Combined Stimulation Mechanisms: Hydraulic Opening and Hydro-Shearing of an Existing Fracture

Abstract

We investigate the growth of a hydro-shearing rupture along an existing fracture where the injection pressure is not controlled to remain below the fracture normal in-situ stress. As a result, hydraulic fracturing of the hydro-sheared fracture occurs (also named hydraulic jacking). We focus on the case of planar three-dimensional circular ruptures, which is geometrically simple yet practically relevant. We demonstrate that two fronts propagate along the existing fracture: a shearing front and an opening front. The latter follows the well-known zero-toughness (viscosity-dominated) solution for a penny-shaped hydraulic fracture, although pore fluid pressure diffuses ahead of the opening front. The shearing front lies ahead of the opening front with an amplification that depends on the stress criticality (in-situ shear stress over initial frictional strength), and the pore-pressure profile resulting from the leakage of fluid ahead of the opening front. Using scaling analysis and fully coupled hydro-mechanical simulations, we show that diffusion ahead of the opening front slowly becomes of order one at large times, with an intensity that mostly depends on the ratio between the characteristic over-pressure for radial fluid flow in the intact fracture and the in-situ normal effective stresses. The extent of the frictional shear rupture ahead of the opening front also depends on stress criticality, defined as the ratio between the initial shear stress and the initial frictional strength of the fracture.