Effects of Surface Composition on the Microbehaviors of CH₄ and CO₂ in Slit-Nanopores: A Simulation Exploration

Abstract

Molecular dynamics simulation studies were employed to investigate the microscopic behaviors of CH 4 and CO 2 molecules in slit-nanopores (SNPs) with various surfaces and different compositions. Three kinds of SNPs were constructed by a pair-wise combination of graphene, silica, and the calcite surface. The grand canonical Monte Carlo and molecular dynamics simulation methods were used to investigate the adsorption and self-diffusion of the gases in the nanopores. It is found that in all three cases, the CH 4 molecules prefer to adsorb onto the graphene surface, whereas the CO 2 molecules prefer to adsorb onto the calcite surface. The adsorption intensity of gases adsorbed onto various surfaces, the adsorption distances, along with the details of adsorption orientations of CH 4 and CO 2 molecules on various surfaces are calculated. The surface characteristics, such as surface roughness and charge distribution, are analyzed to help understand the microscopic adsorption behaviors of the gases on the specific surface. It was found that competitive adsorptions of CO 2 over CH 4 broadly occurred, especially in the SNPs containing calcite, because of the strong adsorption interactions between the CO 2 molecules and the calcite surface. This work provides the microbehaviors of CH 4 and CO 2 in SNPs with various surfaces in different compositions to provide useful guidance for better understanding about the microstate of gases in complex nanoporous shale formation and to give out useful guidance for enhancing shale gas recovery by injecting CO 2 .

The Effect of Hydration on the Structure and Transport Properties of Confined Carbon Dioxide and Methane in Calcite Nanopores

“…In the absence of water, both CO 2 and CH 4 show a layered structure on the calcite surface and the density profiles are symmetric with respect to the pore center ( Figure 3A). These density profiles indicate that CO 2 and CH 4 are adsorbed to create a single layer on the calcite surface in the absence of water which is consistent with previous studies (Franco et al, 2016;Sun et al, 2017;Wang et al, 2017;Simoes Santos et al, 2018). The higher density of CO 2 at the interface compared to CH 4 is attributed to the higher energetic affinity of CO 2 on the calcite interface compared to CH 4 as reported by Wang S. et al (2016b).…”

“…The diffusivity of CO 2 and CH 4 through calcite nanopores without water, one layer of interfacial water composed of 150 water molecules, 500 water molecules, and 1,296 water molecules to mimic the density of bulk water yielded interesting insights. In general, the diffusion coefficients of CO 2 and CH 4 in the absence of water are in reasonable agreement with Sun et al (2017) after accounting for the differences in temperature. The temperature used in this study is 298 K compared with 323 K studied by Sun et al (2017).…”

“…In general, the diffusion coefficients of CO 2 and CH 4 in the absence of water are in reasonable agreement with Sun et al (2017) after accounting for the differences in temperature. The temperature used in this study is 298 K compared with 323 K studied by Sun et al (2017). The diffusion coefficient for CO 2 in the absence of water is 0.4 x 10 −5 cm 2 /s at 298 K compared with 2 × 10 −5 cm 2 /s at 323 K. The diffusion coefficient for CH 4 is ∼20 × 10 −5 cm 2 /s at 298 K compared with 35 × 10 −5 cm 2 /s at 323 K. CO 2 and CH 4 diffusivities in the presence of interfacial water with a density similar to bulk water are in the range reported by Mutisya et al (2017) and Bui et al (2017) ( Table 2).…”

“…Calcite is present in almost all geological systems and represents the main constituent of limestone which forms most of the world's hydrocarbons reservoirs (Meldrum and Cölfen, 2008;Geysermans and Noguera, 2009;Addari and Satta, 2015;Bovet et al, 2015;Côté et al, 2015). For this reason, several studies focused on understanding gas interactions in calcite nano-pores (Franco et al, 2016;Sun et al, 2016bSun et al, , 2017Wang S. et al, 2016a,b;Bui et al, 2017;Simoes Santos et al, 2018). Further, shale formations such as the Middle Bakken region in North America are characterized by a significant fraction of calcite which contribute to pore sizes that are 2 nm or greater (Liu et al, 2017).…”

Exploration of Capturing CO2 from Flue Gas by Calcite Slit‐Nanopores: A Computational Investigation

“…It is found that, the Ds values of gases get enlarged with the pore size increasing at such range of pore size (∼5 Å–∼30 Å) . It can be found that, the Ds values of CO 2 are much smaller than N 2 , and comparing with self‐diffusion of CO 2 adsorbed in other slit‐nanopores, such as the nanopores of silica and montmorillonite, the Ds of CO 2 in calcite slit‐nanopore is much smaller, and in our previous work, we have also demonstrated that the CO 2 molecule has stronger adsorption capacity with the calcite surface than silica and graphene. While the self‐diffusion of N 2 in calcite slit‐nanopore is similar to that in carbon nanotubes (CNTs) and asphaltene nanopores, with the Ds values in the same order of magnitude, and which further indicates that the self‐diffusion of N 2 molecule is much larger comparing with CO 2 no matter in nanopores with organic matter or inorganic minerals.…”