Surface Chemistry and Chemical Structure of Shale Caprocks Exposed to CO<sub>2</sub>: Implication for Sealing Integrity
Ahmed Fatah, Kion Norrman, Ahmed Al‐Yaseri
Abstract
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 CO 2 storage that provide safe CO 2 conditions and prevent possible CO 2 leakage. However, the geochemical CO 2 /shale reactions remain a key challenge that can affect the sealing integrity. Despite the extensive studies on CO 2 /shale geochemical reactions, the impact of CO 2 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 CO 2 exposure on the chemical structure of shale caprocks and the potential implications for seal integrity. A real reservoir subsurface shale sample was treated with CO 2 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 CO 2 and the shale surface was observed, and the composition of the carbon functionalities appeared to be unaffected by the CO 2 exposure. The elemental composition showed insignificant changes, and no major chemical alterations occurred on the surface as a result of CO 2 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 CO 2 . No direct evidence was found for any reaction involving CO 2 . This suggests a high sealing integrity of shales with low risks of CO 2 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 CO 2 to shale caprocks.