The interaction of hydraulic fractures with the pre-existing natural fractures may play a major role in increasing productivity from unconventional formations. When a hydraulic fracture meets a natural fracture, the hydraulic fracture can cross the natural fracture or be arrested. If the natural fracture is permeable, fracturing fluid can leak from the hydraulic fracture into the natural fracture causing elevation of pore pressure in the natural fracture and reducing the effective normal stress acting on the natural fracture, which could then lead to shear failure or slippage along the natural fracture plane. Shear-slip causes dilation, potentially increasing fracture conductivity and enhancing fluid flow deeper into the natural fracture. The conductivity of unpropped shear-induced fractures can play an important role in enhancing the productivity from ultralow-permeability formations like shale. In this paper, we first evaluate analytically the shear-slip condition and its propagation along a natural fracture under remote normal and shear stresses, when it is exposed to the fluid pressure in a hydraulic fracture. Analytical approximations under some limiting conditions are considered. A rigorous 2D numerical model based on coupling between fluid flow and rock deformation using displacement discontinuity method and fluid flow in the fracture is then described. The results of numerical simulations are presented to illustrate the effect of rock stress anisotropy, initial natural fracture conductivity, and fluid properties on the evolution of the fluid and slip fronts along the natural fracture and the associated permeability enhancement.
In the last decade, following the success of horizontal drilling and multistage fracturing in the Barnett Shale, exploration and drilling activities in shale gas and shale oil reservoirs have skyrocketed in the US and abroad. Economic production from these reservoirs depends greatly on the effectiveness of hydraulic fracturing stimulation treatment. Microseismic measurements and other evidence suggest that creation of complex fracture networks during fracturing treatments may be a common occurrence in many unconventional reservoirs [1-3]. The created complexity is strongly influenced by the preexisting natural fractures and in-situ stresses in the formation. To optimize the fracture and completion design to maximize the production from these reservoirs, engineers must have a good understanding of the fracturing process and be able to simulate it to obtain information such as the induced overall fracture length and height, propped versus unpropped fracture surface areas, proppant distribution and its conductivity, and potential enhanced permeability through stimulation of the natural fractures.