Influence of fluid chemical composition on fracture toughness and frictional strength weakening: Sandstone brittle and ductile strength constraints

Under upper crustal conditions, rock pore space is often filled with fluids spanning various chemical compositions. The presence of fluids (water or others) was shown to affect the mechanical strength of porous rocks in both the brittle and ductile regimes. In the brittle regime, micromechanical models predict that porous rock strength is dictated by the frictional parameters and the fracture toughness of the material, while in the ductile regime, only by the fracture toughness of the material. Here, we investigate the influence of fluids and fluid composition on Adamswiller sandstone's mode-I fracture toughness and frictional strength. Different fluid-saturated conditions were tested: dry, water, 6 mol NaCl solution, 0.1 mol HCl solution and 0.1 mol NaOH solution. We found that the mode-I fracture toughness is significantly reduced in the presence of fluids. Most of the weakening occurs when comparing dry to saturated conditions. Additional reductions are observed for acidic and basic solutions. Basic solution allows for the largest toughness weakening. On the contrary, the measured static and peak friction coefficients are unaffected by fluid presence and fluid composition. Incorporating the measured toughness and frictional strength into micromechanical models successfully reproduces fluid-weakening under uniaxial and triaxial conditions. The models capture the effective-pressure dependence of fluid-weakening in both the brittle and ductile regimes, reproducing the observed variations in strength under different fluid compositions. This experimental dataset provides new insight that constrains the micromechanical mechanisms relevant to porous rock deformation in the presence of natural and anthropogenic fluid-saturated environments, necessary for safe geo-reservoir exploitation.