Effects of Pore Fluid Pressure on Extension and Hybrid Fractures in Berea Sandstone

Abstract

Pore fluid pressure in the geological formation at depth varies spatially and temporarily. An increase in pore fluid pressure at depth leads to a reduction in effective normal stress, and thus affects the rock strength and deformation mode. Extremely high pore fluid pressure induces very low normal stress conditions, where extension or extension-shear hybrid fractures are formed. To better quantify the stress states and fluid pressure during fracture formation, it is important to determine mechanical strength and the transition from tensile to shear fracture at low effective stress. However, all previous experimental studies were conducted under dry conditions. This study investigates the effects of pore fluid pressure on tensile and hybrid fractures in Berea sandstone by conducting triaxial extension deformation experiments under pore-fluid-pressure controlled conditions. A series of triaxial extension tests at effective maximum principal stress (1') ranging from 10 to 130 MPa indicate that fracture strength, inelastic strain, and strain at failure, fracture angle to 1', and the amount of comminution increase with 1', and that the transition of extension to shear fracture occurs at 1' = 30 MPa. All the saturated or pore fluid pressure-controlled test specimens exhibit lower fracture strength than dry samples. It implies that the pore fluid pressure coefficient, , may be greater than 1 for the formation of extension and hybrid fractures, and that the use of dry tensile strength data leads to an overestimation of the pore fluid pressure or differential stress based on the attitudes of vein structures or drilling-induced tensile fractures.