Injection-induced stress and fracture orientation changes

Abstract

Examination of the field evidence suggests that induced vertical fractures in the cohesionless, shallow oilsand reservoirs of northeastern Alberta are unlikely to remain vertical when large volumes of hot fluids are injected into them.Numerical models are assembled taking into account the appropriate material properties, model geometries, and boundary conditions in order to simulate as closely as possible the stress response of a reservoir to induced fracturing. The results suggest that generation of horizontal fractures is extremely likely and that thermal input enhances this probability. Overstress (above overburden) may be maintained by an elastic skin effect on the fracture boundary. Continued displacement and pore pressure permeation can overcome this overstress resulting in the cyclic stress buildup–breakthrough pattern reported in the literature.The inadequacy of static geomechanical models (no fracture orientation change) is pointed out; correct simulation requires correct process comprehension.