Study on the Effect of Strain-Softening on Permeability and Gas Pressure in Surrounding Rocks of a Hydraulic Flushing Borehole

Hydraulic flushing gas extraction technology is gradually being applied in coal mines because it can effectively improve the gas extraction efficiency. This study aimed to explore the effect of strain-softening on permeability and gas pressure, due to the deficiency of previous studies on hydraulic flushing that did not consider the strain-softening of surrounding rocks. First, we analyzed the essence of strain-softening based on previous studies and proved the existence of strain-softening in surrounding rocks of the hydraulic flushing borehole by a field example. Subsequently, we established gas extraction models for different borehole diameters with and without considering strain-softening through the permeability evolution equation, the gas migration equation, and the strain-softening equation. We found that strain-softening significantly effects the permeability and gas pressure of the surrounding rocks of the borehole: the larger the diameter, the greater the effect on strain-softening. Compared with strain-softening not considered, the permeability, the permeability increase range, and the gas extraction standard range in surrounding rocks of hydraulic flushing boreholes will increase under the same borehole diameter when strain-softening is considered. The borehole diameter increases from 0.113 to 0.7 m, and the increases in permeability are 592.76 x 10-12, 614.25 x 10-12, 615.13 x 10-12, and 655.29 x 10-12 m2, respectively, the increases in permeability increase radius are 0.050, 0.121, 0.193, and 0.255 m, respectively, and the increase in gas extraction standard range radius under the two indicators are 0.119, 0.124, 0.190, 0.253 m and 0.052, 0.102, 0.150, 0.191 m, respectively. The gas pressure distribution in surrounding rocks of the borehole will also be changed by considering strain-softening, resulting in a slowly increasing gas pressure zone near the borehole: the larger the diameter, the greater the zone. Our research results proved that it is necessary to consider the strain-softening of surrounding rocks when studying gas extraction through hydraulic flushing boreholes. Moreover, we provide a novel explanation for the mechanism of gas extraction through hydraulic flushing boreholes based on the study's results. The finding of this study can help better understand the mechanism of enhanced gas extraction through hydraulic flushing boreholes.