Improving CO₂ Miscibility in Crude Oil Utilizing Viscosity Reducer: Experimental and Molecular Simulation Study

CO₂ injection is typically categorized as either miscible or immiscible, depending on whether the minimum miscibility pressure (MMP) is achieved or not. Miscible CO₂ flooding generally yields higher recovery factors compared to immiscible CO₂ flooding. While heavy oil reservoirs, typically found at shallower depths, are well-suited for thermal-enhanced oil recovery (EOR) methods, deep heavy oil reservoirs, which reach depths of up to 10,000 ft, make thermal recovery methods impractical. In such cases, CO₂ flooding becomes essential. In most CO₂ injection scenarios, MMP exceeds fracture pressure, making its reduction crucial. Therefore, in this study, an evaluation was conducted to determine the effectiveness of a crude oil viscosity reducer in improving the miscibility of CO₂ along with reducing the MMP. The application of n-butanol (C₄H₉OH) to reduce MMP in CO₂–oil systems and its subsequent impact on crude oil viscosity was investigated. Various concentrations of n-butanol (BL), ranging from 2.5 to 10 wt.%, were examined. The MMP between the crude oil and CO₂ was determined before and after the addition of BL using the vanishing interfacial tension technique (VIT). Additionally, the viscosity of crude oil before and after the addition of BL was measured. Molecular dynamics (MD) simulations were employed to elucidate the impact of BL on oil properties and highlight the underlying mechanisms at the atomic level. MMP decreased in the presence of butanol from 5 to 24.5% compared to the reference case of the CO₂–oil system. In addition, a decrease in crude oil viscosities was observed, ranging from 10.4% to 49.8%. MD simulations highlighted the significant influence of aromatics on MMP values in the crude oil–CO₂ system, with elevated aromatic content in the crude oil explaining the observed low MMP compared to other crude oils with higher saturate content. The simulations also revealed the substantial interactions of BL and their impact on MMP and oil viscosity, showing a strong affinity between BL, CO₂, and aromatics. These findings establish criteria for reducing both crude oil viscosity and MMP in CO₂-gas systems, aiding in the selection of optimal chemicals to enhance CO₂ flooding. This, in turn, improves the efficiency and effectiveness of EOR and carbon sequestration processes.