Gas dissolution phenomena in crude oil production

Abstract

Reducing the size of offshore separator vessels can result in large economic and safety advantages, in terms of space saving, mobility and installation costs. Yet the lack of knowledge and understanding regarding the rapid evolution of gas following a sudden reduction in pressure, has so far hampered any significant size reduction in surface separating facilities. Following a review into the needs of the offshore oil and gas industry, and a review of the literature concerning the behaviour of gases in liquids, a fully instrumented thermodynamically enclosed rig facility was designed to study the non steady-state conditions created during, and immediately after, the rapid depressurization of a gas-saturated liquid. The mechanisms of gas evolution and the processes controlling the rate of gas evolution were investigated using techniques predominantly experimental in nature. The test conditions, in what is essentially a pressure vessel, included initial saturation pressures up to 30 bara, liquid temperatures up to 60^oC and salinities up to full saturation. The range of gases, CO₂, N₂, O₂, Ar and CH₄, were investigated in water, brine (NaCl) solutions, two distillate oils, kerosene and gas oil and Statfjord crude, under controlled conditions. Owing to their industrial significance, exploratory tests were carried out using mixed gas compositions of N₂ and CO₂, and oil/water mixes.

Video evidence of events during depressurization and subsequent recovery was recorded and correlated with the pVT data. Gas evolution and hence pressure recovery to equilibrium occurred predominantly by bubbling, although pressure recovery by molecular diffusion was apparent over the latter stages of the approach to equilibrium for all gases. Owing to the dissociation reaction of CO₂ in water, a more complex gas evolution pattern and a slower rate of approach to equilibrium, by ~2 orders of magnitude, was observed with CO₂ in water compared to the other gases in water. In addition, the extent of dissociation of CO₂ in brine, compared with that in tap water, was found to significantly influence the rate of gas evolution. In kerosene, the behaviour of CO₂ was found to be similar to that of the other gases tested, where equilibrium was generally reached within seconds of depressurization. The main factors which were found to significantly increase the rate of gas evolution included the initial liquid temperature, fluid agitation and the addition of solid nuclei, in the form of 5μm uni-sized silica flour particles. The rate of gas evolution was not found to be significantly influenced by the purity of the water.

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