Molecular Simulation of Hydrocarbon Recovery from Calcite Surface with the Implications for CO₂ Storage in Nanopores

Abstract

Carbon neutrality is increasingly becoming a global consensus. To reduce CO₂ emissions and enhance oil-in-place (OIP) recovery, sequestration of CO₂ into reservoirs is an effective solution. However, the microscopic processes involved in the stripping of hydrocarbon molecules by CO₂ and geological storage in carbonate reservoirs are not fully understood. This study utilizes molecular simulations to analyze the recovery of hydrocarbons from calcite surfaces and the behavior of CO₂ storage at the nanoscale. Molecular dynamics simulations were utilized to investigate the characteristics of n-octane adsorption, detachment, and migration from the surface of calcite, as well as the effects of CO₂ under reservoir circumstances. The influence of temperature and CO₂ density on the detachment of n-octane by CO₂ was investigated over a range of 293.15–383.15 K and 0.1–0.9 g/cm³. Grand Canonical Monte Carlo (GCMC) technique is used to examine how the presence of residual oil affects the storage of CO₂ in calcite nanopores. The radial distribution function, diffusion behavior, and interface properties of n-octane and CO₂ are thus analyzed. The density of N-octane decreases as the distance from the surface increases. This is because N-octane molecules create double adsorption layers on the CaCO₃ wall, and the oil layer formed on the calcite surface becomes looser at higher temperatures. The primary process of CO₂ stripping the n-octane molecules is through dissolving, diffusion in the crude oil and adsorption on the CaCO₃ wall. The volume and mobility of n-octane are improved by the dissolution and diffusion of CO₂ in the octane molecules. Since CO₂ interacts with calcite more strongly, CO₂ can substitute octane molecules and adsorb on the surface of calcite, thereby improving oil recovery. The recovery could be efficiently enhanced by raising the temperature and CO₂ density to intensify the interaction between carbon dioxide and n-octane. Residual oil will inhibit CO₂ adsorption trapping in calcite nanopores while it has very little impact on the CO₂ adsorption layer on the calcite wall. This is because the octane, which accumulates in the middle of the pores, compresses the space available for CO₂ molecules, thereby decreasing their storage quantity. This work sheds light on the fundamental mechanisms underlying the microscopic process of CO₂-stripping oil molecules, as well as the sequestration behaviors. The findings can serve as a guide for OIP recovery and CO₂ geological sequestration in carbonate reservoirs.

Copyright 2023, IFEDC Organizing Committee.

This paper was prepared for presentation at the 2023 International Field Exploration and Development Conference in Wuhan, China, 20–22 September 2023.

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