Quantification of Mechanical Integrity of Clay Minerals in Clay-Rich Rocks Under CO2 Geological Storage

Clay minerals are laminated structures with numerous chemically active sites and high surface area to volume ratio which favors CO2 adsorption and storage. However, CO2 adsorption contributes to mechanical degradation of clay minerals. Eventually, clay minerals become vulnerable regions for mechanical failure in clay-rich rocks, affecting both their storage and sealing efficiency. To ensure the long-term sealing performance of clay-rich rocks in CO2 geological storage, it is essential to quantify the level of rock mechanical due to CO2-clay interaction. Clay-rich rocks comprise various clay minerals, each exhibiting different degrees of mechanical degradation under CO2 adsorption. The CO2 adsorption capacity, swelling, and structural changes of clay minerals upon CO2 adsorption have been previously investigated. However, quantifying the impacts of clay type and fluid saturations on the mechanical properties of clay structures during CO2 adsorption remain inadequately understood. Therefore, this paper aims to quantify the impacts of (i) clay type, (ii) CO2 injection pressure, and (iii) water saturation on the mechanical properties of clay minerals upon CO2 adsorption. To achieve the outlined objectives, we construct layered clay structures of different types including illite, kaolinite, and sodium-montmorillonite. We solvate the structures with water at different hydration levels. Subsequently, we perform molecular dynamics (MD) simulations under the isothermal-isochoric (NVT) and the isothermal-isobaric (NPT) ensembles to equilibrate the clay structures. Afterwards, we perform Monte Carlo simulations under the Grand-Canonical ensemble to quantify the CO2 adsorption capacity of the hydrated structures. Moreover, we create synthetic cases with various water-CO2 volumetric saturations in contact with clay structures under reservoir conditions. Then, we equilibrate the fluid-saturated clay structures to swell as a result of fluid-clay interactions. We adopt the strain- controlled loading approach to plot the stress-strain profiles of the structures and quantify their mechanical properties. Throughout this process, we calculate the initial and the subsequent mechanical properties of the clay structures before and after fluid-interaction from the stress-strain curves. Results demonstrate that the mechanical properties of all three clay minerals decrease with increasing hydration level. The exposure of hydrated clay minerals to gaseous CO2 further degrades their mechanical properties. The increase in injection pressure induced a phase change to supercritical CO2 and a decrease in the rate of mechanical degradation of all clay structures. Water has a more pronounced impact on the elastic properties and strength of clay minerals than CO2. Consequently, higher water- to-CO2 volumetric saturation results in lower strength and Young's moduli and higher Poisson's ratios for all clay structures compared to cases with lower water-to- CO2 volumetric saturation. The outcomes of this work highlight the distinct mechanical behavior of each clay mineral under CO2 storage conditions. The molecular- scale investigation of CO2-adsorption-induced deformation on the elasticity of clay-rich rocks enables accurate prediction of the rock mechanical failure during CO2 sequestration. This, in turn, provides reliable quantification of the long-term sealing efficiency and optimal CO2 uptake capacity, helping to prevent mechanical failure.