Numerical simulation of layer-crossing propagation behavior of hydraulic fractures at coal-rock interface

Based on the analysis of coal-rock interface, the vertical propagation behavior of hydraulic fractures at coal-roof interface was studied by using finite element method. The influence of some geological and engineering factors on fracture layer-crossing behavior was analyzed. Results have shown that in-situ stress and shear strength of coal-rock interface are the two main factors affecting the fracture propagation at the coal-rock interface. Greater vertical stress difference coefficient and interfacial shear strength make fractures more easier for crossing the interface. Meanwhile, the difference of elastic modulus and tensile strength between coal and rock has less effect on the fracture vertical propagation behavior. Besides, proper engineering parameters(mainly pump rate and the distance between injection point and interface) can promote fracture to cross the interface. Research results can provide a reference for the application of "multi-stage hydraulic fracturing horizontal well in roof" technology in surface CBM extraction.