Response mechanism of miscibility between multi-component crude oil and CO2 to burial depth: Insights from molecular dynamics simulations

Abstract

The miscibility between oil and gas is a critical factor controlling displacement efficiency in enhanced oil recovery. With increasing interest in deep and ultra-deep oil and gas reservoirs, understanding how miscibility varies with burial depth has become essential. Pressure and temperature, both varying with depth, are key parameters affecting miscibility. Although their individual effects have been extensively studied, the overall influence of burial depth on oil-gas miscibility remains insufficiently explored. In this work, molecular dynamics simulations were conducted to investigate the miscible behavior between multi-component crude oil and CO₂ at different burial depths in Hami, Xinjiang, China. A quantitative method was developed to assess the miscibility of component and overall crude oil. Simulation results reveal a non-monotonic relationship between depth and miscibility, which varies from about 54.8–72.4 %, peaking near 2500 m. The underlying mechanisms were further analyzed in terms of interaction energy and diffusion coefficient. As burial depth increases, oil-gas net interactions weaken, hindering miscibility, with the interaction energy of saturates and aromatics decreasing by 17.7 % and 12.6 %, respectively. In contrast, molecular diffusion is enhanced, with diffusion coefficients of hydrocarbon components increasing by 1.6–5 times, promoting miscibility. The competition between these effects produces an optimal miscibility depth near 2500 m. Therefore, CO₂ flooding can be directly implemented in the preferred area, while injection pressure can be increased in non-preferred zones to enhance miscibility. This work provides a molecular-level understanding of how burial depth influences CO₂-oil miscibility and offers theoretical insights for optimizing CO₂ flooding strategies.