rapidly to a long tail, while gas relative permeability increased nearly linearly as water saturation declined. Note, SJB impiles San Juan Basin.
Production data analysis methods may also be used to estimate relative permeability curves, but strong heterogeneity and low permeability of some coals limits its application.
References
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Cox, Dave, PE 598: Coalbed Methane, 2001, Colorado School of Mines
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Lakshminarayanan, Sunil, The impact of relative permeability on type curves for coalbed methane reservoirs, 2006, West Virginia University
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Gash, B. W. 1991; and Conway, M. W., Mavor, M . J., Saulsberry, J., Barree, R. B., and Schraufnagel, R. A. 1995. Multi-Phase Flow Properties for Coalbed Methane Wells: A Laboratory and Field Study. Paper SPE 29576. Presented at the Joint Rocky Mountain Regional Meeting and Low-Permeability Reservoirs Symposium, Denver, Colorado, March 20–22
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Brian Towler, Mahshid Firouzi, James Underschultz, Will Rifkin Andrew Garnett, Helen Schultz, Joan Esterle, Stephen Tyson, and Katherine Witt. An overview of the coal seam gas developments in Queensland, Journal of Natural Gas Science and Engineering. 2016