Exponential Growth in San Juan Basin Fruitland Coalbed Permeability With Reservoir Drawdown: Model Match and New Insights
- Ji-Quan Shi (Imperial College London) | Sevket Durucan (Imperial College London)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- December 2010
- Document Type
- Journal Paper
- 914 - 925
- 2010. Society of Petroleum Engineers
- 5.8.3 Coal Seam Gas
- coalbed permeability model, coalbed methane
- 1 in the last 30 days
- 862 since 2007
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The exponential-growth behavior of coalbed permeability with reservoir pressure depletion has been observed previously at the Fairway wells in the San Juan basin. More recently, the exponential trend has also been confirmed for a group of 10 wells in a region northeast of Fairway. Increase in the absolute permeability of coalbeds is a result of matrix shrinkage caused by gas desorption, which becomes a dominant factor on cleat permeability over the effective-stress effect during reservoir production.
A permeability model previously developed by the authors has been revisited in an effort to match the recently published field-permeability data. The Shi and Durucan (S&D) permeability model (2004, 2005a) makes use of a stress permeability relationship reported in the literature for an idealized geometry (matchsticks), in which the permeability varies exponentially with changes in the effective horizontal stress through the use of a constant cleat (pore) compressibility parameter. In this study, a stress-dependent cleat volume compressibility defined by an initial compressibility with a negative exponential decline rate was adopted. By tuning these two parameters, it was possible to achieve a close match to all the field-permeability data by use of a common set of reservoir properties (elastic properties and matrix-shrinkage parameters) that are consistent with the available field and laboratory data in the literature.
The matching of the field-permeability data in these two regions of the San Juan basin in this study has further validated the updated S&D permeability model (now with variable cleat-volume compressibility). It has also provided theoretical support for the observed near-exponential growth behavior of coalbed permeability in a producing reservoir. Furthermore, the model has shed new light on the permeability behavior: (1) The exponential growth occurs when the reservoir pressure is reduced to a level generally below the permeability-recovery pressure (the reservoir pressure at which the initial permeability is recovered); (2) The reservoir permeability in these two regions would remain relatively flat at reservoir pressures above the permeability recovery pressure.
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