The exponential growth behaviour of coalbed permeability with reservoir pressure depletion has previously been observed 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 NE 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 re-visited in an effort to match the recently published field permeability data. The Shi and Durucan permeability model makes use of a stress-permeability relationship reported in the literature for an idealised geometry (matchsticks), in which the permeability varies exponentially with changes in the effective horizontal stress through the use of a 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 using 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 has further validated the Shi and Durucan permeability model (with variable cleat volume compressibility). It has also provided theoretical support for the observed near-exponential growth behaviour of coalbed permeability in a producing reservoir. Furthermore, the model has shed new insights into 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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