Molecular Insight into Microbehaviors of n-Decane and CO2 in Mineral Nanopores

Abstract

Nanopores are abundant in unconventional reservoirs. Understanding the microbehaviors of oil confined at such a small scale is favorable to the enhancement of oil recovery. In this study, molecular dynamics simulations were performed to investigate the adsorption behaviors and structural properties of n-decane + CO₂ mixtures confined within quartz, calcite, and clay nanopores. First, comparing the peak of n-decane density distribution in the first adsorbed layer and the interactions between rock surfaces and n-decane, we found that the adsorption strength of n-decane on various minerals follows the order calcite > kaolinite > montmorillonite > quartz > illite. Second, the distributions of n-decane on different mineral surfaces were analyzed, revealing that the strong adsorption of calcite is due to the strong interactions between Ca atoms in calcite and n-decane, and the flexible H atoms on the surfaces of quartz and kaolinite make n-decane distribute evenly. Then, the effects of pore size, filling fraction and pressure on the structural properties of n-decane were explored. Results show that the adsorbed layers remained unchanged when the pore size is big enough for n-decane (∼3 nm) to form a bulk region, while the peak in the adsorbed layers and the number of adsorbed layers vary with the pore size when the pore size becomes smaller. The reduction of alkane filling fraction decreases the density of n-decane in the region far from the wall in advance, and then the density in the adsorbed layers. The density of n-decane in the adsorbed layer decreases greatly with the increase of pressure (by adding CO₂), showing good oil displacement by CO₂. This work provides new molecular insights to CO₂ enhanced oil recovery.