Study on mechanical and seepage characteristics of water-saturated coal under rapid cold and heat shock by liquid nitrogen freeze-thaw and microwave heat injection

This study proposes to explore the coal mechanics and seepage characteristics under the coupling of water-saturation, freeze-thaw and different temperatures. The original coal of 9105 working face in Wangzhuang Coal Mine was taken as the research object, and non-contact digital image processing was used to carry out the triaxial loading test of the coal rock under the conditions of liquid nitrogen freeze-thaw and microwave heat injection. Results showed that: ① in the original coal samples after freeze-thaw, water-saturated, and water-saturated freeze-thaw conditions and gradient temperature treatment, there is increase in the compressive strength, modulus of elasticity, Poisson's ratio, and shear strain value of the coal rock; ② the original coal rock in the compression density, elasticity, yield and destruction of the four stages of the stress concentration phenomenon shows no obvious developmental changes, but by water-saturated, water-saturated freeze-thaw, water-saturated freezing and thawing conditions of 100 ℃ and 150 ℃ temperature treatment, the local stress of the specimen concentrates in the destruction phase with obvious amplification; ③ compared with the initial permeability of the original coal rock, the initial permeability of the coal sample underwent a small increase of 1.44×10^-3μm² after the liquid nitrogen freeze-thaw treatment. After the water-saturated liquid nitrogen freeze-thaw, water-saturated liquid nitrogen freeze-thaw treatment at 50 ℃ and 100 ℃, there is more significant increase in the initial permeability of the coal rock, and after the water-saturated liquid nitrogen freeze-thaw treatment at 150 ℃, the initial permeability of the coal sample underwent a significantly higher increase and reaches 7.40×10^-3μm². The study provide references for increasing the permeability of gas in deep low-permeability coal beds, which could improve the efficiency of downhole gas extraction.