Abstract: In order to further analyze the complicated propagation path of hydraulic fracture in combined rock masses, the coal-rock combined models were established from the aspects of hard roof-floor management, permeability enhancement of coal seam, and fracturing coal-shale unconventional reservoirs. The propagation of hydraulic fractures in different cases was investigated, and breakdown pressures were compared correspondingly. Complex and tortuous fractures were induced when fracturing coal seam, which were suitable for coal seam permeability enhancement while not for hard surrounding rock management. Straightforward fractures were obtained when hydraulic fracturing was performed in hard surrounding rocks, which was beneficial to roof management instead of coal permeability enhancement, since limitation 2019年 - 第1卷第3期于永军,等:北山隧道式大跨度地下四季滑雪场围岩稳定性研究 \=-was detected as fracture penetrated into coal seam. Setting pairing wellbores could increase the fracture penetrating into coal seam, whereas mutual interference among wellbores and fractures was also emerged. Wellbores should be configured rationally based on softening surrounding rocks and permeability enhancement for coal seam, aimed at an optimized fracturing effectiveness and reducing engineering costs. Both the leak-off of injected fluid and pore pressure distribution were influenced by configuration of natural fractures embedded abundantly in shale. The propagation of hydraulic fractures was affected by both natural fracture distributions and in-situ stress regimes. Comparisons were made between numerical and analytical solutions of breakdown pressure in different cases. Divergent data of breakdown pressure were obtained by analytical equations while numerical results were in the range of analytical solutions. Furthermore, more reservoir geological and fracturing engineering factors can be incorporated during simulations. The research may provide insights regarding of hard surrounding rock management, coal permeability enhancement and shale reservoir stimulation.
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2019, Vol. 1 
