Gas Lift Operations Require Accurate Predictions of Downhole Annulus Pressure
Authors: Feder Judy 
Published: February 2019

This article, written by JPT Technology Editor Judy Feder, contains highlights of paper SPE 190929, “Gas Lift Annulus Pressure,” by Kenneth Decker, SPE, retired, and Robert P. Sutton, SPE, consultant, prepared for the 2018 SPE Artificial Lift Conference and Exhibition—Americas, The Woodlands, Texas, USA, 28–30 August. The paper has not been peer reviewed.

Downhole annulus pressure is required for any gas lift design. This paper presents several methods of determining annulus pressure at depth and helps determine which method is most appropriate for specific conditions. It also provides advice on the accuracy of a combination of different critical properties and compressibility correlations, offers an alternative design technique to account for changing annulus temperature during unloading, and provides guidelines for modeling changes in annulus pressure during unloading.

Introduction

While many methods of computing gas pressure in a flowing environment are available, what is needed in the case of gas lift is the static annulus pressure at depth.

Historically, the method of using average temperature and pressure to compute compressibility factors and assuming linear well-temperature gradients made gas lift design relatively simple. The small errors were tolerable and within the normal design safety mar-gins of operating systems with surface pressures of less than 1,500 psi.

When surface injection pressures exceed 1,500 psi, calculations become more important. Computers are now used to calculate downhole annulus pressure, and the designer must trust the number that appears on the computer screen. That trust may not be warranted, depending on the method of calculation. The small errors that creep into computer calculations increase linearly as surface injection pressure increases. Additionally, the choice of which correlation to use to model the critical properties of the gas and which compressibility factor correlation to use also affect the accuracy of the calculation.

As a gas lift well transitions from a geothermal to a flowing temperature profile during unloading and lifting, the annulus pressure at depth decreases despite surface injection pressure remaining unchanged. The change in annulus pressure is governed by the volume of gas present in the annulus. Reduction in gas volume decreases pressure.

Traditional design techniques ignore this reality and substitute a simplified analogy of valve performance that incurs errors and misconceptions about how the annulus pressure changes during unloading. A more-detailed discussion of these issues and proposed questions for program developers is presented in the complete paper.

Annulus Pressure Calculation Methods

Methods of determining downhole annulus pressure include monographs, density equations used to full depth with average pressure and temperature, and density equations used in small depth increments with average temperature and pressure within the increment. An examination of one of these monographs is shown in Fig. 1. The choice of which method to use comes with limitations on the accuracy of the predicted pressure at depth.