Effects of pore fluid chemistry on localised and ductile deformation of porous rocks.

In geological reservoirs, pore fluid chemistry significantly impacts rock strength through mineral dissolution, precipitation, and surface charge modifications. Understanding these interactions is crucial for geological applications such as CO₂ storage and geothermal energy applications, where fluid chemistry controls reservoir integrity, exploitability, and aging.

With increasing depth, porous rocks transition from localized to ductile deformation regimes, with the latter characterized by compactant behavior that drastically reduces permeability and affects reservoir exploitability. The role of fluid chemistry in controlling this transition remains poorly understood and varies with fluid composition, mineralogy, and stress conditions.

We performed triaxial deformation experiments on Tavel limestone (84% calcite, 7.5 % quartz and 6% phyllosilicates,14% porosity) at effective confining pressures of 20 MPa (localised regime) and 100 MPa (ductile regime) under constant strain rate (10⁻⁶ s⁻¹). Samples were tested dry and saturated with: deionized water, 0.01 M HCl solution, CO₂-water solution, CaCO₃-saturated solution, 0.1 M MgCl₂, 6 M NaCl and 0.1 M NaOH solutions. During deformation, we continuously monitored spectral electrical conductivity (0.1 Hz–1 MHz), permeability, and P-S wave velocities. Pore fluid chemistry variations were analyzed using ICP-OES, and post-mortem sample were characterized at SEM.

Results reveal water weakening in the localised regime, while in the ductile regime water or fluid chemistry only marginally affect rock strength. These findings contrast sharply with previous results on sandstone under identical conditions (Lazari et al., 2025), where chemical effects were negligible in the localized regime but caused 30-35% weakening during ductile deformation.

In the localized regime, the presence of Mg⁺² or CaCO₃ leads to a slight increase of peak stress, while the presence of HCl creates dissolution patterns on the sample, though without altering the mechanical properties of the rock in the observed timescale.

Increased pore connectivity is evidenced by increasing electrical conductivity with deformation, while calcite dissolution is testified by increased Ca⁺² concentration in the fluid after deformation.

Our results have critical implications for reservoir management: (1) carbonate integrity in shallow reservoirs is more sensitive to formation water chemistry than siliciclastic rocks; (2) CO₂ injection requires careful assessment and evaluation of long-term processes; and (3) rock-specific understanding of chemical-mechanical coupling is essential—behaviors cannot be extrapolated across lithologies. These findings underscore the importance of accounting for specific rock-fluid interactions in geological reservoir management.