Evaluation of micro-dispersion on oil recovery during low-salinity water-alternating-CO2 processes in sandstone cores: An integrated experimental approach

Abstract

Low-salinity water (LSW) and CO₂ could be combined to perform better in a hydrocarbon reservoir due to their synergistic advantages for enhanced oil recovery (EOR); however, its microscopic recovery mechanisms have not been well understood due to the nature of these two fluids and their physical reactions in the presence of reservoir fluids and porous media. In this work, well-designed and integrated experiments have been performed for the first time to characterize the in-situ formation of micro-dispersions and identify their EOR roles during a LSW-alternating-CO₂ (CO₂-LSWAG) process under various conditions. Firstly, by measuring water concentration and performing the Fourier transform infrared spectroscopy (FT-IR) analysis, the in-situ formation of micro-dispersions induced by polar and acidic materials was identified. Then, displacement experiments combining with nuclear magnetic resonance (NMR) analysis were performed with two crude oil samples, during which wettability, interfacial tension (IFT), CO₂ dissolution, and CO₂ diffusion were quantified. During a CO₂-LSWAG process, the in-situ formed micro-dispersions dictate the oil recovery, while the presence of clay minerals, electrical double-layer (EDL) expansion and multiple ion exchange (MIE) are found to contribute less. Such formed micro-dispersions are induced by CO₂ via diffusion to mobilize the CO₂-diluted oil, alter the rock wettability towards more water-wet, and minimize the density contrast between crude oil and water.