Abstract: Based on the theoretical analysis of crack initiation mechanism and crack direction of hydraulic fracturing, the hydraulic fracturing process of coal seam under five different in-situ stress situations has been simulated by using the seepage-stress coupling analysis system of coal and/or rock damage & fracture process, and hence, the crack initiation direction, initial pressure and expansion pressure have been obtained. Studies have found that, the crack direction is vertical, with the initial pressure as well as the expansion pressure presenting as gradual increasing trend when the lateral pressure coefficient λ<1. When the lateral pressure coefficient λ>1, the crack direction is horizontal, with the initial pressure and the expansion pressure presenting as gradually decreasing trend. When the lateral pressure coefficient λ=1, by carrying out three different kinds of yield criterion contrastive analysis, the minimum initial pressure are respectively obtained under the different kinds of yield criterion. Through the further stress analysis, i.e., when coefficient λ=1.2, studies have proposed that the antireflection range of hydraulic fracturing is the summation of macro-crack zone, micro-crack crossing zone, micro-crack generation zone (tension zone) and native crack disturbed zone (pressure recovery zone); the growth trend of anti-reflection area which has been obtained by digital image processing accords with quadratic function.

Key words: hydraulic fracturing of the coal seam, crack initiation direction, crack length, crack expansion process, anti-reflection range