Image and ultrasonic analysis-based investigation of coal core fracturing by microwave energy

Formation microwave heating treatment (FMHT) is a potential method for degassing coalbed methane. Camera pictures and ultrasonic method were used to evaluate the crack propagation on the surface and in the interior of the coal cores, respectively. The results suggest that the fracture induced by microwave energy would remarkably increase with increased heating time. New fractures and pore sizes produced by microwave energy decreased P-and S-wave velocity in coal cores. The amplitude of the frequency spectrum of coal cores decreased after microwave energy treatment. The frequency distribution domain in the frequency spectrum of coal cores was also affected by microwave energy treatment. The density, bulk modulus and shear modulus of coal decreased with microwave energy treatment. Specifically, the slopes of declines in the bulk modulus and shear modulus were larger than that in the coal density. The increasing fracture network could improve the permeability of coal for degassing coal seams. In a word, the microwave energy has potential for assisting coalbed gas drainageing.