A Novel IR-Based Workflow for Quantifying CO2 Competitive Adsorption and Geochemical Interactions in Reservoir Caprock

Abstract Competitive adsorption of CO2 and reservoir fluids near caprocks impacts CO2 trapping and leakage. Conventional adsorption quantification techniques fail to distinguish between physisorption and chemosorption interactions. Therefore, we aim to (i) develop a novel Fourier-transform infrared (FTIR) spectroscopy workflow to quantify the CO2:CH4 competitive adsorption within caprocks, (ii) evaluate the influence of rock chemical compositions on CO2:CH4 competitive adsorption, and (iii) investigate the geochemical changes in clay minerals when exposed to CO2 under varying concentrations. Attenuated total reflectance (ATR) FTIR was used to estimate CO2 and CH4 adsorption in caprock components such as Na-montmorillonite (Na-MMT), kaolinite, and activated carbon as a porous shale proxy. The solid samples were crushed and sieved to the same grain size, and positioned on the ATR surface. Pure CO2, CH4, and their mixtures were flowed over the samples while monitoring spectral infrared changes at different gas concentrations. Gas adsorption was quantified by calibrating the peaks of the FTIR spectra to the relative changes in gas concentration in the sample. The analysis of the FTIR spectra showed that the ATR technique could quantify the adsorption peaks of CO2 and CH4, and detect shifts in the material-specific peaks caused by gas adsorption. In the case of CO₂ adsorption on the tested rock components, a consistent increase was observed in the physisorption-related IR peaks, particularly within the C=O bending (at approximately 667 cm⁻¹ wavelength) and asymmetric stretching (at approximately 2349 cm⁻¹ wavelength) regions. These increases reflect enhanced CO₂ population near the solid surfaces with increasing partial pressure. Among the tested materials, Na-MMT exhibited the most prominent peak increase, followed by kaolinite, and then activated carbon, indicating a higher adsorption affinity driven by its greater surface area, interlayer spacing, and exchangeable cation sites. Additionally, CH₄ showed significantly weaker IR activity and a plateauing trend in peak heights, consistent with limited physisorption and the low IR-activity nature of the molecule. These observations confirm a stronger interaction between CO₂ and the tested solid surfaces compared to CH₄. These results not only highlight the physisorption behavior of gases in caprock components, but also provide critical insights into the geochemical interactions at the gas-solid interface. The introduced experimental method leverages IR spectroscopy to quantify the competitive adsorption and interfacial interactions of CO2 with reservoir fluids and rock components. Unlike conventional methods, this method accurately quantifies the multi-gas adsorption capacity of caprock systems without requiring sophisticated gas chromatography setups. These advancements offer a transformative approach to optimizing CO2 storage processes while enabling a precise assessment of CO2 leakage potential, thereby addressing key challenges in carbon sequestration projects.