Molecular simulation of the solubility of hydrocarbon oligomers in supercritical CO2 for direct viscosification

Abstract

Direct viscosification of CO₂ offers promising alternative for mobility control and reduction in residual brine saturation, thus to improve the CO₂ trapping in saline aquifers. Hydrocarbon oligomers, recognized for their exceptional properties, are considered as one of the most promising viscosifiers in displacement of brine-saturated porous media. However, the molecular-level mechanisms governing the solubility and viscosification of hydrocarbon oligomers in scCO₂ remain poorly understood. In this study, we employ coarse-grained molecular models to advance our understanding in the effects of molecular structure of hydrocarbon oligomers on their solubility in scCO₂. The coarse-grained models of five hydrocarbon oligomers with different numbers of methyl-branch (n-C32, P1D-2, P1D-3, P1D-6 and squalane) are established to investigate their effects on solubilization in scCO₂. We demonstrate that the number of methyl groups has a monotonic correlation with the solubility of hydrocarbon oligomers when the molecular weights of oligomers are comparable. The radial distribution function reveals n-C32, P1D and squalane are uniformly dispersed with separation distances of approximately 1.0–2.0 nm. The interaction energy between hydrocarbon oligomers and CO₂ shows that the number of methyl-branch in hydrocarbon oligomers can directly influence their solubility in scCO₂. Molecular simulation results demonstrate that the interaction distances between the methyl-branch and CO₂ are smaller than those of other molecular fragments. There are approximately 20% more CO₂ molecules interacting with methyl-branch than with other parts. This work sets the stage for our future molecular dynamics study in viscosification by hydrocarbon oligomers with different branching length and interfacial phenomena in multiphase systems.