Numerical modeling and mechanism analysis of a cemented natural fracture on hydraulic fracture

Mineral-filled cemented natural fractures are abundantly distributed in unconventional reservoirs such as shale. The interaction mechanism between hydraulic fractures and cemented natural fractures plays a key role in controlling the generation of complex fracture network. Using the cohesive zone model based on flow-deformation coupling, this study establishes the numerical model of the interaction between hydraulic fractures and cemented natural fractures. The cementing strength of natural fracture is simply characterized using the parameter of fracture energy. The feasibility of the proposed method has been verified by comparing with the asymptotic solutions of a single hydraulic fracture propagation model in limiting regimes. Furthermore, this study investigates the effects of in-situ stress, approaching angle, cementing strength ratio, fracturing fluid viscosity, injection rate and other factors on the interaction between hydraulic fracture and natural fracture. Research results show that both the horizontal in-situ stress difference and the minimum horizontal in-situ stress jointly control the crossing behavior of hydraulic fracture. Under the same horizontal in-situ stress difference, different minimum horizontal in-situ stresses may lead to a difference in the final geometries and pressure distribution inside the hydraulic fractures. The less the approaching angle is, the more easily the hydraulic fracture will deflect into the natural fracture. The larger the cementing strength ratio is, the more difficult the hydraulic fracture will turn into the natural fracture. The same product value of the injection rate and fluid viscosity would lead to similar fracture geometry and pressure profiles at the injection point if the internal fluid leak-off is ignored. In addition, the low pore pressure zone in front of the crack tip is relevant to the size of the cohesive zone:the smaller the cohesive zone is, the lower the pore pressure will be. © 2019, Editorial Office of ACTA PETROLEI SINICA. All right reserved.