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-diluated oil, alter the rock wettability towards more water-wet, and minimize the density contrast between crude oil and water.
Published Version
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