Gas Well Performance at Subcritical Rates

Abstract

Gas wells producing late in their life are normally subject to liquid loading problems. As rates fall below the critical rate necessary for unloading, a static liquid column will often develop in the well. This can result from condensed water out of the gas phase or formation water being produced into a well having insufficient gas velocity to clear the liquid from the wellbore. The presence of this liquid column impairs the well performance by imparting additional back pressure on the reservoir. The water saturation around the wellbore can increase causing a reduction in the near well effective gas permeability which further compounds the problem of keeping the well unloaded. Traditional methods of evaluating well performance do not properly capture this phenomena; therefore, the existence of additional back pressure on the reservoir can go undetected. Wells continue to produce, but at a reduced rate because of the liquid column. Numerous analysis techniques are available to model static liquid columns in wellbores. A review of these methods and an evaluation of these techniques is presented. Comparisons with field data are made to ascertain the accuracy of the methods and to select an appropriate method to model well performance. Incorporation of this method into a well design allows for the optimization of well productivity. Introduction Static liquid columns will form if the well is producing at an insufficient gas velocity to clear the free liquid from the well. The presence of a static liquid column impairs the well performance in the following two ways. First, the column places additional back pressure on the formation as the well is produced and second, standing water across from the sand face is spontaneously imbibed into the formation reducing the near well effective gas permeability. Recognizing the existence of the static liquid column is the first step in improving future well performance. Accurately estimating the flowing bottom hole pressure in the presence of a static liquid column is a critical part of properly characterizing the actual flow capacity of the well. This paper will discuss several methods previously presented in the literature comparing the calculated bottom hole pressure from each to field examples. Two conditions are required to form a static liquid column in a flowing gas well. The first requirement is the presence of free water. Free water in the completion can be a result of water condensation from the gas phase or formation water. The presence of condensed water can be determined in Fig 11 or analytically from a simple method published by GPA2. Fig. 2 shows a depiction of the equilibrium water content of the gas stream over the length of the wellbore. Gas saturated with water enters the wellbore from the reservoir. As pressure and temperature changes along the well, the equilibrium water content of the gas decreases allowing water to condense. In this instance, free water can be expected to condense out of the gas above 9,000 ft. The second requirement to form a static liquid column is that gas velocity in the wellbore is below the critical velocity to continuously lift the free water in the completion to the surface. If the well's production string consists of multiple tubing/casing diameters actual velocity in each string must be considered in the analysis. Furthermore, upsets in production can aid in the accumulation of free water at the bottom of a well even though the production rate is above the critical rate at the surface. Critical Unloading Velocity A method for calculating the minimum flow rate for continuous liquid removal from a gas well was proposed by Turner et al.3 in 1969. The method is based upon a model of liquid droplets entrained in a high velocity gas stream. The minimum velocity required to keep the liquid moving up the well is derived from the terminal fall velocity of the largest droplet which can exist. Droplet size is controlled in part by interfacial tension. Turner devised an equation to calculate the terminal fall velocity of the largest droplet. After testing this equation on over 480 wells, Turner suggested an upward adjustment of 20 percent resulting in the following equation.

Inflow Profiles Obtained With Pulsed Neutron Logs in Subcritical-Velocity Wells

“…It has been observed by Turner et al 3 11 that the flow stream velocity required to continuously move the liquid film along the conduit wall is consistently greater than that required to suspend and lift the entrained liquid droplets upward in the flow stream. As noted by Turner et al 3 , as well as from field observations of wellbore liquid unloading conditions [4][5][6][7][8][9][10][11] , that the criteria for the translation of entrained liquid droplets in the flow stream generally provides a better indication of the ability of a well to continuously transport wellbore liquids to the surface.…”

“…If left unchanged for a period of time, a well that produces at subcritical velocities may load up and cease flowing under its own potential due to an increase in the liquid accumulation in the wellbore. Sutton et al 7 An adaptation of the well liquid loading analyses and conditions discussed in the previous paragraphs for a subcritical velocity analysis that is applicable to Pulsed Neutron production log interpretation in sub-critical velocity environments has been addressed in this study. The ability to reliably and accurately consider sub-critical velocities in a production log evaluation is required in low productivity gas wells that produce significant quantities of liquids.…”

Numerical and Analytical Modelling of the Gas Well Liquid Loading Process

“…Recently, Sutton et al (2003) noted that wells may operate at subcritical rates and gave a review of the methods for analyzing this phenomenon.…”

Investigation of Gas Carryover With a Downward Liquid Flow

“…Liquid loading and critical flow rate issues have been well documented in the literature (Lea et al 2003). The liquid accumulation phenomenon was discussed by Sutton et al (2003), and a method was developed to predict the resulting pressure gradient. An example is shown in Fig.…”