Numerical study on the effect of directional perforationin coal on crack propagation in hydraulic fracturing

Gas drainage is the key to restrict the efficient production of coal mines. In order to improve the gas extraction efficiency in low-permeability and difficult-to-drain coal seams, the extended finite element method was used to solve the fluid-solid coupling model of hydraulically fractured coal seams according to the influence of directional perforation in coal on the propagation of fracturing cracks. The research reveals that the effect of stress difference, perforation azimuth and perforation depth on crack propagation law, crack initiation pressure and crack width. The study results show that when the perforation is switched to fracturing, the cracks begin at the bottom of the perforation hole first and eventually turns to the direction consistent with the maximum horizontal principal stress and continue to expand. When the plane stress difference increases, it is easier to form long and wide cracks. When the plane stress difference increases from 2 MPa to 7 MPa, the initiation pressure increases by 94.4%, but the final crack width increases by 375%. The larger the azimuth angle of the hole, the shorter the crack formed. When the azimuth angle of the perforation is increased from 0° to 60°, the crack initiation pressure increases by 187%, and the width of the crack eventually formed is reduced by 84%. Deeper perforations can form longer and wider cracks. When the ratio of perforation depth to drilling radius increases from 1.0 to 3.5, the crack initiation pressure of the cracks decreases by 51.4%, and the width of the cracks increases by 145%. Therefore, it is recommended that in actual construction, the distribution of in-situ stress of coal seam should be obtained as accurately as possible, perforating in the direction consistent with the maximum principal stress, and appropriate perforation parameters should be selected to achieve a more ideal perforation depth.