Experimental study on the extension of hydraulic fracture through the layer in coal-rock complex

In order to further study the law of fracture extension of hydraulic fracture through the layer in coal - rock complex,a true triaxial coal -rock complex hydraulic fracturing experiment 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 expansion direction of the hydraulic fracture is limited by the maximum principal stress,and when the maximum horizontal principal stress is close to the maximum vertical stress,the fracture expands along the direction of the combined force of the stresses. The increase in stress difference facilitates the extension of the hydraulic fracture through the layer,and the extension distance after the penetration increases,while the effect on the initial fracture initiation pressure and time is smaller. After the fracture spreads from the rock layer into the coal seam,the fracture pressure will show a sudden drop and secondary lift,and most of the acoustic emission events are located in the coal seam;if the penetration fails,the pressure shows a continuous fluctuation. In addition,the fracture pipe is arranged in the rock seam in the direction perpendicular to the intersection,and the fracture initiation point of water injection is located in the rock seam,so the fracture initiation difficulty is lower,and the fracture seepage channel formed in the coal seam after the extension through the seam is more complete,which provides the theoretical basis for the application of coal seam seepage increase and roof fracturing to unload pressure in engineering.