Numerical study on the influence of coal-roof interface on vertical propagation of hydraulic fracture

To extract coalbed methane (CBM) from tectonically deformed coal seams, a horizontal well was drilled in the roof of coal seam. Staged hydraulic fracturing was then conducted to connect the horizontal wellbore and underlying coal seam. A finite element model that coupled seepage, stress, and damage theories was built to investigate the influence of coal-roof interface on vertical propagation of hydraulic fracture. Results showed that the coefficient of friction and crossing stress ratio were the two primary factors controlling the fracture penetration. A higher interfacial shear strength is beneficial to fracture penetration. The crossing stress ratio required for fracture penetration decreases as the interface friction coefficient increases. The numerical simulation results agree well with the field pilot test and can provide theoretical support for effective CBM development in similar coal seams. [Received: October 28, 2020; Accepted: March 10, 2021]