Impact of undrained pore pressure response on expected failure stress in anisotropic shales

Undrained pore pressure response to stress changes in shales may have impact on drilling problems, fault reactivation and microseismicity observed at significant distances from hydrocarbon reservoirs and injection zones. This mechanism has not been thoroughly studied and is usually disregarded in geomechanical modelling workflows. We examine to what extent the inclusion of the pore pressure response affects the amplitude of total stress changes expected to cause shear failure in several overburden and outcrop shales. Poroelastic pore pressure coefficients and Mohr-Coulomb failure envelope parameters are estimated through triaxial laboratory experiments and are combined to model the total stress increases at failure for a wide range of loading scenarios and rock orientations. Differences in modeling output are explored on the basis of fully anisotropic and simplified isotropic pore pressure parameters as well as a case where pore pressure changes are neglected altogether. As a result, shear failure in shales is expected at significantly lower stresses changes and is plausible in a much wider range of loading paths once the undrained pressure response is considered. Therefore, we postulate that it should be taken into account during injection or production operations safety assessment.