Assessing the Effect of Carbonated Water on the Geochemistry of CO₂-Storing-Bed Minerals

Global warming and climate change are influenced by the discharge of carbon dioxide into the Earth's atmosphere. CO₂ can be injected into underground storage locations such as depleted oil and gas reservoirs or saline aquifers to reduce emission impacts. Injected CO₂ will be located next to the reservoir phases in place: brine, pore surface minerals, and any residual oil. CO₂ in brine forms carbonic acid, which could affect the stability of minerals. Short-term and long-term geochemical alteration processes should be screened to improve the understanding of mineral dissolution and in-situ mineralization mechanisms, giving improved quality of the numerical models needed for large-scale simulations. This study investigated the chemical interactions between sandstone, chalk minerals, and carbonated water (CW) at static high-pressure/temperature conditions. Feldspar and carbonate minerals batches with different surface areas were exposed to CW for 1 and 3 months. Fluid properties before and after CW exposure were measured using ion chromatography (IC) and pH tests, and the integrity of the rock grains was studied by scanning electron microscopy (SEM) and a laser diffraction analyzer. Subsequently, the compositions of the exposed minerals were examined using energy-dispersive X-ray spectrometry (EDX). In addition, CW core flooding tests were conducted on outcrop chalk, as chalk was the mineral showing the highest reactivity in the static batch experiments. At the final stage, the static CW exposure test results were modeled by PHREEQC. The results showed that the static batch experiments only revealed minor dissolution effects in chalk after CW exposure. Dynamic core flooding tests using an outcrop chalk core showed that injection of CW can cause higher rock dissolution at the inlet of the core. Exposing reactive minerals to CW can cause chalk dissolution and ionic exchange in feldspars. However considerable changes in sample integrity and grain geometry during the experiments were not observed. PHREEQC modeling made an acceptable match between the experimental and the simulated data. This research shows that the dominant mechanisms between CW and the exposed minerals were ionic exchange and mineral dissolution. When these processes consume CO₂, it leads to improved CO₂ storage due to increased dissolution trapping. The study's results can be used to assess the integrity of the storing bed minerals after CW exposure.