Significant increases in permeability of coal with continued production of coalbed methane is a well-accepted phenomenon, particularly in the San Juan basin in the US and Surat basin in Australia. Modeling this increase is either based on the resulting increase in fracture porosity of coal or, the associated changes in stresses as a result of the sorption-induced strain. This paper combines the experimental results of a study that measured sorption-induced coal matrix volumetric strain with depletion and a model proposed to estimate the associated changes in stress. The overall changes in stress, resulting from the combined effects of poro-mechanical behavior and sorption-induced strain, were estimated by introducing a Biot-like coefficient relating the volume of sorbed gas and the associated stress. Plotting the stress path followed during depletion along with the failure envelope for the coal-type clearly showed that shear failure of coal is possible due to the anisotropic loading of coal as a result of reduction in the horizontal stresses. This, in turn, results in a large increase in permeability.
US coalbed methane (CBM) production was 1.4 trillion cubic feet (TCF) in 2014 and, as of December 31, 2014, proven reserves were estimated to be 15.7 TCF (EIA, 2015). Leading in production in the US were the states of Wyoming, Virginia, Utah, Oklahoma, New Mexico and Colorado. Although production has dipped from the high of 1.9 TCF in 2010, CBM continues to be an important source of energy in the US.
The controlling mechanism when producing CBM is the coal permeability. Although, typically believed to decrease with continued production (Unconventional Oil & Gas Production, 2010), coal exhibits a unique behavior, termed “matrix shrinkage”, associated with desorption of gas, resulting in increased permeability (Harpalani and Chen, 1997; Levine, 1996; Mitra et al., 2012). The result is a negative declining trend when producing CBM.