Simulator Optimizes Gas Lift Unloading

Abstract

This article is a synopsis of paper SPE 29450, "Gas-Lift Unloading and Operating Simulation as Applied to Mandrel Spacing and Valve Design" by James W. Hall, SPE, Producing Solutions and Kenneth L. Decker, SPE, Decker Technology, originally presented at the 1995 SPE Production Operations Symposium, Oklahoma City, OK April 2-4. Please read the full-length paper for additional detail, illustrations, and references. Gas lift has been practiced for many years with principles developed on experience and "rules of thumb" with little regard for actual valve performance. Available software now simulates the unloading and operation of gas lift wells by use of scientific principles and valid "valve-performance" data. In a conventional configuration, the tubing/ casing annulus is initially filled with a completion fluid from the packer to the surface. To initiate gas lift, the fluid must be removed from the annulus to uncover the installed flow-control device at the desired point of gas injection into the tubing. This process of removing the completion fluid is referred to as "unloading" the annulus. Various procedures for unloading the annulus have been recommended. When recommending changes of injection rate and pressure, these procedures ignore the actual performance of the gas lift valve under downhole conditions. For this paper, valve performance data are defined as the quantitative measure of a valve's flow-rate response to changes in casing and/or tubing pressure for a given pressure and temperature setting. Vendors supply tables and figures to predict gas-flow rates for their equipment. The information is presented in different formats and no algorithm is available to compare valve performance of different manufacturers. Additionally, the data are focused on gas-flow, making liquid-flow predictions difficult. Therefore, the unloading process cannot be optimized with these data. API RP 11 V2 outlines a procedure for obtaining performance data of gas lift valves. The procedure is not required and must be requested by the end user. Tests are being performed and results are available. Computer applications applying the valve-performance data have been developed. Another error in some applications is the assumption that gas lift valves are fully open immediately after opening and before closing. This is true only for pilot and differential- pressure-operated valves. The application of valve-performance data to engineering solutions is still not a standard practice. Tools are available to design annulus-unloading procedures for the unique parameters for each well. Prudent operators will be able to examine mandrel spacing and valve design before installation to ensure proper performance, to reduce valve damage, and to eliminate multipoint injection.