Analysis of permeability variation and stress sensitivity of liquid nitrogen fracturing coal with different water contents

At present, the technology of fracturing coal by liquid nitrogen is a new type of anti-reflection technology of coal seam. Water content, coal rank and temperature gradient all affect the anti-reflection of liquid nitrogen. Among them, water content has a significant effect on the anti-reflection effect of liquid nitrogen cracking. In order to study the effect and mechanism of water content on liquid nitrogen cracking, four groups of coal samples with water contents of 0%, 2%, 4% and 6% were treated by liquid nitrogen leaching respectively, permeability tester is used to test gas phase permeability of coal samples, and the permeability and stress sensitivity of coal samples at different pressure points were discussed and analyzed. Some findings were obtained. (i) Under the same confining pressure, the variation trend of gas phase permeability with effective stress generally accords with negative exponential function k = ae(-b sigma) + c, and the correlation indexes are all greater than 0.95. (ii) When the effective stress exceeds a certain critical value, the gas permeability of coal sample is affected by Klinkenberg effect and presents an upward trend with the increase of the effective stress. (iii) The increase of water content of coal sample can significantly promote the fracturing effect of liquid nitrogen. The most obvious is that when water content increases from 2% to 4%, the increase of permeability before and after fracturing increases from 92.88% to 357.12%. (iv) The normalized permeability of coal samples before and after liquid nitrogen fracturing is treated, and it is found that coal moisture has both a negative effect of blocking seepage and a promoting effect of improving the fracturing effect. The normalized permeability of coal samples before and after fracturing is complicated under the combined effect of moisture. (v) Under the same pore pressure, the effective stress sensitivity of coal decreases gradually with the increase of effective stress, and it can be significantly improved by liquid nitrogen fracturing.