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 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 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 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 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 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 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 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 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 ASME/API Gas-Lift Workshop

10. Pressures Acting on an Unchoked Valve 2001 ASME/API Gas-Lift Workshop

11. Pressures Acting on a Choked Valve 2001 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 will verify 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 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 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 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 yet to be confirmed.) • Use of a gas-lift valve/choke model is recommended when designing to provide an accurate prediction of flow through the valve. 2001 ASME/API Gas-Lift Workshop