Importance and Inclusion of Gas Diffusion in CO2 Emulsion Population-balance Model

Abstract

Summary Unlike conventional foams made of nitrogen or methane, CO2 emulsion exhibits more complex behaviors in porous media. For instance, CO2 emulsions are relatively weak and do not exhibit sudden loss of apparent viscosity at very high foam quality. Compared to conventional foams in porous media that capillary suction is the main mechanism of bubble coalescence, gas diffusion is significantly enhanced in CO2 emulsion, but the relative contribution of the two emulsion destruction mechanisms and how to account for gas diffusion are rarely seen in the literature. To better understand the key underlying mechanisms, a comprehensive investigation of the CO2 emulsion stability is carried out and bubble coalescence due to gas diffusion is introduced in a population-balance model. First, analytical models are set-up to evaluate the characteristic times of capillary suction and gas diffusion under the same conditions. The analytical solutions suggest that the characteristic time of gas diffusion is comparable to that of capillary suction for CO2 emulsion, while it is one to two orders longer in case of N2 foam. Based on these analyses, a foam/emulsion model is developed through incorporating an additional gas diffusion term as a function of gas solubility, diffusivity, capillary pressure, temperature and several other variables. The new foam/emulsion model is used to fit a set of experiments of CO2 and N2 foams ranging among different foam qualities in the same core. The fittings were carried out using three different selections of the coalescence terms, i.e., the capillary suction term only, the gas diffusion term only, and both terms, for N2 foam and CO2 foam. The results reveal that using the original coalescence model (only capillary suction) can fit the N2 foam data but leads to mismatch with the CO2 data, while using the gas diffusion term only leads to mismatch with the N2 foam data but better match with the CO2 foam data. Using both terms was found optimum for the CO2 emulsion model. In addition, having the gas diffusion term enables to capture the gradual change of the foam strength with foam quality for CO2 foam instead of the abrupt change of foam strength for N2 foam near the limiting capillary pressure. Our research on this subject has unveiled the fact that gas diffusion is important for CO2 emulsion instability. This methodology is a key to evaluate the feasibility of improving CO2 EOR through foaming and to optimize such a process.