Coupling Between Fluid Pressure and Opening During Fluid Injection into a Pre-existing Fracture

Fractures have been shown to play a significant role in fluid transport in a number of georesource applications. Here, we study the fluid-driven reopening of a pre-existing fracture when the deformation along the fracture is significant, which has important implications for the initiation and migration of induced seismicity. We compare semi-analytical calculations of the pressure propagation in an elastically deformable fracture due to fluid injection with numerical simulations using the Distinct Element Method (DEM) implemented in the 3DEC code (ITASCA). We show that when the normal stiffness of the fracture is low, a pressure front attached to a tensional stress pulse emerges. This front propagates along the fracture at large distances from the injection point with a square root time propagation mimicking a pseudo-diffusion process. The propagation is slower when fracture normal stiffness decreases. Moreover, the normal stiffness together with plane strain modulus and the pressure gradient highly influence the elastic response of the host material. We also show that for low normal stiffness fracture, the calculated aperture profile is similar to the asymptotic crack-opening profile obtained from linear elastic fracture mechanics. Lastly, the effective stiffness of the fracture, which is the normal stiffness of the fracture as a whole (as opposed to an element scale stiffness), decreases during fluid injection. Our results suggest that fractures can be categorized as ’soft’ when they exceed a few tens of meters in size.