Smith Leggett and Erasmus Mensah - Bob L. Herd Department of Petroleum Engineering, Texas Tech University
Converting a well from continuous gas lift to intermittent gas lift (IGL) changes the flow behavior entering the separator from a relatively steady flow regime to periodic high-rate liquid slugs. The resulting slugs can challenge the available gas capacity, liquid retention volumes and dump valve capacity of the existing separator. Before this conversion to IGL, operators must determine whether the existing separator can accommodate these transient slug loading conditions as the daily-average production rates can overlook these short-duration facility constraints.
This study presents a screening workflow anchored by an IGL model that predicts slug volume per cycle, slug frequency, and the vertical slug velocity in the tubing. The continuity relationship, A_1 V_1=A_2 V_2, is then used to translate the predicted vertical wellbore velocity to the velocity in the flowline and separator inlet. This translated velocity, the flowline area, and slug volume defines the instantaneous inlet rate and slug-entry duration. The resulting outputs are now combined with fluid properties, separator geometry, operating conditions and installed dump valve data, in a screening workbook.
The study applies separator sizing criteria and correlations published by Sivalls; to evaluate separator performance under given slugging conditions. The separator geometry and operating liquid levels are used to determine available phase volumes and effective gas-flow area, while retention time, gas-handling capacity, and dump-valve throughput are assessed to determine separator suitability following IGL conversion.
The results demonstrate that evaluating intermittent gas lift (IGL) conversions using cycle-average production rates alone can grossly underestimate the short-duration peak inlet rates imposed on the separator by individual slug events. The model also provides guidance on potential mitigation measures. Where liquid discharge capacity is limiting, larger dump-valve trims may reduce liquid accumulation and shorten required separator recovery time. However, limitations in liquid retention volume or gas-handling capability cannot be resolved through valve changes alone and may require operating-level adjustments, vessel modifications or separator replacement.