The effect of fluid chemistry on sandstone’s fracture toughness and frictional strength: Implications for brittle and ductile strength

In the upper crust, rock pore spaces may be occupied by fluids of diverse chemical compositions. Pore spaces can be naturally filled with water, carbon dioxide, oil or gas, or artificially saturated with reactive fluid for geo-engineering purposes, including geothermal energy, wastewater disposal, carbon dioxide or hydrogen storage. The presence of water and other fluids modifies the mechanical strength of porous rocks in both the brittle (i.e., localised) and ductile (i.e., distributed) regimes. According to micromechanical models, the strength of porous rock in the brittle regime is controlled by both frictional parameters and fracture toughness of the material, while inelastic compaction by cataclastic pore collapse is governed exclusively by fracture toughness. Experimental studies indicate that the presence of fluid affects the fracture toughness and static friction of limestones and sandstones. Accordingly, for a given rock type, fluid-induced weakening of the rock strength should be explained entirely by a decrease in fracture toughness and/or frictional parameters.

This interpretation is supported by measurements of the mode-I fracture toughness (K_Ic) and static friction (µ_s) of sandstones and limestones, under both dry and water-saturated conditions, which allow for the estimation of the uniaxial compressive strength and quantification of the degree of water-weakening. In this context, we investigate the influence of fluids and fluid composition on the mode-I fracture toughness and frictional strength of Adamswiller sandstone. This sandstone was selected because its mechanical behaviour is well-documented in the literature, and because both fluid presence and fluid composition have been shown to affect its response under uniaxial and triaxial compression. We tested a range of fluid-saturated conditions, including dry, deionised water, 6 mol NaCl solution, 0.1 mol HCl solution and 0.1 mol NaOH solution. For K_Ic, most of the weakening occurs between dry and fluid-saturated conditions, with additional reductions observed for acidic and basic solutions, with the greatest under basic conditions. For a saline solution, the extent of weakening relative to water-saturated conditions is unclear. In contrast, the measured static and peak friction coefficients are unaffected by either the fluid presence or the fluid composition. Incorporating the measured toughness and frictional strength into micromechanical models (wing crack model and pore collapse model) 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 strength variation associated with different fluid compositions. This experimental dataset provides new insight that constrains the micromechanical mechanisms governing porous rock deformation in natural and anthropogenic fluid-saturated environments, with direct implications for the safe exploitation of geo-reservoirs.