Molecular insights into mass transfer and adsorption behaviors of hydrocarbon and CO2 in quartz multiple-nanopore system

Abstract

Exploitating liquid hydrocarbons from unconventional reservoirs is playing an ever-increasing role in addressing global energy supply issues. CO2 huff-and-puff is an environmental and effective method to enhance hydrocarbon recovery and achieve CO2 geo-sequestration simultaneously, playing an important role in carbon capture, utilization, and sequestration (CCUS) projects. The optimization of huff-n-puff operations requires clarifying the adsorption state and migration behaviors of hydrocarbons and CO2 in unconventional reservoirs dominated by nanopores. In this paper, we used molecular dynamics (MD) simulation to investigate adsorption behaviors and mass transfer of CO2and n-decane (nC10) molecules in multiple quartz nanopores by carefully considering pore size and structure effects. It shows that the adsorption behaviors and replacement efficiency of oil molecules are closely related to pore aperture. Interestingly, we found that the surface adsorption of nC10 demonstrates a non-monotonic trend of rising after falling as the pore size decreases from 5 nm to 0.7 nm. Specifically, when the pore size is reduced to ∼ 1 nm, nC10 molecules exhibit a pseudo-double layered distribution state. In such a nanopore, the surface adsorption of nC10 is significantly weakened but thesurface adsorption of CO2 is enhanced, leading to the maximum oil recovery efficiency. Although the reduction of pore width adversely affects the extraction speed of oil, it is generally favorable for improving final oil recovery. In a double-nanopore system, the exchange between nC10 and CO2presents a counter-current flow pattern. By contrast, in the triple-nanopore systems, nC10 and CO2form their respective dominant migration paths and the large pores dominates the equilibrium time. These observations provide a more in-depth understanding of CO2-enhanced oil recovery (CO2-EOR) mechanisms from the molecular level, and reveal new insights into CO2and oil adsorption and migration behaviors in complex nanopores, thus aiding in the efficient implementation of CO2-EOR project.