Threshold Horizontal Stress Contrast for Fully Cemented Natural-Fracture Reactivation: Dependence on Intersection Angle and Initial Reservoir Pressure

Stimulation of pre-existing natural fractures is considered to contribute significantly to hydrocarbon production from unconventional shale reservoirs. An effective natural-fracture simulation may occur in mixed normal and shear modes due to the misalignment of the natural fractures with respect to the principal stresses. This mixed-mode failure is triggered especially upon intersection of these natural fractures with hydraulic fractures and the fracturing fluid infiltration into the natural fractures. Among various influential parameters on the fracture intersection behavior, hydraulic-natural fracture intersection angle and initial reservoir pore pressure carry the highest uncertainty in shale resources characterization. To address this uncertainty, we adopted poroelastic fracture-intersection and propagation models on the basis of a commonly used pore-pressure cohesive zone model. For the natural fracture reactivation, we obtained a threshold horizontal stress contrast beyond which the natural fracture does not undergo slip. We propose an iterative algorithm using numerical experiments to retrieve the threshold horizontal stress contrast for 16 permutations of initial reservoir pore pressure and intersection angle. Our simulations reveal that with increasing intersection angle, the natural fracture reactivation may occur at a higher or lower horizontal stress contrast depending on the initial reservoir pore pressure.