Experimental study on evolution of pore structure in coal after cyclic cryogenic fracturing

For the ineffective gas drainage in single low-permeability coal seam in China, the method of cyclic cryogenic fracturing coal is proposed.In the process of cyclic cryogenicfluid injection, the coal freezes. Then the frozen coal thaws by absorbing heat from surrounding coals.So the freezing and thawing cycles emerge in C oal mass and the pore structure and permeability of coal will also be changed.The experiment which coal samples were treated by freezing and thawing in a total of 20 cycles was carried out. The nuclear magnetic resonance test was used to the coal samples.The evolution of pore structure characteristics at different freeze-thaw cycle s was researched.The results show that the macropores and fractures' proportion, the total porosity, the effective porosity, the permeability is rising with increasing free ze-thaw cycles. This proves that the pores of coal samples have bigger quantities, larger size and be more connected after multiple freezethaw cycles, so the pore-fract ure network forms.Finally, the gas permeability of coal seam increases significantly.