Subcritical crack growth and arrest in the presence of a material interface

Abstract

Reservoir systems often consist of layered rock formations that are naturally fractured. These can include both fractures that are confined within the main reservoir layer, and ones that penetrate adjacent confining layers or cut through into nearby reservoir strata. Layer-bound fractures have the effect of improving the overall permeability of a reservoir layer, however through-cutting fractures act as pathways for both fluid leakage and pressure communication between otherwise isolated formations. In this study, we investigate the subcritical propagation of a single crack from a stiff inner layer into more compliant and ductile outer layers. We model the crack growth numerically, by making use of recently formulated correction factors that account for the effect of mechanical boundaries on the crack tip energy release rate. Furthermore, we utilize a modified arc-length method to handle the stiff behaviour of the differential equations governing crack tip motion in a layered medium. Under different conditions, a fracture may be arrested permanently or temporarily at the layer boundary, propagate a short distance into the outer layer before arrest, or propagate critically in the outer layer forming a through-cutting fracture. We find that fractures are most likely to remain layer-bound in layers less than 5 m thick, a subcritical index below 10, and a low critical energy release rate relative to the confining layers. If the outer layer modulus is less than half that of the inner layer, the fracture is more likely to propagate a short distance into the confining layer before arrest. In the case of tectonic deformation characterized by steadily increasing far-field extensional strain, crack arrest at the layer boundary is likely to be only temporary, and the occurrence of layer-bound versus through-cutting fractures will be determined by the duration of said deformation together with the thickness and mechanical properties of the different layers.