Simulation Study On Crack Propagation In Granite Reservoir Networks

Bedrock reservoir contains a large number of oil and gas, which accounts for 15% of the world's known reserves of oil and gas. Granite reservoir, which accounts for 75% of the total reserves of bedrock reservoir, is the hotspot of exploration and development. However, the porosity and permeability of granite matrix is low. Natural fracture is the main storage space and flow channel. The key to effective development of such reservoir is to make natural fracture shear slip and connect with each other by hydraulic fracturing. The traditional symmetrical two-wing fracture propagation model is not applicable when simulating shear slip dominated fracture network propagation. Most of the existing fracture network propagation model is aimed at tensile failure mechanism. So it is necessary to carry out research on fracture network propagation under shear failure mechanism.Understand the seepage, stress and strain characteristics and failure mechanism by analysis of physical and mechanical property of granite reservoir. Choose suitable fracture characterization parameters for granite reservoir, and establish seepage flow mathematical model considering dual media, permeability anisotropy and stress sensitive effect. Constitutive model of fractured rock before and after failure is obtained by superposition of tensor, combined with the balance equations and geometric equations, fractured formation stress field mathematical model is established. Use Mohr coulomb failure criterion and weak surface failure criterion as shear failure criteria. The finite element method is introduced to solve mathematical model of the seepage field and stress field. The weak coupling analysis method is adopted to deal with fluid-stress coupling. Nonlinear equations are solved by Newton iteration. Based on the finite element numerical format of fracture network propagation model of granite reservoir, calculating program is compiled. Fracture propagation example is analyzed after model validation.Factors affecting fracture network propagation are analyzed. High injection rate is beneficial to the expansion of the joint network, results in better permeability of formation near the injection point. Fracture azimuth has great influence on the rock mass failure mode and fracture network propagation. The fracture network is a parallelogram when failure mainly occurs in the natural fractures and dominant propagating direction is along the natural fracture direction. Fracture network approximates for rectangular form when failure is mainly rock matrix shear failure.In this paper, the fracture network fluid-solid coupling propagation model of granite reservoir under shear failure mechanism is established, which provides a theoretical basis and methods support for fracturing optimization design and is beneficial for pertinence and effectiveness of fracturing.