Abstract: To study the propagation process of hydraulic fracture in compound hard roof, a propagation model was built and the propagation mechanism and mode were analyzed theoretically. A compound hard roof in Datong Mining Area was taken as the research object. Then the propagation process and influencing factors of hydraulic fracture were studied by coupling system of flow and solid in the rock failure process analysis (RFPA^2D-Flow). The numerical simulation results were verified by a similar model test. The results show that the fracture propagation process is mainly influenced by rock mechanical properties, principal stress difference, angle and mechanical properties of bedding plane. Among others, that the fracture propagates preferentially into the marl layer with the weakest strength, that an increase in the principal stress difference weakens the differences in the processes of fracture propagating to different rock layers,that with the increase of bedding plane angle, the fracture propagation mode changes gradually from crossing the bedding plane to extending along it and be confined in the fractured layer,and that an increase in the bedding plane elastic modulus promotes is helpful for the fracture to propagate into the coarse sandstone.

Key words: hydraulic fracture, compound hard roof, propagation law, principal stress difference, bedding plane