Experimental study on the permeability evolution mechanism of fractured coal-rock combination under ground stress and gas pressure

In view of the gas flow problem in the hydraulic fracturing engineering of L-type horizontal well and in the process of gas production in coalbed methane development, the permeability stress test of single fracture coal-rock combination was carried out. The permeability evolution law of fractured coal-rock combination under axial compression, confining compression, and gas pressure is analyzed. The axial compression has little effect on the permeability of coal-rock combination with fractures, and the permeability has no obvious change. Taking 0.25 MPa gas pressure as an example, the permeability of N1 fluctuates between 0.004 and 0.005 mD, and the permeability of N2 and N3 samples fluctuates between 0.002 and 0.0035 mD. With the increase of confining compression, the permeability decreases with the increase of confining compression. Taking the N4 sample as an example, when the gas pressure is 0.50 MPa, the permeability of N4 rapidly decreases from 0.351 to 0.0025 mD. The permeability has decreased by 99.3%. With the increase of gas pressure, the permeability and stress sensitivity of fractured coal-rock combination decrease gradually. In the process of gas pressure loading, the permeability decreases greatly due to the existence of gas slippage effect in the low-pressure stage. When the gas pressure exceeds 1 MPa, the joint action of slip effect and velocity-sensitive effect makes the permeability almost unchanged. Taking the N4 sample as an example, when the axial and confining pressures are 6 MPa, the permeability decays from 1.298 to 0.382 mD, with a decay ratio of 70.6%. Finally, the permeability calculation model of single fracture coal-rock combination under in situ stress and gas pressure can well match the experimental data and clarify the influence of each permeability on the overall permeability. The permeability model shows that the overall permeability depends on the part of the smaller permeability, and the higher permeability only makes the overall permeability increase slightly.