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

• Published in: Accounts of chemical research 2023-07, Vol.56 (13), p.1862-1871
• Main Authors: Bowers, Geoffrey M., Loganathan, Narasimhan, Loring, John S., Schaef, Herbert Todd, Yazaydin, A. Ozgur
• Format: Article
• Language: English
• Subjects: Cations; Energy; Fluids; INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; Layered materials; Molecular modeling
• ISSN: 0001-4842; 1520-4898
• DOI: 10.1021/acs.accounts.3c00188

Conspectus In the mid 2010s, high-pressure diffraction and spectroscopic tools opened a window into the molecular-scale behavior of fluids under the conditions of many CO2 sequestration and shale/tight gas reservoirs, conditions where CO2 and CH4 are present as variably wet supercritical fluids. Integrating high-pressure spectroscopy and diffraction with molecular modeling has revealed much about the ways that supercritical CO2 and CH4 behave in reservoir components, particularly in the slit-shaped micro- and mesopores of layered silicates (phyllosilicates) abundant in caprocks and shales. This Account summarizes how supercritical CO2 and CH4 behave in the slit pores of swelling phyllosilicates as functions of the H2O activity, framework structural features, and charge-balancing cation properties at 90 bar and 323 K, conditions similar to a reservoir at ∼1 km depth. Slit pores containing cations with large radii, low hydration energy, and large polarizability readily interact with CO2, allowing CO2 and H2O to adsorb and coexist in these interlayer pores over a wide range of fluid humidities. In contrast, cations with small radii, high hydration energy, and low polarizability weakly interact with CO2, leading to reduced CO2 uptake and 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 strongly influenced by the cation properties, framework properties, and fluid humidity. The silicate structural framework also influences CO2 uptake and behavior; for example, smectites with increasing F-for-OH substitution in the framework take up greater quantities of CO2. Reactions that trap CO2 in carbonate phases have been observed in thin H2O films near smectite surfaces, including a dissolution–reprecipitation mechanism when the edge surface area is large and an ion exchange–precipitation mechanism when the interlayer cation can form a highly insoluble carbonate. In contrast, supercritical CH4 does not readily associate with cations, does not react with smectites, and is only incorporated into interlayer slit mesopores when (i) the pore has a z-dimension large enough to accommodate CH4, (ii) the smectite has low charge, and (iii) the H2O activity is low. The adsorption and displacement of CH4 by CO2 and vice versa have been studied on the molecular scale in one shale, but opportunities remain to examine behavioral details in this more complicated, slit-pore inclusive