Characterization of Oil/Water Flows in Horizontal Pipes

Abstract

Abstract The dynamic characteristics of oil-water flow systems have not been fully understood. The need for improved design methods has led researchers towards its continuous investigation. The objective of this study was to characterize oil-water flow in horizontal pipes. The tests were conducted in a 2-in. horizontal test section using Tulsa City tap water and a mineral oil (density = 0.85 g/cm3 and viscosity = 15 cp) with superficial velocities ranging from 0.025 m/s to 1.75 m/s. Various data were acquired on flow patterns, pressure drop, phase fraction and droplet size as function of flow patterns and were used in characterization of the flow and performance evaluation of an oil-water model. A high speed video camera was used to identify flow patterns, measure droplets and ten conductivity probes were used to obtain phase distributions. New experimental data on pressure drop, holdup, phase distribution and droplet size distribution in oil-water flows are presented. Three probabilistic distributions were tested for fully dispersed flows. Sauter Mean Diameter (SMD) analysis were conducted across the pipe diameter. Droplet size data were used to evaluate existing models such as Hinze 1, Kubie and Garner2, Angeli and Hewitt3, and Kouba4. An empirical correlation to predict the SMD profile of droplets across the pipe cross section was developed for flow pattern of dispersed oil in water and water (D o/w & w). Log-normal distribution was the best probabilistic distribution for representing the data for fully dispersed systems. The empirical correlation gave acceptable results. Model comparisons revealed that none of them could accurately represent the experimental data. The new data can lead to better modeling and design of dispersed systems and the new information on droplet sizes can have significant impact on separator design. Moreover, the interpretation of production logs in horizontal wells heavily relies on the flow behavior. Introduction Two phase liquid-liquid flow can be defined as the simultaneous flow of two-immiscible liquids. It can be encountered in a wide range of industries including petroleum where it commonly occurs in production and transportation of oil and water during the later years of production. When heterogeneous fluids are flowing together, they are characterized by the existence of diverse flow configurations and flow patterns or geometrical arrangement of the phases in the pipe. The flow patterns differ from each other in the spatial distribution and position of the interface, resulting in different flow characteristics, such as velocities and holdup profiles as well as pressure gradients. These internal flow structures depend on variables such as flow rates of liquids, pipe geometry and physical properties of the liquids involved. The flow characteristics of oil-water mixtures are generally different from gas-liquid systems. The differences in characteristics are caused mainly by the large momentum transfer capacity, small buoyancy effects, lower free energy at the interface and smaller dispersed phase droplet size in liquid-liquid flows5. Therefore, the characteristics of gas-liquid flow cannot be applied directly to oil-water flow in most cases. Generally, knowledge of the distinctive features of oil-water systems, together with those of gas-liquid systems, can be used in the future for understanding the more complex case of gas-oil-water mixtures which occur daily in petroleum industry production systems. From the different existing flow patterns in oil-water flows, stratified flow in particular has received the most attention, since the low flow velocities and well-defined interface favor both experimental and theoretical investigations. For fully dispersed systems, information is available mainly from studies in stirred vessels. The available information is even more limited for the intermediate flow patterns between stratified and fully dispersed flows6. The pressure drop for two-phase liquid-liquid pipe flows strongly depends on the flow regime and hence the distribution of the two liquids in the cross sectional area of the pipe. Turbulent mixing in the pipe can be sufficient to disperse the initially separated phases, so that dispersions are formed, resulting in higher pressure drops. The flow behavior of dispersions of oil and water depends on the volume fraction and the droplet distribution of the dispersed phase7. Drop size depends on the competition between breakup and coalescence phenomena. Knowledge of drop size and distribution would improve understanding of dispersed systems and contribute to their better design and modeling.