The development of microstructure of coal by microwave irradiation stimulation

Abstract Microwave fracturing has been proposed as an alternative approach to stimulate coalbed methane reservoirs as it is environmentally friendly, widely applicable and economical. Different microwave parameters (microwave power and treatment time) and saturation conditions of coal have great effect on the microwave heating effect, which further results in different microwave fracturing effects. In this study, repeated nuclear magnetic resonance experiments and quantitative analysis were performed to investigate the effects of microwave power, irradiation time and saturation condition on microstructure development of coal. Three indicators including macropore and fissure proportion variation (MFPV), variation rate of water porosity (VRWP) and moisture loss were proposed to evaluate microwave's effect on pore structure. It was found that the pore size, water porosity and moisture loss increase with rising microwave power and irradiation time. Microwave's effect on coal sample with saturations ranged from 0% to 100% was first studied in this paper. The results show that microwave irradiation has great effect on pore structure of coal samples when water saturation is less than 25% and becomes insignificant when the water saturation is larger than 50%. Surface topography measurements and SEM experiments were also performed before and after microwave treatment to observe the microstructure development. New fissures and cracks were observed on the surface of samples and the surface become smooth with less accumulated particles after microwave irradiation.