Evolution of Coal Petrophysical Properties under Microwave Irradiation Stimulation for Different Water Saturation Conditions

Coalbed methane (CBM) reservoirs generally have very low permeability and require stimulation to make gas extraction economical. Hydraulic fracturing has been widely applied to enhance CBM productivity, but this technology has a number of key limitations, including the drawback that fractures only propagate along existing joints, the large amount of water usage, and the potential environmental impact. Microwave irradiation technology can likely overcome the above limitations. In this study, the effect of microwave irradiation on the petrophysical properties of an unconstrained bituminous coal was comprehensively investigated through a suite of integrated diagnostic techniques including nuclear magnetic resonance and X-ray computed tomography. A series of experiments were conducted both on centrifuged samples and on samples with different water contents ranging from 1 to 15%. The mineral removal and moisture evaporation due to the microwave selective heating lead to the enlargement, opening, and interconnection of coal pores. The NMR-determined porosity increases linearly with the microwave power while grows exponentially with respect to water contents. When the water content is above 6%, the porosity increases by around 98–211%. The fracture volume and coal permeability increase while the P-wave velocity decreases with increasing water contents. Microwave irradiation is effective in enlarging mesopores and macropores and in enhancing the pore connectivity. The significant enhancement of coal permeability and pore fracture structure indicates that the microwave irradiation is effective in improving gas productivity thus has the potential to become a new CBM reservoir simulation technology.