2001 ASME/API Gas-Lift Workshop

1. 2001 ASME/API Gas-Lift Workshop Using Chokes in Unloading Gas-Lift Valves by Ken Decker, Decker Technology Cleon Dunham, Oilfield Automation Consulting Burney Waring, Shell EP Technology 2001 Fall ASME/API Gas-Lift Workshop

2. Using Chokes inUnloading Gas-Lift Valves • Outline • Rationale for using chokes • Reason for a gas-lift valve/choke model • Development of the model • Implementation of the model • Improving performance of unloading gas-lift valves using chokes – a comparison • Outstanding issues • Summary of benefits • Conclusions 2001 Fall ASME/API Gas-Lift Workshop

3. Why Use Chokes inUnloading Gas-Lift Valves? • Rationale for this practice • Shell has used chokes in unloading gas-lift valves for many years • Primary reasons are: • Injection can be limited to desired rate • Valve remains fully open, thus preventing throttling • So, during critical unloading phase, injection is controlled – not too high, not too low • Another potential advantage – reduced risk of erosion to the valve ball/seat • Much of pressure drop is across the choke 2001 Fall ASME/API Gas-Lift Workshop

4. Decision to Develop a Model forChoked Gas-Lift Valves • Historical perspective • Historically, Shell “knew” advantages of using chokes • But, design was limited to Thornhill-Craver • Advent of API RP 11V2 • This provided opportunity to model gas-lift valve behavior • Shell asked Decker Technology to develop a model for choked gas-lift valves based on API experience 2001 Fall ASME/API Gas-Lift Workshop

5. Objectives of a Model ofChoked Gas-Lift Valves • Objectives are to have a: • Rigorous tool for designing choked unloading gas-lift valves • Rigorous method of analyzing existing wells that use choked valves • Way to develop accurate models of choked valves without having to test every valve port/choke size combination 2001 Fall ASME/API Gas-Lift Workshop

6. Development of the Model forChoked Gas-Lift Valves • Developing the model • In simple terms • Develop an API RP 11V2 model of the valve • Develop a Thornhill-Craver model of the choke • Mathematically add them together • Tricky part • Pressure upstream and downstream of the port is held higher than normal by the choke • An iterative solution is needed to determine the pressures upstream and downstream of the valve and thus upstream and downstream of the choke • Accuracy is within 15% of test results 2001 Fall ASME/API Gas-Lift Workshop

7. Implementation of theGas-Lift Valve/Choke Model • Shell has installed the model in WinGLUE • For use in gas-lift design • To size gas-lift valve ports & chokes • To predict performance during unloading • For use in gas-lift analysis • To help determine how much gas is being transmitted through open unloading valves • Shell has donated the model to the Valve Performance Clearinghouse (VPC) 2001 Fall ASME/API Gas-Lift Workshop

8. Comparison Between Choked and Unchoked Valves Plot of Injection Rate vs. Pressure Unloading Gas-Lift Valve with Choke vs. Valve with no Choke Using Gas-Lift Valve/Choke Model Macco R-1D 3/16" port 10/64" choke Pc = 1100 psi Pt = 325 psi Choked Unchoked Valve is open when tubing pressure is above about 900 psi. It "snaps" closed when closing pressure is reached. 2001 Fall ASME/API Gas-Lift Workshop

9. Comparison Between Choked and Unchoked Valves Plot of Injection Rate vs. Pressure Unloading Gas-Lift Valve with Choke vs. Valve with no Choke Using Gas-Lift Valve/Choke Model Macco R-1D 3/16" port 10/64" choke Pc = 1200 psi Pt = 325 psi Choked Unchoked Note that choked valve remains open over entire range and actually transmits much more gas. It "snaps" closed when closing pressure is reached. 2001 Fall ASME/API Gas-Lift Workshop

10. Pressures Acting on an Unchoked Valve The gas injection rate through the valve is reduced as the valve throttles closed. Large pressure drop, large Injection Pressure. This pressure is greater than the pressure downstream of the ball. suction force on ball, small gap Injection Pressure > P between ball > Tubing pressure and seat The pressure down-stream of ball is nearly equal to the tubing (production) pressure. P ~ Tubing Pressure Production Pressure 2001 Fall ASME/API Gas-Lift Workshop

11. Pressures Acting on a Choked Valve The gas injection rate through the valve is higher because the valve ball is held off of the seat by the higher pressure beneath the ball. Small pressure drop, small suction force on ball, more gap between ball Injection Pressure and seat The pressure down-stream of the ball is held higher by the choke. Large Choke pressure drop P ~ Tubing P ~ Tubing Pressure Pressure 2001 Fall ASME/API Gas-Lift Workshop

12. Outstanding Issues • Outstanding issues to be resolved • Model was developed for 1" IPO valves • Needs to be checked for 1.5" IPO valves, for PPO valves • Shell is verifying the model for 1.5" IPO valves in 2001 • Chokes may limit risk of erosion during unloading • This needs to be checked • VPC will evaluate this in 2001 • The model may help to reduce cost of testing and modeling new gas-lift valves • This needs to be verified 2001 Fall ASME/API Gas-Lift Workshop

13. Summary of Benefits • Known and Suspected Benefits • Injection rate during unloading is limited to desired amount • Throttling during unloading is eliminated • This helps assure that unloading process will be successful in reaching desired lift depth • It may have other benefits, e.g. more rapid unloading • Because rates can be “tuned” with chokes, each field need only stock one port size • Likelihood that risk of erosion is reduced 2001 Fall ASME/API Gas-Lift Workshop

14. Conclusions • Use of Chokes in Unloading Gas-Lift Valves Strongly Supports Objectives of Continuous Gas-Lift • To avoid both over and under injection during unloading • To successfully reach bottom and stay there • To prevent valve damage due to erosion • To minimize costs of maintaining gas-lift valve inventories 2001 Fall ASME/API Gas-Lift Workshop

15. Conclusions • Use of chokes in 1” IPO gas lift valves is strongly recommended based on valve modeling and experience in Shell. (The case for 1.5" valves is now being confirmed.) • Use of a gas-lift valve/choke model is recommended when designing to provide an accurate prediction of flow through the valve. 2001 Fall ASME/API Gas-Lift Workshop