Experimental study on the hydraulic fracture penetration propagation through the layer in coal-rock complex

To further study the law of hydraulic fracture penetration propagation through the layer in coal-rock complex, a true triaxial hydraulic fracturing experiment of coal-rock complex was carried out to compare the fracture extension effect and fracture initiation difficulty by changing the stress difference, the arrangement direction of fracture pipe and the location of water injection point.The research results show that the propagation direction of hydraulic fracture is restricted by the maximum principal stress, and when the maximum horizontal principal stress is close to the maximum vertical stress, the fracture expands along the resultant force direction of the stresses.The increase of stress difference is conducive to the penetration propagation of hydraulic fracture through the layer, and the propagation distance after the penetration increases, while a little influence on the initial fracture initiation pressure and time.After the fracture expands from the rock layer into the coal seam, the fracturing pressure will drop sharply and then rise again, and most of the acoustic emission events are located in the coal seam.If the penetration fails, the pressure fluctuates continuously.In addition, the fracture pipe is arranged in the rock seam in the direction perpendicular to the interface, and the water injection initiation point is set in the rock seam, the fracture initiation is much easier, as well as the fracture seepage channel formed in the coal seam after the penetration propagation through the seam is more complete, which provides a theoretical basis for the application of coal seam seepage enhancement and roof fracturing relief in engineering project.