Prediction of Slug-to-Annular Flow-Pattern Transition
Authors: Bybee Karen 
Published: February 2015

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 100615, "Prediction of Slug-to-Annular Flow-Pattern Transition (SAT) for Reducing the Risk of Gas Lift Instabilities and Effective Gas-Liquid Transport From Low-Pressure Reservoirs," by P. Toma, SPE, P.R. Toma Consulting Ltd., and E. Vargas and E. Kuru, SPE, U. of Alberta, prepared for the 2006 SPE Gas Technology Symposium, Calgary, 15–17 May.
Slug-flow to annular-flow transition occurring during upward gas/liquid flow is a source of flow instabilities often experienced in conventional gas lift as well as in unloading water accumulated at the bottom of gas wells. In both situations, a significant decrease in tubing pressure from perforations to wellhead is associated with a significant increase in superficial gas velocity and may induce flow-pattern transitions. The full-length paper uses field data and laboratory measurements to suggest that flow-pattern transition can result in flow instabilities and should be avoided.

Introduction
Understanding and prediction of slug-flow to annular-flow transition are essential for designing effective gas lift or unloading strategies. Mechanistic modeling of gas/liquid-flow systems including descriptions of major flow patterns and transitions is essential to develop suitable production strategies in both depleted and large offshore gas/oil reservoirs. These include the ability to control the stability of a suitable gas/liquid-flow pattern from perforations to wellhead and achieve the designed production volumes. To minimize the pressure drop, large-liquid-volume gas lift primarily uses a slug flow pattern, while production of gas with relatively small amounts of condensate or water uses an annular flow pattern.
Slug-to-annular flow-pattern transition including the intermediate churn condition is considered to be a potential source of instabilities. Fig. 1 shows flow patterns for upward vertical flow. Difficulties visually assessing the hydrodynamic evolution of this transition because of highly turbulent gas/liquid-flow reversals are a source of controversy, mainly related to the definition of churn as a standalone flow pattern or as a transition stage between slug and annular flow.
Slug Flow Pattern. The slug flow pattern is characterized by a chain of bullet-shaped, rising Taylor bubbles. A relatively large amount of liquid and much smaller gas bubbles are contained in a slug found between two consecutive Taylor bubbles. The population of much smaller bubbles found in the slug subpattern is formed continuously through turbulent breaking of the trailing edge of large Taylor bubbles and disappears through coalescence (back into Taylor bubble). The slug-subpattern volume is essential to transport a large amount of liquid upward during conventional gas lift operations. A back-flowing liquid film (found between Taylor bubbles and the tubing) is an equally essential feature of any slug flow pattern and limits the depth from which liquid can be transported in a slug flow pattern from extremely low-pressure reservoirs.