Microwave fracturing of frozen coal with different water content: Pore-structure evolution and temperature characteristics

Liquid nitrogen (LN2) freezing has widely been studied for the production of coal-bed methane (CBM) in coal reservoirs, but there is little study involving the thawing process. In this paper, the effects of microwave thawing on the LN2-freezing coals with different water content are studied through magnetic resonance imaging (NMR), p-wave velocity, and infrared thermal imaging (ITI). The results prove that the increase rates of the seepage pores of the air-thawing samples increase with the increasing water content. Compared with air thawing, the increase rates of the seepage pores after microwave thawing increase by 17.37%, 9.56%, and 9.32% for the dried, wet, and saturated conditions, respectively. The results of the wave velocity are similar to the increased rates of the seepage pores. The uniformity of the surface temperature of the samples thawed by microwave is related to the initial water content. The low initial water content contributes to the development of surface cracks during LN2 freezing. After microwave thawing, the pore water reduces by 21.46%, 44.85%, and 21.31% for the dried sample, wet sample, and saturated sample, respectively. The research shows that microwave thawing of the frozen coal not only enhances the pore structures but also removes the pore water.