Characterization of Oil-Water Flow Patterns in Vertical and Deviated Wells
Authors: Flores Jose G. , Chen X. Tom , Brill James P. 
Published: December 2007

Abstract
An oil-water flow pattern classification and characterization for wellbores is proposed based on the integrated analysis of experimental data, including frictional pressure drop, holdup and spatial phase distribution, acquired in a transparent test section (2 in. ID, 51 ft long) using a refined mineral oil and water (=0.85, =20.0 and =33.5 dyne/cm at 90 F). The tests covered inclination angles of 90, 75, 60 and 45 from horizontal.
The oil-water flow patterns have been classified into two major categories given by the status of the continuous phase, including water dominated flow patterns and oil dominated flow patterns. It was found that most water dominated flow patterns show significant slippage but relatively low frictional pressure gradients. In contrast, all the oil dominated flow patterns exhibit negligible slippage but significantly larger frictional pressure gradients. Six flow patterns have been characterized in upward vertical flow; three were water dominated and three were oil dominated. In upward inclined flow there were four water dominated flow patterns, two oil dominated flow patterns and a transitional flow pattern. Flow pattern maps for each of the tested inclination angles are presented. A mechanistic model to predict flow pattern transitions in vertical wells is proposed. The transitions to the very fine dispersed flow patterns were evaluated by combining the concepts of turbulent kinetic energy with the surface free energy of the droplets, while the transitions to the churn flow pattern and the phase inversion were predicted based on the concept of agglomeration. The model compares favorably with the measured data.

Introduction
Two-phase flow of oil and water is commonly encountered in wellbores; however, its hydrodynamic behavior under a wide range of flow conditions and inclination angles constitutes a relevant unresolved issue for the oil industry. Multiphase flows are characterized by the existence of diverse flow configurations or flow patterns, which can usually be identified by a typical geometrical arrangement of the phases in the pipe. Inherent to each flow pattern are characteristic spatial distributions of the interface, flow mechanisms and distinctive values for design parameters such as pressure gradient, holdup and heat transfer coefficient. There is clear evidence that accurate knowledge of oil-water flow patterns, their ranges of existence as a function of flow rates and pipe inclination angles, and values for their associated hydrodynamic parameters are crucial in a number of production engineering applications. These include production optimization, optimum string selection, production logging interpretation, -downhole metering and artificial lift design and modeling. Additionally, the understanding of oil-water flow in wellbores is fundamental in determining the volumes of free water in contact with the pipe that could cause scaling and corrosion of the pipe after prolonged exposure. This paper addresses the fundamental problem of identifying and characterizing oil-water flow patterns and predicting flow pattern transitions for conditions pertinent to oil-water producing wells.

Literature Review
Despite being a subject of permanent interest for the petroleum industry, the issue of oil-water flow patterns in wellbores has barely been addressed in the technical literature. A limited number of experimental studies have been found that provide a description of the flow patterns for low-medium viscosity oil and water. Govier et al. and Zavareh et al. conducted investigations in vertical flow, while Scott and Zavareh et al. covered inclinations near the horizontal and near the vertical, respectively. A larger number of related studies concentrated on measuring the holdup, mainly for oil dispersed systems, focusing on finding expressions for the slip velocity that could be used in the interpretation of production logs. P. 601^