Hydro-mechanically coupled FDEM framework to investigate near- wellbore hydraulic fracturing in homogeneous and fractured rock formations
Résumé
This paper reports on a numerical study that was conducted to investigate near-wellbore behaviour of Hydraulic Fracture (HF) growth using a Finite-Discrete Element Modelling (FDEM) approach. In particular we consider the effect on fracture propagation of pre-existing defects (joints) within the rock medium, as well as physical mechanisms that induce microseismic events. Initial modelling was conducted for an isotropic and homogeneous rock mass subject to isotropic and anisotropic far-field stress states. The introduction of single isolated joints for a rock mass subject to anisotropic far-field stresses that did not intersect the wellbore, such as those created by previous fracturing stages, were seen to impose a lateral stress gradient. The presence of stress gradients leads to asymmetric fracture initiation and growth, generally away from the pre-existing joint, accompanied by an increase in the fluid pressure required to initiate fractures. However, the influence of pre-existing joints diminishes with distance from the wellbore. Due to the high permeability of pre-existing rock joints, fractures prefer to initiate at joint tips after the joint is intersected by a fluid-driven fracture. Based on our simulations, a set of pre-existing, randomly distributed joints around the wellbore leads to microseismic events that are primarily induced by shear slippage on critically stressed joints. For a given injection energy, the presence of multiple joints around a wellbore increases the extent to which fluid-driven fractures can grow. Finally, comparison of our simulation with measured results from low-volume injection test confirms that our approach appears to capture salient aspects of elastic deformation and breakdown pressures in the field.