China possesses vast potential in coalbed methane (CBM) resources, yet more than half of the CBM bearing formations are heavily crushed into large amounts of cataclastic or pulverized coal, which usually means low strength, developed coal cleats and natural fractures. Field experiences show that it is difficult to generate long hydraulic fractures in this type of coal seam due to low mechanical strength and heavy fracturing fluid leakoff into the natural fractures. A technology called “indirect fracturing”, which means that fractures initiate in the adjacent layers and penetrate the coal seam after a period of propagation, has been proposed to circumvent this difficulty. In this paper, we established a three-dimensional finite element model for analyzing the hydraulic fracture propagation behavior during the indirect fracturing of coal. Fracture initiation and propagation is characterized by the cohesive zone model. Parametric studies have been carried out to unveil the effects of in-situ stress difference between the adjacent layers and the coal seam, the distance from the horizontal wellbore to the interface between the coal seam and the adjacent layer, the fracture energy and the elastic modulus of the coal seam as well as the injection rate and the viscosity of fracturing fluid on the fracture propagation. Numerical results indicated that indirect fracturing can significantly increase the propagation length of the fractures in coal seams compared with direct fracturing, which demonstrates that indirect fracturing technique is beneficial to improving coal seam stimulation performance. Results presented in this paper are expected to be helpful to indirect fracturing design and optimization in coalbed methane reservoirs.

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