Predicting permeability change of fractured coals during methane depletion

Assessing the evolution of apparent permeability of coal matrix during gas injection/extraction is crucial in coalbed methane (CBM) production. It is commonly found that permeability increases upon the primary depletion of CBM reservoirs. Such increase is mainly attributed to the desorption of methane molecules from the coal matrix which will then exhibit notable volumetric shrinkage and changes of the pore structures. Various permeability models exist for such system but often equipped with oversimplified the geomechanical representation of coals. This study attempts to improve this aspect by adopting the concept of adsorption stress to formulate a mechanistic theory for adsorption-deformation coupling in coals. The constitutive theory is implemented in a finite element (FE) scheme and then coupled with explicit representation of fractures by FE mesh. Coal methane depletion tests are simulated by the model and the results are compared against existing experimental data. Through parametric studies, the permeability curve is found to be controlled by a few factors. The proposed model has such flexibility to capture various types of permeability evolution trends that a cluster of measured permeability data from San Juan basin can be well covered.