An Experimental Study of Liquid Loading of Vertical and Deviated Gas Wells

Abstract

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 164516, ’An Experimental Study on Liquid Loading of Vertical and Deviated Gas Wells,’ by G. Yuan, E. Pereyra, and C. Sarica, SPE, University of Tulsa, and Robert P. Sutton, Marathon Oil, prepared for the 2013 SPE Production and Operations Symposium, Oklahoma City, Oklahoma, USA, 23-26 March. The paper has not been peer reviewed. An experimental study of liquid loading of gas wells has been conducted with air/water flow in a 3-in. pipe at well deviations of 0, 15, and 30° from vertical. A total of 131 tests have been carried out; differential-pressure gradient and liquid holdup were measured. Flow pattern was observed, and critical gas velocity, which defines the threshold for liquid accumulation in a well (commonly termed liquid loading), has been identified. Introduction During the production of gas wells, liquid loading is considered one of the most important production problems. Symptoms of liquid loading include liquid slugging at surface equipment, declining or erratic water- and gas-production rates, and dramatic changes in wellhead pressure. Left unchecked, the water accumulation in the well increases the backpressure against the reservoir and provides a damage mechanism by saturating the near-well region with water. Critical gas velocity is the key parameter in describing liquid loading. There are three definitions of critical gas velocity used in the literature: The minimum gas velocity at which the largest liquid droplets entrained in the gas core can be transported upward, as used in the criterion of Turner et al. (1969). The gas velocity at which the flow pattern changes from annular flow to intermittent flow. The gas velocity at which liquid film starts to move up the pipe wall continuously. The Turner et al. criterion has been used widely in the industry to predict critical gas velocity for the past several decades. This criterion is based on a force balance on a single droplet entrained in the gas core. It was proposed that the gas well loads up when the entrained droplet moves downward, which is called droplet-flow reversal. Turner qualified the use of the model by stating that the velocity result should be increased by approximately 20% to ensure liquids do not accumulate in the well. The Turner et al. droplet model is based on a vertical configuration and does not include the influence of well deviation on critical gas velocity. This may explain the poor behavior of the Turner et al. model on field data from deviated wells. The liquid-loading mechanism is still under debate. The majority of existing models are modifications of the Turner et al. droplet model, each with some degree of success in matching the field data. However, film-flow reversal now seems to be the main mechanism, on the basis of observations in the most recent experimental studies. This criterion is adopted in this study to describe the loading initiation in vertical and deviated wells.