(37a) Kinetics of Gas Evolution from a Supersaturated Oil at Elevated Pressures.

An uncertainty in the oil and gas industry is the time required for degassing or the rate of gas evolution from supersaturated solution within a gas-liquid separator at elevated pressure. The time required for separation is usually determined by assuming thermodynamic equilibrium between the two phases and estimating the time required for entrained gas bubbles to rise out of the liquid. However, gas-liquid separator design using the above approach does not fully account for the dissolved solution gas present within the liquid. Separator performance is critically important for the safe and reliable operation of downstream equipment. In the production of crude oil, various levels of pressure drops (supersaturation ratio) are experienced by the oil as it passes through a series of gas-liquid separators. Therefore, it is critical to investigate the influence of the supersaturation ratio on the rate of gas evolution.

The impact of the supersaturation ratio on the rate of gas evolution was investigated using a model oil as the liquid phase, and methane was used as the gaseous phase. A high pressure vessel was pressurized to 1250 psia, and the temperature was maintained at 50 ^oC. The oil was saturated with the gas by using the mixer and then depressurized to create a supersaturated oil. The rate of gas evolution from the supersaturated oil was investigated at different supersaturation ratios (0.15 to 3). The presence or absence of gas bubbles in the liquid was observed using a camera through the viewports present in the high pressure vessel. We hypothesize that at low supersaturation ratios, the rate of gas evolution (from the oil ), and the rate of absorption (into the oil) will be similar where no bubbles would be present. Our results also showed that the rate of gas evolution and absorption was within 20% of each other at low supersaturation ratios. While at higher supersaturation ratio, a significant difference was observed between the rate of absorption and gas evolution.

Key words: Gas evolution (desorption), absorption, gas-liquid separators, supersaturation ratio.