ABSTRACT:

This paper presents numerical simulations of hydraulic fracture propagation in a naturally-fractured rock at large scale using the displacement discontinuity method. The model has been developed to address hydraulic fracture propagation; it’s interaction with natural fractures (crossing/arrest) and transition of natural fractures (closed joint elements) to hydraulic fractures. The natural fracture elements (fracture opening supported by asperities and fluid pressure) and hydraulic fracture elements (fracture opening supported fully by fluid pressure) are treated separately in the model. Using a relationship between fracture closure and effective stress and by specifying stiffness of asperities, the amount of the closure experienced by natural fractures is determined. The effect of pore pressure distribution and induced stresses due to fracture deformation on natural fracture slip is studied. The results include injection pressure response, network geometry and fracture conductivities for the rock masses under different in-situ stress anisotropy. We find fluid diversion into natural fracture can initiate wing fracture growth while the parent fracture is still mechanically closed. Induced stresses alone have negligible effect on slippage of isolated natural fractures. Also, small deformations of natural fractures due to the initial in-situ stress state has negligible impact on hydraulic fracture propagation.

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