Two-Phase Slug Flow Experiments with Viscous Liquids

Abstract

The challenges behind the multiphase transport of oil and gas mixtures are increasing as the oil and gas industry is moving towards production from non-conventional reservoirs and in remote locations. Transport of high viscosity fluids in long multiphase pipelines is a particular challenge. Previous experiments have shown that gas-liquid slug flow is a frequent two-phase flow pattern at high liquid viscosities. The slug flow regime is an unstable flow, which may lead to operational problems, as the slug lengths and velocities can become very large. Most of the available experimental data and flow models are based on low viscosity fluids, and therefore, the computational models have some uncertainty regarding the viscous effects. With this in mind, the main objective of this thesis is to extend the knowledge of the twophase flow transport of slug flow with viscous liquids by experimental work, data analysis of available information and evaluation of certain flow concepts within slug flow models, such as bubble propagation, wake effect, severe slugging stability, and flow regime transition. All the experiments were carried out at the Multiphase Flow Laboratory of the Norwegian University of Science and Technology and the experimental data were used to evaluate the performance of existing models. Dedicated set-ups and measurement methods were designed for each of the studied phenomena. Instrumentation has included an array of video cameras with image analysis, as well as external and internal impedance ring probes. Flow stability was evaluated for gas-liquid severe slugging in a lazy wave shaped riser with different liquid viscosities. The experimental results show flow stability at lower gas velocities as the liquid viscosity is increased. The history effect of the flow in an upstream pipe connected to a downwards inclined pipe was studied. The results confirm the existence of a metastable region in the flow regime map where both stratified and slug flow can be stable flow regimes depending on the flow pattern at the inlet. This effect is stronger for liquids that are more viscous. It is demonstrated that slug tracking models in principle can capture this phenomenon. The bubble propagation velocities for horizontal flows at Reynolds numbers for laminar flow and for transition to turbulent flow have been measured. An empirical correlation for estimation of the bubble front velocity in the full range from laminar to turbulent flow is suggested, based on the experimental results. The wake effect between two consecutive bubbles in laminar horizontal flow has been measured. In comparison with the turbulent case, an earlier interaction of the bubbles was observed in terms of the slug length between them. A new multi-beam gamma densitometer was also designed and fabricated to measure three phase fractions along the cross-sectional area of acrylic pipes. The new gamma densitometer is subject to additional testing. Further works might address to extend the metastable region study to include stronger pipe inclinations and much longer pipes, in order to verify the sustainability of the slugs generated on this region. Moreover, additional sensitivity studies can be made with dynamic slug tracking models, regarding the effects of the bubble velocity relations and the wake effect on the simulations of the slug flow evolution in a pipes.