Impact of CO2 Inherent Solubility on Aqueous Nanobubble Dispersion and Its Oil Displacement

Abstract

High-pressure aqueous nanobubble (NB) dispersions contain gaseous species in two modes: a cluster of bubbles and molecular dissolution in the external aqueous phase. The presence of small bubbles enables supersaturation of the aqueous phase, with both modes collectively contributing to the total gas content. As NB-based enhanced oil recovery (EOR) gains more attention, it is important to determine which mode of gas containment is likely to play important roles in EOR mechanisms. This research addresses this question by controlling the carbon dioxide (CO2) distribution between the two modes in a high-pressure CO2–NB fluid using an organic carbonate additive. A comparative study using two corefloods with CO2–NB, with and without the additive, clarified the impact of CO2 distribution in the CO2–NB fluid on CO2 mass transfer to the resident oil, one of the known oil-recovery mechanisms in simultaneous gas and water injection. Phase behavior experiments showed that ethylene carbonate (EC) enhanced the inherent solubility of CO2 in water without affecting the total CO2 concentration in high-pressure CO2–NB fluid. Detailed characterization of CO2–NB fluid samples using 5-wt % KCl brine revealed that CO2–NB with no EC additive (NB-1) yielded a greater fraction of CO2 in the bubble phase than CO2–NB with the 20-wt % EC additive in the brine (NB-2). NB-1 (i.e., a greater nanobubble-phase fraction) showed accelerated bypassed-oil recovery relative to NB-2; that is, CO2 mass transfer into the bypassed oil was more rapid with a greater CO2 fraction as bubbles under the same total CO2 concentration in the CO2–NB fluids. The more rapid CO2 mass transfer to the resident oil was also indicated by the oil recovery factor at breakthrough, which was higher with NB-1 than with NB-2. This finding is important for designing CO2–NB fluid properties for effective applications and highlights the difference between CO2–NB and carbonated-water injection.