Gas Evolution Rates in Supersaturated Water-in-Oil Emulsions at Elevated Pressures

Abstract

In the petroleum industry, emulsions are encountered in nearly every stage of oil production, transportation, and operation. An understanding of the mass transfer rates during gas evolution from supersaturated solutions is critical for enabling better design and operation of gas–liquid separators. The objective of this work was to elucidate the influence of surfactant and water droplet sizes present in water-in-oil emulsions on the rate of gas evolution (volumetric mass transfer coefficient) from supersaturated systems. The volumetric mass transfer coefficient during gas evolution at elevated pressure (3.45 ± 0.00689 MPa) was determined using a batch stirred tank system. The volume of the liquid phase (0.0005 m3), the initial saturation pressure (3.45 ± 0.00689 MPa), the liquid-phase temperature (298.15 ± 0.5 K), and the mixing speed during gas evolution (250 rpm) were kept constant during the experiments. Ultrahigh purity methane was used as the gas phase. Pure model oil (Tech 80) and water-in-oil emulsions (30 wt % water) were used as the liquid phases. Water-in-oil emulsions with two different droplet sizes with average droplet sizes of 6.6 ± 2.6 and 21.7 ± 7 μm were used to investigate the influence of the droplet size on gas evolution rates. We hypothesize that the presence of water droplets in the continuous oil phase increases the diffusional path length, which would result in a decrease in the volumetric mass transfer coefficient. At the investigated emulsion concentrations, our results showed that the presence of the surfactant (Span 80 at 0.1 wt %) did not affect the volumetric mass transfer coefficient of methane leaving the model oil. However, the volumetric mass transfer coefficient decreased with a decrease in the initial droplet size. The decrease in the volumetric mass transfer coefficient results in an increased time required for gas evolution. Based on our data set, one can infer that a decrease in the initial droplet size might increase the time required for the solution gas to exit the liquid in a gas–liquid separator. In addition, our results showed that gas evolution from supersaturated water-in-oil emulsions led to the destabilization of the water-in-oil emulsions. We hypothesize that this increase in the droplet size is due to the coalescence of the emulsion droplets during gas evolution.