Effect of Imidazolium-Based Ionic Liquids on the Sandstone Reservoir’s Wettability: Implications for CO₂ Geo-Storage

In the context of CO₂ geological storage, understanding the interplay of interfacial tension (IFT) and wettability dynamics within the CO₂-brine-rock system plays a huge role in improving their storage capacity and ensuring their secure containment. Ionic liquids (ILs) emerge as a promising surface-active agent that has the potency to alter the reservoir’s wettability more toward the water-wet state, thereby increasing the oil recovery. However, the potential of ILs has remained unexplored for enhancing CO₂ geological storage. Hereby, we assess the effect of ILs on the wettability modification of the sandstone reservoirs by measuring the rock-brine-CO₂ contact angles (CA). In this context, we conducted CA measurements on both the clean and crude oil-aged samples, both with and without ILs, at broad ranges of IL concentrations (0-1000 ppm), temperatures (25-80 °C), and pressures (14.7-3000 psi). This approach is designed to evaluate the CO₂ storage capacity in both the saline aquifers and oil-depleted sandstone reservoirs. Additionally, we also explored the impact of ILs on the CO₂-seawater (SW) IFT. Findings indicate that the clean sandstone sample maintained a water-wet nature; however, the IL treatment shifted its wettability more toward a strongly water-wet state. While the oil-wet samples were initially identified to be CO₂-wet, upon treatment with ILs, they shifted to an intermediate water-wet state. Molecular dynamics (MD) simulations were undertaken to get atomistic insights into the rock wettability changes and the CO₂-SW interface. MD revealed that CO₂ on a clean sandstone alters the rock surface charge, which induces wettability changes. Such charge development is a result of pH modifications induced by CO₂ that affect the silanol group density. The addition of an IL was found to reduce the effect of CO₂ on the rock surface charge. IL is adsorbed on the rock surface and screens the rock-CO₂ interactions, maintaining the water-wet state. Such wettability alteration is poised to significantly improve CO₂ storage efficiency, thereby reducing the security risks that are associated with it. The finding highlights that enhancing the CO₂ storage efficiency can be greatly improved by preliminarily injecting ILs before CO₂ flooding at a minor concentration. It is worth highlighting that the observed SW-CO₂ IFT changes with ILs are relatively minimal, indicating a favorable condition for capillary trapping. Furthermore, the investigated ILs exhibit exceptional stability, solubility, detectability, scalability, and economic viability; thus, the preinjection of these solutions will make them robust solutions to improve the CO₂ storage potential and containment security at the reservoir conditions.