Chemistry and Dynamics of Supercritical Carbon Dioxide and Methane in the Slit Pores of Layered Silicates

Conspectus In the mid 2010s, high-pressurediffraction and spectroscopic toolsopened a window into the molecular-scale behavior of fluids underthe conditions of many CO2 sequestration and shale/tightgas reservoirs, conditions where CO2 and CH4 are present as variably wet supercritical fluids. Integrating high-pressurespectroscopy and diffraction with molecular modeling has revealedmuch about the ways that supercritical CO2 and CH4 behave in reservoir components, particularly in the slit-shapedmicro- and mesopores of layered silicates (phyllosilicates) abundantin caprocks and shales. This Account summarizes how supercriticalCO(2) and CH4 behave in the slit pores of swellingphyllosilicates as functions of the H2O activity, frameworkstructural features, and charge-balancing cation properties at 90bar and 323 K, conditions similar to a reservoir at & SIM;1 km depth.Slit pores containing cations with large radii, low hydration energy,and large polarizability readily interact with CO2, allowingCO(2) and H2O to adsorb and coexist in these interlayerpores over a wide range of fluid humidities. In contrast, cationswith small radii, high hydration energy, and low polarizability weaklyinteract with CO2, leading to reduced CO2 uptakeand a tendency to exclude CO2 from interlayers when H2O is abundant. The reorientation dynamics of confined CO2 depends on the interlayer pore height, which is stronglyinfluenced by the cation properties, framework properties, and fluidhumidity. The silicate structural framework also influences CO2 uptake and behavior; for example, smectites with increasingF-for-OH substitution in the framework take up greater quantitiesof CO2. Reactions that trap CO2 in carbonatephases have been observed in thin H2O films near smectitesurfaces, including a dissolution-reprecipitation mechanismwhen the edge surface area is large and an ion exchange-precipitationmechanism when the interlayer cation can form a highly insoluble carbonate.In contrast, supercritical CH4 does not readily associatewith cations, does not react with smectites, and is only incorporatedinto interlayer slit mesopores when (i) the pore has a z-dimension large enough to accommodate CH4, (ii) the smectitehas low charge, and (iii) the H2O activity is low. Theadsorption and displacement of CH4 by CO2 andvice versa have been studied on the molecular scale in one shale,but opportunities remain to examine behavioral details in this morecomplicated, slit-pore inclusive system.