Improved Model for the Liquid-Loading Process in Gas Wells

Abstract

This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 106699, "An Improved Model for the Liquid-Loading Process in Gas Wells," by Frank van Gool, SPE, and Peter K. Currie, SPE, Delft University of Technology, prepared for the 2007 SPE Production and Operations Symposium, Oklahoma City, Oklahoma, 31 March–3 April. An improved model for describing the liquid-loading process in gas wells has been introduced. The new model improves how the Dousi model handles the downhole inflow and outflow between the well and the reservoir and makes more-realistic and -detailed assumptions with respect to the production and injection interval. Also, the new model can be used to predict future behavior of the well in a more realistic fashion. The difference between the new model and the Dousi model is that flow-rate changes occur slower when liquid loading begins, reflecting realistic inflow-performance assumptions. Introduction Liquid loading is a serious problem in maturing gas fields. The liquid-loading process occurs when the gas velocity within the well drops below a certain critical gas velocity. The gas then is unable to lift the water that is coproduced with the gas (either condensed or formation water) to surface. The water will fall back and accumulate downhole. A column is formed that imposes a backpressure on the reservoir and reduces gas production. The process eventually results in intermit-tent gas production, and the well dies. Methods of reducing liquid loading include production-string resizing in which a smaller tubing size is chosen to increase the gas velocity above the critical Turner rate, compressor installation to lower the tubinghead pressure thereby increasing the gas velocity above the critical Turner rate, plunger lift to lift all the liquids by use of the gas pressure during shutdown of the well, pump installation to pump the liquids during production, foaming the liquids so that it is easier for the gas to lift all the fluids thus reducing the critical Turner rate, and gas lifting with gas from other wells to decrease the pressure loss in the tubing and increase the velocity. The best solution for a given well depends on the properties of that particular well. Metastable Flow During the liquid loading of a mature gas well, the gas velocity in the tubing drops when the reservoir pressure drops, and, in time, the gas velocity may fall below the critical Turner rate. At that time, the produced liquids fall back down the wellbore and accumulate at the bottom of the well. This liquid column forms a backpressure on the reservoir, thus slowing the production rate and accelerating the problem. However, in parts of the well, the pressure can be lower than in the reservoir such that production still occurs even though at the bottom of the well the pressure from the liquid column is high enough that liquid injection can take place. If the liquid-injection rate equals the rate of liquid coproduction with the gas, metastable production occurs. This metastable flow rate is observed in the field. Data analyzed from gas wells in The Netherlands, including production profiles, temperature, and pressure, showed several examples indicating the existence of a metastable flow rate.