Role of seepage forces on hydraulic fracturing and failure patterns

Abstract

The mechanical role of seepage forces on hydraulic fracturing and failure patterns was studied both by the analytical methods of the continuum mechanics and by numerical simulations. Seepage forces are frictional forces caused by gradients of pore-fluid pressure. Formation of different failure patterns (localized shear bands or tensile fractures) driven by the localized fluid overpressure in the poro-elasto-plastic medium was studied using a numerical code specially developed for this purpose. The pre-failure condition for different failure patterns and fluid pressure at the failure onset was predicted using a new analytical solution.In the analytical solution the elliptical cavity filled with fluid in the non-hydrostatic far-field stress-state is considered. Since, the fluid pressure inside cavity differs from the far-field pore-fluid pressure; the poroelastic coupling is taking into account in the calculation of the deformation. Using Griffith’s theory for failure and this analytical solution, the generalized equation for the effective stress law was obtained. This generalized effective stress law controls the failure in the fluid-saturated porous medium with a non-homogeneous fluid pressure distribution.