Mechanism of alcohols enhancing CO2-oil miscibility for carbon storage and enhanced oil recovery: Molecular insights

Abstract

Carbon capture utilization and storage (CCUS) is considered one of the most promising approaches for carbon neutrality. Thereinto, it is more realistic and feasible to inject CO₂ into hydrocarbon reservoirs for geological storage and simultaneously enhanced oil recovery (CO₂-EOR). CO₂ miscible flooding is far superior to immiscible flooding in storing more CO₂ and producing more oil, but some reservoirs face the challenge of miscible flooding due to a high minimum miscibility pressure (MMP). Facing this issue, this work analyzes the impact of straight-chain (ethanol, pentanol) and branched-chain alcohols (2-pentanol) on the CO₂-oil MMP by employing molecular dynamics simulation for revealing the mechanism of different types of alcohol additives enhancing the CO₂-oil miscibility from the molecular aspects of interaction energies, and radial distribution function (RDF). Results indicate that the studied alcohols can significantly lower the CO₂-oil MMP due to the ‘amphipathicity’ characteristic of oil-philicity and CO₂-philicity. Ethanol has the best effect to enhance miscibility. The molecular structure of alcohols plays a key role in reducing MMP, and straight-chain alcohols are more effective in aggregating CO₂ around their hydroxyl groups, compared to branched-chain alcohols. The hydroxyl groups of straight-chain alcohols are less hindered by spatial constraints, showing stronger affinities with CO₂, thereby more effectively disrupting the cohesion within the oil phase and reducing MMP. The findings of this work provide some new insights into achieving miscible CO₂ flooding and offer guidance on employing suitable alcohol additives to improve the effect of CO₂-EOR.