Geochemical Reactions of High Total Organic Carbon Oil Shale during CO₂ Treatment Relevant to Subsurface Carbon Storage

Subsurface carbon dioxide (CO2) storage is being widely studied as a suitable solution for reducing greenhouse emissions. Shales are regarded as effective storage rocks and caprock seals of stored CO2; however, the induced CO2/shale interactions remain a key challenge that can affect storage and sealing behaviors. Despite the current devotion directed toward understanding the CO2/shale geochemical interactions, only limited work is available on the reactivity of CO2 to shale rocks with high total organic carbon (TOC) contents. In this work, we experimentally investigate the CO2/rock geochemical reactions for high-TOC shales and address the impact on the surface morphology and storage potential. A high-TOC shale sample (14.6 wt %) was treated with CO2 at 75 degrees C and 1400 psi for 60 days, and various analytical methods, including X-ray diffraction, TOC analysis, scanning electron microscopy, and energy-dispersive X-ray analysis, were performed. The results indicate a minor alteration in mineral composition after CO2 treatment, suggesting that mineral dissolution/precipitation induced by CO2 was very low. The dissolution behavior of calcite indicates that carbonate solubility in shales in the presence of CO2 is likely to occur, which in the long term can provide an efficient trapping mechanism through mineralization. The contents and distribution of organic matter on the shale surface did not exhibit a significant change after CO2 treatment, suggesting that decomposition/oxidation of organic matter was not sufficient during the treatment duration. We observed only minor changes in the pore distribution of the volume after CO2 treatment, indicating that neither pore expansion nor shrinkage occurred. The obtained results confirm the stability and suitability of high-TOC shales for CO2 subsurface storage applications under the experimental conditions. Future works can build on the obtained results of this experimental work to assist in the understanding of the nature of shale reactivity to CO2.