This paper deals with the shear-induced dilatation mechanism when a plane-strain fluid-driven fracture propagates on a plane of weakness, subjected to shear and compressive stresses. The fracture surfaces are in contact and their sliding over each other gives rise to normal opening because shear across rough surface generates opening dilation. The analysis assumes an incompressible Newtonian fluid with zero viscosity which is injected into the fracture in an impermeable elastic medium. On the basis of the plane strain elasticity, the resulting slip, crack length and net shear stress, which is defined as the difference between the applied shear stress and the friction stress based on the Coulomb law, are calculated. A slip-weakening friction law is implemented to account for crack surface roughness changes with shear displacement. Using a slip-weakening law means no stress singularity exists at the shear crack tip for propagation along the plane of weakness. The governing equations are derived for equilibrium cracks and a scaling is proposed to simplify those equations. Numerical results based on a Chebyshev polynomial expansion show that the size of slipping region can grow under negative net shear stresses as a result of the slip-weakening mechanism which helps explain the success of hydraulic fracturing in promoting shear fracturing along planes of weakness. A critical length for shear fracture initiation exists as a result of use of a slip-weakening friction law and the removal of stress singularity. Similar to dislocation cores, expenditure of energy is required for crack nucleation from this critical size. For stable shear crack growth, the net shear stress should follow a critical curve obtained numerically. If the net shear stress is larger than its critical value, the shear crack will propagate unstably, otherwise it will be arrested as lack of driving forces. In addition, since the net shear stress is bounded, there is another critical length for the onset of unstable crack growth.
Shear dilatation is recognized as a mechanism that can enhance permeability of fluid-driven shear fractures. In contrast to the conventional tensile or opening mode fractures in which the fracture is kept open by an internal pressure that exceeds the minimum stress, the coupling between fluid pressure and conductivity in a shear fracture is via shear displacement or slippage. The fluid pressure in the fracture acts to reduce the effective normal stress acting across it which promotes shear of the fracture. The shear displacement changes the conductivity by causing shear-induced dilation.
The shear dilation mechanism has been exploited to stimulate hot dry rock reservoirs and gas reservoirs (Pine and Batchelor, Vychytil and Horii, Mayerhofer et al.). In addition, microseismicity generated by shearing is commonly observed during conventional hydraulic fracture treatments. Recently, hydraulic fracturing has been applied to caving inducement and preconditioning rock masses for mining by block caving methods (van As and Jeffrey). The preconditioning is aimed at modifying the rock mass strength and the size of rock fragments formed during later caving.