Poroelastic Analysis of Natural Fracture Propagation and Coalescence

摘要

Propagation of natural fractures under a compressive in-situ stress state often involves mode I and mode II propagation. Shear slip on the fracture surfaces initiates the so-called wing cracks which kink at approximately 70 degrees from the crack tip. To date, the literature is mostly concerned with the propagation of dry cracks under increasing compressive stresses. This, however, is not a true representation of the actual in-situ stress path since the far-field stresses remain unchanged in most petroleum, and geothermal operations. Instead, injection pressure and the associated thermo-poroelastic stresses are the main driving forces for propagation. Hence, a coupled poroelastic hydro-mechanical model is proposed to study the reservoir stimulation mechanism in naturally fractured reservoirs. The model is used to study wing crack growth considering pore pressure and poroelastic stresses. The results indicate that the injection pressure required for wing crack propagation is higher in a poroelastic rock than in an elastic one due to the back stresses caused by the pore fluid diffusion into the matrix.