Molecular insight into the competitive adsorption mechanism of supercritical CO₂/Crude oil in shale composite model

Abstract

With the continuous growth of global energy demand, shale oil, as an important unconventional oil and gas resource, is increasingly highlighting its strategic position. As one of the key means of shale oil development, supercritical carbon dioxide (CO₂) flooding technology has significant advantages in reducing crude oil viscosity, improving reservoir pore structure and fluid occurrence state, thereby effectively enhancing oil recovery. The molecular simulation method provides an effective way to reveal the microscopic mechanism of CO₂ flooding. In this paper, based on the molecular dynamics simulation method, a multi-component system model including supercritical CO₂, organic-inorganic composite wall and crude oil is constructed. The microscopic mechanism of competitive adsorption between supercritical CO₂ and components in both single-component and multi-component oil phase is systematically investigated. The effects of wall type, temperature and CO₂ injection pressure on competitive adsorption behavior are clarified. The results show that the competitive adsorption of supercritical CO₂ varies significantly among different shale oil compoents. The competitive adsorption efficiencies of supercritical CO₂ for the single-component systems, n-hexane, toluene, acetic acid, and n-dodecane, are 55.17%, 51.72%, 44.83%, and 27.59%, respectively. In the multi-component oil system, the competitive adsorption efficiencies of supercritical CO₂ are as follows: n-hexane (66.58%), n-dodecane (47.06%), toluene (49.46%), and acetic acid (78.78%). This indicates that the competitive adsorption efficiency is closely related to the molecular polarity, molecular weight and the component's position in the adsorption layer. In addition, the competitive adsorption efficiency of CO₂ for n-hexane first increases and then decreases with increasing temperature, reaching a maximum of 50.57% at 383.15K and the injection pressure of 10.5 MPa. The increase of injection pressure significantly improves the competitive adsorption efficiency, and 10.5 MPa is considered as the minimum miscibility pressure (MMP). This study reveals the competitive adsorption mechanism between supercritical CO₂ and shale oil at the molecular scale, thus providing theoretical support for the technical optimization and scheme design of shale oil of efficienyt development through CO₂ injection.

Keywords

Supercritical CO₂ / Shale oil / Competitive adsorption / Composite wall / Molecular dynamics simulation

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