ABSTRACT:

Increased coal permeability with continued gas production from coalbed methane reservoirs, typically encountered in the San Juan basin, has been explained by the phenomenon of matrix shrinkage, which results in opening up the fracture system and promoting flow. Recently, sudden permeability jumps have been observed in the deeper part of the basin after significant drawdown. The behavior has been explained by the desorption induced loss of horizontal stress, resulting in stress anisotropy and shear failure of coal. The permeability models currently used are capable of predicting coal permeability prior to failure, that is, in the elastic zone, but not beyond that. This paper is an attempt towards developing a permeability model beyond the linear elastic behavior of coal in the reservoir. The model is able to predict the sudden uptick in permeability with increased dilatancy and coal failure. At this time, the permeability model is empirical, and piece-wise for the three zones: elastic, dilatancy and post-failure. The model is able to match experimental data available for San Juan coal well. Advancing the model would not only enable prediction of coal permeability over the entire life of a producing reservoir but it can be extended to shale gas formations as well.

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