Experimental study of microwave thawing on the LN2 frozen coals for enhancing coalbed methane extraction

Abstract Liquid nitrogen (LN 2 ) fracturing has been studied widely in coal-bed methane (CBM) stimulation. Nevertheless, the thawing effect on the frozen coal has been rarely considered. The thawing behaviors of the frozen coal by microwave were researched using nuclear magnetic resonance (NMR), ultrasonic wave, and infrared thermal imaging. The evolution of the pore structure, temperature, water content, and surface cracks of the coal samples treated by freezing and thawing is discussed. NMR results illustrate that microwave thawing not only improves coalʼs permeability by increasing seepage pores but also removes the water from the coal. On the contrary, air thawing treatment increases the moisture of the coal sample. The losing-water rate of the samples thawed at high power is smaller than that of samples thawed at low power. The microwave thawing treatments generate cracks and reduce the wave velocity of the coal samples, and higher thawing power on the frozen is in more favor of forming macro-cracks under the same input energy. Therefore, microwave thawing on frozen coal can eliminate water blocking damage and provide the flow space for the gas. The study analyzed the feasibility of microwave thawing on the frozen coal and provided a reference method for CBM production.