Evaluation of a complex fracture network in deep coalbed methane reservoir considering natural fracture tensile and shear failure

Natural fractures develop in deep coalbed methane reservoirs, and the formation mechanism of complex fractures is still unclear. This paper establishes a finite element model of hydraulic fracture propagation in deep coalbed methane reservoirs containing natural fractures in Ordos, China, to simulate and evaluate the complex hydraulic fracture under natural fracture tensile shear failure. Second, the theoretical criterion of hydraulic expansion of deep coalbed methane reservoirs is deduced, and the mechanism of hydraulic fracture expansion is explained. Finally, the relationship between tension/shear failure and hydraulic fracture complexity is analyzed using microseismic data of deep coalbed methane reservoir hydraulic fracturing. The results show that both tensile and shear failure can increase the complexity of hydraulic fractures in deep coalbed methane reservoirs, and shear failure plays a leading role in increasing the complexity of hydraulic fractures. Field microseismic results show that the proportion of shear events positively correlates with the complexity of hydraulic fractures, and the correlation coefficient R2 is 0.9564. Using large injection rate (greater than 20 m3·min−1) is conducive to the shear failure of natural fractures and improves the complexity of hydraulic fractures.