The effect of pore fluid chemistry on limestone deformation.

Active use of the subsurface alters the in-situ pore-fluid composition. For limestone, chemical interaction between the pore fluid and the rock has been shown to alter some of the mechanical parameters, though the exact nature of this mechano-chemical interaction is not yet fully understood. To address this, we performed tri-axial compressive experiments on two highly pure (>97% CaCO₃) and well documented limestones: Indiana Limestone and Edwards White, which were saturated with different fluid compositions. We selected our samples to have a porosity within a narrow range: 23.2 ± 0.3% for Edwards White and 12.9 ± 0.5% for Indiana Limestone. Prior to testing, the rock samples were saturated under vacuum with a fluid solution, and left to equilibrate under vacuum at room temperature for 18 hours. The fluids used in our experiments are 1) CaCO₃-saturated water, 2) a brine which has a composition representative for the Dutch subsurface, and 3) a solution of industrial corrosion inhibitor. Samples were tested at room temperature and confining pressures of 2.5, 5 and 10 MPa. In addition to the stress and strain data observed from these experiments, thin sections were made from the deformed samples to perform micro-structural analysis on the damage zone.

Our results show no mechano-chemical effects for Edwards White. However, the rock strength of the Indiana limestone samples changes due to the different pore fluids: At a confining pressure of 2.5 MPa the sample saturated with to CaCO₃ solution failed at 49 MPa, compared to 47 MPa and 54 MPa for the samples that were saturated with the brine and the inhibitor solution respectively. At a confining pressure of 5 MPa both the sample tested with the CaCO₃ solution and the brine solution failed at 57 MPa and the sample exposed to the inhibitor solution failed at 56 MPa. The samples tested at a confining pressure of 10 MPa respectively failed at: 76, 79 and 73 MPa.  These differences of 5 to 10% lead to a shift in the resulting failure envelopes depending on the pore fluid used in the experiments when describing the failure behaviour of these samples using the Mohr-Coulomb failure criterion: The group tested with CaCO₃ solution had a cohesion of 11 MPa and the coefficient of friction of 0.67. For the samples tested with brine solution these values are 10 MPa and 0.71 respectively. For the group tested with inhibitor solution these values equal 15 MPa and 0.47 respectively. The experiments presented here serve as a baseline from which we can further determine which ions or compounds interact with the rock, and the nature of this interaction. For our follow-up work, we will continue by performing a detailed microstructural analysis to better understand the overall controls on the mechano-chemical interactions or the lack thereof. In follow-up experiments, we will narrow down the complexity of the fluid solutions so we can identify the effect of specific ionic species.