Supercritical fluid behavior at nanoscale interfaces: Implications for CO₂ sequestration in geologic formations

Injection of CO² into subsurface geologic formations has been identified as a key strategy for mitigating the impact of anthropogenic emissions of CO². A key aspect of this process is the prevention of leakage from the host formation by an effective cap or seal rock which has low porosity and permeability characteristics. Shales comprise the majority of cap rocks encountered in subsurface injection sites with pore sizes typically less than 100 nm and whose surface chemistries are dominated by quartz (SiO ²) and clays. We report the behavior of pure CO² interacting with simple substrates, i.e. SiO² and muscovite, that act as proxies for more complex mineralogical systems. Modeling of small-angle neutron scattering (SANS) data taken from CO²-silica aerogel (95% porosity; ∼7 nm pores) interactions indicates the presence of fluid depletion for conditions above the critical density. A theoretical framework, i.e. integral equation approximation (IEA), is presented that describes the fundamental behavior of near-critical adsorption onto a non-confining substrate that is consistent with SANS experimental results. Structural and dynamic behavior for supercritical CO² interaction with muscovite (KAl ²Si₃AlO₁₀(OH)₂) was assessed by classical molecular dynamics (CMD). These results indicate the development of distinct layers of CO² within slit pores, reduced mobility by one to two orders of magnitude compared to bulk CO² depending on pore size and formation of bonds between CO² oxygens and H from muscovite hydroxyls. Analysis of simple, well-characterized fluid-substrate systems can provide details on the thermodynamic, structural and dynamic properties of CO² at conditions relevant to sequestration.