Pore structure and fractal analysis of coal subjected to multiple treatments based on nuclear magnetic resonance.

Microwave heating (MW) and freeze fracturing (FF) have been widely used for enhancing the permeability of coal reservoirs. In this paper, the effects of combined MW–FF treatments on coals are experimentally investigated through nuclear magnetic resonance. T2 distributions and fractal dimensions of coals under MW, FF, and the combined MW–FF treatments are compared and analyzed. The results show that the T2 area, porosity, and fractal dimensions of coals increase to varying degrees after MW, FF, and MW–FF treatment. The T2 spectrum peaks increase from 2 to 3 and macrofractures can be observed after the coal was treated with the combined MW–FF treatments. However, these phenomena are not observed under MW or FF treatment. The combined MW–FF treatments can effectively fracture coals with less microwave energy and less liquid nitrogen freezing cycle owing to its intensive thermal shock fracturing. Therefore, the combined MW–FF treatment is favorable for fracturing the coals and hence favorable for the CBM production. [ABSTRACT FROM AUTHOR]