Mechanics Analysis of Interaction Between Hydraulic and Natural Fractures in Shale Reservoirs
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Copyright 2014, Unconventional Resources Technology Conference (URTeC). The recent advances in micro-seismic monitoring suggest that hydraulic fracturing stimulation in unconventional reservoirs (shale-gas) has often caused complex fracture networks. The most important factor that might be responsible for the fracture complexity is the interaction between natural and hydraulic fractures. Optimization of hydraulic fracture treatments considering natural fractures requires accurately modeling the interaction and investigating fluid flow in the networks. In this study, we present numerical results that quantify the nature of a hydraulic fracture (HF) propagating interactions with natural fractures (NFs). The numerical model is based on a simplified three dimensional displacement discontinuity method and finite difference method. The Newton-Raphson method and Picard iterative method are used to handle the coupling between rock mechanics and fluid flow. Fracture propagation and fluid invasion into pre-existing fractures are both driven by an incompressible, non-Newtonian fluid in a permeable homogenous reservoir. Numerical results are obtained for injection pressure, fracture geometry and stress shadow effects. The results demonstrate that additional pressure is required to start fluid deflecting into natural fractures. The larger the relative angle between HF and NF, the higher the additional pressure required. Injection pressure is increased upon intersection with a natural fracture and decreases as propagation moves along the natural fracture segment. Fracture width restriction is more pronounced when the hydraulic and natural fracture are perpendicular, and is more severe for shorter natural fracture segments. The longer the HF diversion length along the NF, the lesser disparity in orientations when the hydraulic fracture breaks out from the natural fracture. The elevation in injection pressure and width reduction is also dependent on the differential stress (SHmax-Shmin). Analysis shows that increased treating pressure and fracture width reduction can be attributed to higher closure stress acting across misaligned pre-existing fractures and stress shadow effects from the large hydraulic fracture segments. The interaction effects reach the maximum values just when the hydraulic fracture intersects the natural fracture and decrease as the tip moves beyond the intersection point. The up-and-down trend in the injection pressure is an indication that the hydraulic fracture has intersected natural fractures and the fluid has invaded natural fractures. The fracture width is restricted along the offsets of natural fracture segments. Detailed pressure and aperture distributions from this work provide critical insights into fracture propagation mechanisms under the complex conditions.
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Proceedings of the 2nd Unconventional Resources Technology Conference
