Molecular Insights into the Growth/Dissociation of CH4 Hydrate under the Calcite Slit-Pore Confinement: Effect of Pore Size Variation

Abstract In nature, CH4 hydrates occur within marine sediments characterized by varying pore size, geometry, and composition, all of which influence hydrate phase transition. Although molecular dynamics (MD) studies of confined hydrates often focus on silicate pores, hydrate reservoirs also contain carbonates, which may exhibit hydrate behaviour distinct from silicates. Hence, in this MD study, we investigate the effect of calcite slit-pore size variation on the dissociation temperature (T3) of CH4 hydrate, as well as the dynamics of confined water and methane molecules within calcite pores. To this end, we perform direct phase co-existence MD simulations of CH4 hydrate confined in calcite slit-pores of three different pore sizes (i.e., ∼3.8, ∼6.2, and ∼8.6 nm), in the NVT ensemble at 270, 280, 290, and 300 K. Our results indicate that decreasing slit-pore size shifts the T3 range for CH4 hydrate to lower temperatures, with the smallest pore exhibiting the fastest dissociation. The mechanisms of hydrate cage distortion near the hydrate–calcite surface interface and the formation of the bound interfacial water layer have been elucidated for the small slit-pore system, along with hydrogen bond analysis to explain the microscopic origins of our simulation observations. In contrast to the hydrophilic silica slit-pores studied previously (Fluid Phase Equilibria 2025, 588, 114218), the CH4 cluster confined in the calcite slit-pore tends to remain away from the calcite surface. This behavior arises from the strong electrostatic interaction between the calcite surface and water molecules, which creates a layer of ordered bound water. The potential of the mean force calculation reveals that this bound water layer imposes a higher energy barrier for the CH4 molecule near the calcite surface. The relative location of the CH4 hydrate with respect to the CH4 cluster and/or the calcite substrate influences the hydrate growth and dissociation mechanism.