Experimental investigation of the intermittent injection of brine-scCO₂ to mitigate salt precipitation during CO₂ storage in saline aquifers

Saline aquifers, the primary option for carbon dioxide (CO₂) storage, face a significant challenge in the form of salt precipitation from water evaporation during geological CO₂ sequestration. This salting-out phenomenon can alter reservoir porosity and permeability, impacting injectivity. While previous experimental studies have focused on how salt precipitation negatively affects CO₂ injectivity, research on mitigation strategies is less prevalent. This study, however, places a strong emphasis on investigating the intermittent injection of brine and scCO₂ as a potential mitigation strategy for salt precipitation in saline aquifers. The results of the experiments point to the significant potential of this method as a practical solution. Two high-pressure, high-temperature core flooding experiments were designed to look into how salt precipitation affects injectivity. The effect of formation water salinity was also studied, considering salinities of 2 g/L and 200 g/L. In the lower salinity experiment, no significant salt precipitation was observed at the end of the test. In contrast, the higher salinity experiment showed a 44 % decrease in CO₂ relative permeability at residual water saturation, manifested as an increase in differential pressure. Following the dynamic experiments, Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES) was performed on the effluent brine, revealing that geochemical interactions between the rock and fluids had a negligible impact on the outcomes. Additionally, the conditions of the plug samples before and after supercritical CO₂ (scCO₂) injection were analysed using Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS), revealing visible NaCl salt crystals and accumulation within the pore space of the rock. Subsequently, two additional core flooding experiments were conducted to investigate the effect of intermittent injection of brine and scCO₂ on injectivity and mitigation salt precipitation. The injection procedure involves a continuous process of temporarily stopping scCO₂ injection, followed by water injection, and then resuming scCO₂ injection at the same rate. Two salinities, 200 g/L and 38 g/L, were evaluated for the intermediate injection. At a water salinity of 200 g/L, the CO₂ relative permeability at the end of the test decreased by 26 % compared to the endpoint of the first CO₂ injection period. In contrast, using a salinity of 38 g/L (seawater) resulted in a 22 % increase in CO₂ relative permeability. The findings of this study strongly suggest that the intermittent injection method is a highly effective solution for enhancing injectivity and mitigating salt precipitation, underscoring its importance in the field of CO₂ storage in saline aquifers.