Experimental investigations on effects of pore fluid pressure on extension and extension-shear mixed-mode fracture in Berea sandstone

Pore fluid pressure in the geological formation at depth varies spatially and temporarily. An increase in pore fluid pressure 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 an extension or extension-shear hybrid fractures are formed. To better quantify the stress states and fluid pressure during fracture formation, it is crucial to document mechanical strength and the transition from tensile to shear fracture at low effective stress with elevated pore fluid pressure. However, all previous experimental studies were conducted under dry conditions. Here, we investigate 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 at effective maximum principal stress (σ₁' = σ₁ - P_p, where σ₁ is total maximum principal stress and Pp is pore fluid pressure) ranging from 10 to 130 MPa. Fracture strength, inelastic strain, strain at failure, fracture angle to σ₁', and the amount of comminution increase with σ₁'. The transition of extension to shear fracture occurs at σ₁' = ~ 30 MPa, based on the fracture angle and the degree of comminution. All the saturated or pore fluid pressure-controlled test specimens exhibit lower fracture strength than dry samples, and the difference is distinct when the minimum principal stress is tensile (i.e., σ₃' < 0). This implies that pore fluid pressure more effectively assists the breakage of the bonds and opening of the microcracks in the extension fracture regime. A series of triaxial extension experiments at σ₁' = 20 and 50 MPa with various combinations of σ₁ and Pp indicate that the fracture angle to σ₁' is independent of σ₁ and Pp in the extension fracture regime at σ₁' = 20 MPa, and that fracture angle increases with σ₁ and Pp in the extension-shear hybrid fracture regime at σ₁' = 50 MPa. This implies that the estimation of in-situ stress and pore fluid pressure from natural or human-induced deformation at low effective pressure (such as joints, veins, and drilling-induced tensile fractures) requires careful consideration of the mode of fractures formed.