Surface Chemistry and Chemical Structure of Shale Caprocks Exposed to CO2: Implication for Sealing Integrity

Subsurface carbon dioxide storage is being widely studied as an effective solution for minimizing greenhouse gas emissions. Shale caprocks are regarded as effective seals during CO2 storage that provide safe CO2 conditions and prevent possible CO2 leakage. However, the geochemical CO2/shale reactions remain a key challenge that can affect the sealing integrity. Despite the extensive studies on CO2/shale geochemical reactions, the impact of CO2 on the surface chemistry and chemical structure of shale caprocks has rarely been addressed. In this work, we provide a detailed experimental analysis of the impact of CO2 exposure on the chemical structure of shale caprocks and the potential implications for seal integrity. A real reservoir subsurface shale sample was treated with CO2 at 75 °C at 1400 psi for 35 days and examined using chemical characterization techniques X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry. Based on the XPS results, no detectable reaction between CO2 and the shale surface was observed, and the composition of the carbon functionalities appeared to be unaffected by the CO2 exposure. The elemental composition showed insignificant changes, and no major chemical alterations occurred on the surface as a result of CO2 exposure. TOF-SIMS analysis revealed that the saturated hydrocarbons accumulated on the surface, which consequently reduces the occurrence of inorganic and aromatic components since these get partly covered by the saturated hydrocarbons. This process is presumably a result of the experimental conditions (75 °C, 1400 psi, 35 days) rather than the presence of CO2. No direct evidence was found for any reaction involving CO2. This suggests a high sealing integrity of shales with low risks of CO2 leakage. Future work can build on the findings in this work and consider it as a suitable experimental framework to assist in evaluating the reactivity of CO2 to shale caprocks.