Impact of chemical subprocesses during calcite precipitation in sandstones on the measured SIP response and their identification

Interactions between mineral phases and fluids in the subsurface inevitably lead to mineral precipitation reactions and dissolution. While these processes are the major drive behind many geochemical changes in aquifer systems, their detection, monitoring and characterization is difficult. Geoelectrical methods provide potential to investigate precipitation and dissolution reactions in rocks non-invasively. However, with the measurement of the DC electrical conductivity alone, changes in pore water salinity, mineralogy, or pore space characteristics can hardly be differentiated. The ambiguity in the identification of these processes can be reduced by also measuring the spectral induced polarization (SIP) response, i.e., the frequency-dependent complex electrical conductivity, given the sensitivity of especially the imaginary component to textural and chemical characteristics. In order to assess the capability of this approach, we conducted multiple laboratory experiments on quartz-rich sandstone samples in which different precipitation scenarios were provoked under controlled conditions while monitored with SIP. The experimental setup consists of two reactant solutions in contact with both sides of the sample, leading to a reaction within the sample as diffusion from each side into the rock goes on. We used reactant solutions of NaHCO₃and CaCl₂in varying molality, the mixing of which in the sample’s pore space results in CaCO₃formation. By varying samples and solutions, three different components contributing to the complex conductivity response during the ongoing precipitation could be identified. The onset of the chemical reaction is clearly visible in the temporal evolution of imaginary conductivity at relatively low frequencies. The observed temporal peak can be associated with changes in the pH value due to the infiltration of the reactant at earlier times and the reduction in pH with calcite precipitation. This explanation is supported by additional experiments performed on a similar sample, where pH was altered by infiltration of NaHCO₃only. A second spectral high-frequency peak shows up at later stages of the experiments, suggesting that here the main changes of the pore surfaces in response to the precipitation are occurring. This phenomenon could not be recreated by using the infiltration of a pure electrolyte solution or the infiltration of NaHCO₃. The last component in the complex conductivity response is the continuous increase of the real component due to the increasing salinity of the pore water, which also could be reproduced in comparative measurements. Our results show the potential of complex conductivity measurements for precipitation monitoring in rocks, including improved textural and chemical characterization. Given the applicability of complex conductivity imaging at the field scale, the method thus holds promise for monitoring tasks in the context of, for example, carbon capture and storage, enhanced geothermal energy, soil stabilization, and capture of dissolved contaminants, which are of increasing societal relevance.