Effects of pore fluid chemistry on the localized to ductile transition of sandstone.

In geothermal reservoirs, pore fluid chemistry can affect mechanical and hydraulic properties of rocks inducing mineral dissolution, precipitation, weakening and alteration. Moreover, with increasing pressure and temperature deformation mode can change from localized to ductile (diffused), leading to a major decrease in permeability, thus affecting the exploitability of the reservoir. The effect of fluid chemistry on the transition between localized and ductile deformation of rocks is still marginally understood.

To investigate the effect of water presence and fluid chemistry on localized and ductile deformation, two sets of triaxial experiments (at 20 and 100 MPa effective confinement pressure, in the localized and ductile field respectively) were performed on a porous silicate sandstone (Adamswiller sandstone), dry, with deionized water and with a 6 M NaCl solution (Na⁺ rich solution), with a 0.1 M HCl solution (pH 1 solution) and a 0.1 M NaOH solution (pH 13 solution). To complement the mechanical properties, complex spectral electrical conductivity was measured during deformation to monitor pore fluid ion content; pore fluid was collected at the beginning and at the end of the experiments and analyzed with ICP-MS; post-mortem microstructural analyses were performed.

In the localized domain, both water presence and pore fluid chemistry had a marginal effect on the strength of the rock, leading to a 5/10% strength reduction over dry rock strength indipendently of the pore fluid composition. In the ductile domain, de-ionized water weakens the rock by 25%, a Na⁺-rich or pH 1 fluid leads to a 35% weakening and a pH 13 fluid weakens the rock by 40% over dry rock strength. Spectral electrical conductivity does not change during localized deformation, while it increases by 2 to 5 times during ductile deformation when the rock is saturated with deionized water; conductivity does not change with the Na⁺-rich fluid regardless of the deformation mode and conductivity decreases by half an order of magnitude with both pH 1 and pH 13 fluids both with localized and ductile deformation.