Fluid pressure induced shear slip and permeability changes in fractured wellbore cement

Abstract

Fluid pressure changes can induce shear displacements within a fractured wellbore cemented annulus which may, in turn, alter the fracture permeability and consequently the leakage rate through the wellbore system. This study examined the effects of fluid pressure on shear displacements and permeability of fractured cement samples through a series of pore pressure induced shear tests. The samples were subject to shear stress in a triaxial cell, and a fluid pressure gradient was maintained across the sample. The fluid pressure was increased, reducing the normal stress and fracture shear strength, which eventually induced fracture shear slip. The shear displacement was measured, and the fracture permeability and hydraulic aperture were interpreted. The fluid pressure at which the shear displacements occur was found to depend on the applied external stress, and the fracture permeability changed significantly with the shear displacements. A shear slip criterion and fracture friction coefficient of μ=0.66 were interpreted from the pore pressure induced shear tests. Direct shear tests on comparable samples produced relatively higher friction coefficient which was attributed to the accumulation of wear products in the fracture. Results from the experimental work were used as input to an analytical model to evaluate the possibility of pore pressure induced shear displacement under external stress and internal pressure conditions of a fractured cemented annulus associated with an underground storage facility. Additionally, the critical pore pressure that could cause shear slip in the cement fracture was found to be impacted by the fracture location and orientation.