An experimental study on coal permeability enhancement by water freezing cycles without effects on produced gas compositions: Implications for enhancing coalbed methane production

Freezing treatment with liquid nitrogen enhancing coal permeability is widely adopted, but its evaporation reduces the produced gas quality when freezing is terminated. The artificial ground freezing technology utilizes cycling brine as the media, which does not affect gas compositions. However, the temperature formed in ground freezing (approximately-20 degrees C) is significantly higher than that in liquid nitrogen treatment, and the effects of this temperature range on coal permeability remain unclear. This study conducted three 7-day cycles freezing at-20 degrees C on six coal samples, and the permeability/pore structure was determined before/after freezing. The results show that freezing cycles have weak effects on stress sensitivity, determined by the low changes in compression coefficient. The initial permeability (k0, at atmospheric pressure) increases and then fluctuates with increasing freezing cycles, and the average change ink0 in three cycles is 159.25 %, 1251.73 %, and 2037.06 %, respectively. The change in k0 decreases with increasing ash yield and positively correlates with the change in porosity. The water in pores/fractures generates swelling stresses due to freezing, driving the pore/fracture formation, original fracture extension, and dead pore connection, jointly leading to the permeability enhancement, and the coal with low ash yield and extensive fracture development is more suitable for freezing cycles to enhance permeability under similar geological conditions. The results offer insights for evaluating the permeability enhancement potential by freezing without effects on produced gas compositions.