Fully coupled hydromechanical flow simulation of water-induced dynamic fractures in low-permeability reservoirs

摘要

Water-induced dynamic fractures are the new features affecting the reservoir heterogeneity in low-permeability reservoirs. Due to the high injection pressure, water-induced fractures will generate and propagate when the formation pressure exceeds fracturing pressure during water-flood of low-permeability reservoirs. Based on field production data and water-flood behavior analysis in low-permeability reservoirs, propagation mechanism of dynamic fractures is identified and the relationship between fracture aperture and permeability variation with normal and shear stresses is derived. We developed a dynamic fracture model that implicitly coupled fluid flow with the stresses induced by fracture deformation. A crossing criterion is implemented that predicts whether propagating dynamic fracture will cross natural fractures or terminate against them. With dual porosity model, the dynamic fracture model is fully coupled with the fluid flow in matrix and fractures. Simulations were performed of a single propagating fracture to validate the model against classical analytical solutions based on PKN model. Numerical simulation results show that formation pressure controls matrix rock deformation and creation, opening, propagation and closure of dynamic fractures. Unlike the former studies which mainly focus on the normal stress, the study in this paper found that shear stimulation also plays an important role in controlling fracture aperture and permeability. Dynamic fractures increase heterogeneity in low-permeability reservoirs so they significantly affect residual oil distribution after water-flood. The sweep efficiency and ultimate oil recovery can be increased by controlling injection pressure and the range of hydraulic fracturing and optimizing the match between well pattern and dynamic fracture orientation.