Modeling the Gas-Well Liquid-Loading Process

Abstract

This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 95282, "Modeling the Gas-Well Liquid-Loading Process," by N. Dousi, Delft U. of Technology; C.A.M. Veeken, SPE, Shell E&P Europe; and P.K. Currie, SPE, Delft U. of Technology, prepared for the 2005 Offshore Europe, Aberdeen, 6-9 September. Liquid loading is a serious problem in maturing gas fields. Analysis shows that these wells can operate at two different rates, a stable rate at which full production is taking place and a lower metastable rate at which liquid loading affects production. A model was constructed that enhances understanding of the process of water buildup and drainage in gas wells. A metastable flow rate occurs when the water-reinjection and water-coproduction rates are equal and the water-column height stabilizes. Introduction Many of the mature offshore gas fields in the southern North Sea have experienced considerable pressure depletion resulting in liquid loading and significantly reduced gas production. Insight into the flow characteristics of these liquid-loaded wells could help manage tail-end production (i.e., help define the most effective means of accelerating and maximizing ultimate recovery). All wells that produce from depleting gas reservoirs will eventually exhibit so-called liquid loading. The liquid-loading process occurs when the gas velocity within the well drops below a critical velocity. The gas then is unable to lift the water coproduced with the gas (either condensed or formation water) to surface. The water will fall back and accumulate downhole. A hydrostatic column is formed that imposes a backpressure on the reservoir, thus reducing gas production. The process eventually results in intermittent gas production, then production ceases. A widely applied method for predicting liquid loading is based on analysis of droplet transport in vertical turbulent gas flow leading to the prediction of a critical flow rate, which the authors refer to as the Turner rate. Metastable Flow Production data from mature gas fields was analyzed for evidence of stable flow rates less than the Turner rate. The full-length paper details five examples to illustrate the effects of liquid loading. Several techniques were used to analyze these production data. Study of individual-well behavior confirmed that when certain wells were liquid loading, the gas-flow rate could drop to a lower metastable flow rate. Furthermore, the data indicate that the time span in which liquid loading occurs varies. The objective of the modeling work was to determine the processes behind the field observations and to quantify them for various given well conditions. Two methods are presented to analyze liquid loading. First, a simple numerical model was constructed to simulate the transient behavior of liquid loading and its effect on various parameters. The second method encompassed a procedure to deter-mine the metastable flow rate analytically, assuming a steady-state situation. To predict how water buildup takes place in a gas well, a simple conceptual model was created to illustrate the effect of liquid loading. The model incorporates well behavior (buildup and drainage of a liquid column) but does not take into account reservoir behavior (depletion and repressurization). The numerical program takes discrete timesteps and calculates and solves the model equations in an iterative manner.