Effects of High Oil Viscosity on Oil/Gas Flow Behavior in Horizontal Pipes

Abstract

Abstract Oil-gas pipe flows are expected to exhibit significantly different behavior at high oil viscosities. Effects of high viscosity oil on flow pattern, pressure gradient and liquid holdup are experimentally observed and differences in flow behavior of high and low viscosity oils are identified. The experiments are performed on a flow loop with a test section of 50.8-mm ID and 18.9-m long horizontal pipe. Superficial liquid and gas velocities vary from 0.01 to 1.75 m/s and from 0.1 to 20 m/s, respectively. Oil viscosities from 0.181 to 0.587 Pa•s are investigated. The experimental results are used to evaluate the performances of existing models for flow pattern and hydrodynamics predictions. Comparisons of the data with the existing models show significant discrepancies at high oil viscosities. Possible reasons for these discrepancies are carefully examined. Some modifications are identified and implemented to the closure relationships employed in the Zhang et al.1model. After these modifications, the model predictions provide better agreement with experimental results for flow pattern transition, pressure gradient and liquid holdup. Introduction Gas-liquid two-phase flow in pipes is a common occurrence including the petroleum, chemical, nuclear and geothermal industries. In the petroleum industry, it is encountered in the production and transportation of oil and gas. Accurate prediction of the flow pattern, pressure drop and liquid holdup is imperative for the design of production and transport systems. High viscosity oils are discovered and produced all around the world. High viscosity or "heavy oil" has become one of the most important future hydrocarbon resources with ever increasing world energy demand and depletion of conventional oils. Almost all flow models have viscosity as an intrinsic variable. Two-phase flows are expected to exhibit significantly different behavior for higher viscosity oils. Many flow behaviors will be affected by the liquid viscosity, including droplet formation, surface waves, bubble entrainment, slug mixing zones, and even three-phase stratified flow. Furthermore, the impact of low Reynolds number oil flows in combination with high Reynolds number gas and water flows may yield new flow patterns and concomitant pressure drop behaviors. The literature is awash with two-phase studies addressing mainly the flow behavior for low viscosity liquids and gases. However, very few studies in the literature have addressed high viscosity multiphase flow behavior. In this literature review, the state-of-the-art of two-phase flow is first summarized. Then, the studies addressing the effects of liquid viscosity on two-phase oil-gas flow behavior are reviewed. Taitel and Dukler2 proposed first mechanistic model to predict flow pattern transitions for horizontal and near-horizontal gas-liquid flow. Later, Barnea3 proposed a unified model for predicting steady state transition boundaries for the whole range of pipe inclinations. Xiao et al.4 developed a comprehensive mechanistic model for two-phase flow in horizontal and near-horizontal pipelines. The comprehensive mechanistic model incorporated a flow pattern prediction model and separate models to calculate the flow variables, such as pressure drop and liquid holdup, for individual flow patterns. Zhang et al.1 conducted a detailed review of existing two-phase flow prediction models, and developed a unified hydrodynamic model to predict flow pattern transitions, pressure gradient and liquid holdup and slug characteristics in gas-liquid pipe flows for all inclination angles from -90° to 90° from horizontal. The model was based on the dynamics of slug flow and was applicable to all pipe geometries and fluid physical properties. Weisman et al.5 experimentally studied the effects of fluid properties and pipe diameter on two-phase flow patterns in horizontal pipes. Air-glycerol water solutions having viscosities of 0.075 Pa•s and 0.15 Pa•s were used as test fluids. They concluded that liquid viscosity had little effect on flow pattern transition boundaries. Taitel and Dukler6 conducted an investigation of the effect of pipe length on flow pattern transition boundaries for high viscosity liquids. In their study, the liquid viscosity ranged from 0.09 Pa•s to 0.165 Pa•s. Contrary to Weisman et al.'s claim, Taitel and Dukler6 concluded that pipe length can have a considerable effect on transition boundaries for high viscosity liquids.