Molecular Behavior CO2 and CO2-H2O Mixtures at Interfaces

Injection of CO2 into subsurface geologic formations has been identified as a key strategy for mitigating the impact of anthropogenic emissions of CO2. Regardless of the formation type, the CO2 will encounter a complex heterogeneous porous matrix with widely varying pore size and pore distribution, interconnectivity, and surface composition. A small but non-trivial percentage of the pore space is comprised of voids that range from 100 nm down to a few nm in size. These nanoporous environments are more dominant in the cap or seal rocks, such as shale or clay-rich mudstones that act as confining barriers to leakage of CO2 out of the storage reservoir. A concern 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 (SiO2) and clays. We investigated the behavior of pure CO2 and CO2-H2O mixtures interacting with simple substrates, e.g. SiO2 and muscovite, that act as proxies for more complex mineralogical systems. SANS results were described for sorption properties of supercritical CO2 inside mesoporous silica aerogel (95% porosity; 5-40 nm pores), a proxy for the quartz sub-system. The Adsorbed Phase Model (APM) allows, for the first time, a means to quantify the physical properties (e.g. excess, absolute and total adsorption) of the adsorbed phase formed by fluids inside porous media in terms of the mean density and volume of the sorption phase. The results show clear evidence for fluid depletion for conditions above the critical density. Classical molecular dynamics (CMD) modeling of CO2-silica aerogel interactions also indicates the presence of fluid depletion for conditions above the critical density consistent with SANS results. CMD was also used to assess the microscopic behavior of CO2-H2O mixture (2.3 mol % CO2) at a silica surfaces and within silica 1 nm slit pores at 45oC and 200 b. For the case of a silica plate we observed significant layering within 10 Å from the mineral surface, promotion of CO2 co-sorption with H2O on hydroxylated surfaces and stronger H2O adsorption at non-hydroxylated surfaces. In the 1 nm slit pore case, nano-confinement strongly influences the sorption behavior wherein for hydroxylated surfaces H2O is preferentially enriched in the pore but for non-hydroxylated surfaces, H2O dewetting occurs with enhanced capillary uptake of CO2. Structural and dynamic behavior for supercritical CO2 interaction with muscovite (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 magnitudes compared to bulk CO2 depending on pore size and formation of bonds between CO2oxygens 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 and CO2-H2O mixtures at conditions relevant to sequestration.