Confinement in nanometer-size pores affects structural and transport properties of water and coexisting volatile species. It has for example been reported that confinement can enhance the solubility of gases in water. We report here equilibrium molecular dynamics simulations for aqueous H 2 S confined in slit-shaped silica pores at 313 K. We investigated the effect of pore width on the H 2 S solubility in water. We quantified the molecular distribution of the fluid molecules within the pores, the hydration structure for solvated H 2 S molecules, and the dynamical properties of the confined fluids. The results show that confinement reduces the H 2 S solubility in water and that the solubility increases with the pore size. Our analysis suggests that these results are due to perturbations on the coordination of water molecules around H 2 S due to confinement. Confinement is found to dampen the dynamical properties of aqueous H 2 S as well. Comparing the results obtained for aqueous H 2 S to those, reported elsewhere, for aqueous CH 4 , we conclude that H 2 S permeates hydrated slitshaped silica nanopores faster than CH 4 . These observations contribute to understand fluids in the subsurface and could have important implications for applications in catalysis and perhaps for developing new natural gas sweetening technologies.

Although enhanced oil recovery (EOR) is often achieved by CO2 injection, the use of acid gases has also been attempted, for example in oil fields in west Canada. To design EOR technologies effectively, it would be beneficial to quantify the molecular mechanisms responsible for enhanced recovery under various conditions. We report here molecular dynamics simulation results that probe the potential of recovering n-butane confined from silica, muscovite and magnesium oxide nano-pores, all proxies for subsurface materials. The three model solid substrates allow us to identify different molecular mechanisms that control confined fluid behavior, and to identify the conditions at which different acid gas formulations are promising. The acid gases considered are CO2, H2S, as well as their mixtures. For comparison, in some cases we consider the presence of inert gases such as N2. In all cases, the nanopores are dry. The recovery is quantified in terms of the amount of n-butane displaced from the pore surface as a function of amount of gases present in the pores. The results show that the gas performance depends on the chemistry of the confining substrate. While CO2 is more effective at displacing n-butane from the protonated silica pore surface, H2S is more effective in muscovite, and both gases show similar performance in MgO. Analysis of the interaction energies between the confined fluid molecules and the surface demonstrates that the performance depends on the gas interaction with the surface, which suggests experimental approaches that could be used to formulate the gas mixtures for EOR applications. The structure of the gas films at contact with the solid substrates is also quantified, as well as the self-diffusion coefficient of the fluid species in confinement. The results could contribute to designing strategies for achieving both improved hydrocarbon production and acid gas sequestration.

Because gas injection into geological formations is a common technology deployed for enhanced oil recovery (EOR), it is important to understand at the molecular level the relations between competitive adsorption and fluid mobility at the single-pore level. To achieve such an understanding, we report here molecular dynamics simulation results to document structural and dynamical properties of n-octane confined in slit-shaped alumina and graphite pores in the presence of CO 2 or H 2 S. The substrates are chosen as proxy models for natural hydrophilic and hydrophobic substrates, respectively. It was found that CO 2 and H 2 S could displace n-octane from alumina but not from graphite surfaces. Analysis of the results demonstrates that more attractive n-octane -surface and weaker CO 2 /H 2 S -surface interactions in graphite compared to alumina are responsible for this observation. Regardless of pore type, the results suggest that adding CO 2 or H 2 S suppresses the diffusion of n-octane due to pore crowding. However, the mechanisms responsible for this observation are different, wherein preferential adsorption sites are available on the alumina surface for both CO 2 and H 2 S, but not on graphite. To contribute to designing advanced EOR technologies, possible molecular mechanisms are proposed to interpret the results.