Fluid Chemistry Effect on Localized and Ductile Deformation of a Porous Sandstone

Abstract In geological reservoirs, pore fluid chemistry can affect rock strength by inducing mineral dissolution, precipitation, and alteration, among other processes. Increasing pressure and temperature can shift deformation from localized to ductile, decreasing permeability and affecting reservoir exploitability. The role of fluid chemistry in this transition remains marginally understood. Deformation experiments were performed on a porous silicate sandstone (Adamswiller sandstone) at effective confinement pressures of 0, 20, and 100 MPa, under dry conditions and saturated with deionized water, pH 1 (0.1 M HCl), 6 M NaCl, and pH 13 (0.1 M NaOH) solutions. Spectral electrical conductivity was measured during deformation, and pore fluid chemistry variations were determined with ICP‐MS and ICP‐OS on pore fluid samples collected before and after the experiments. SEM microstructural analyses and chemical characterization were conducted post‐mortem. Water presence and fluid chemistry had a marginal effect on localized deformation, reducing peak strength by 5%–10%. In the ductile regime, deionized water reduced strength by 25%, pH 1 and NaCl solutions by 30%, and pH 13 by 35%. Surface electrical conductivity increased during ductile deformation due to mineral alteration. Greater weakening in ductile samples was hypothesized to be linked to increased silica solubility and dissolution rates in chemically enriched solutions under higher effective pressures.