Feb. 6, ASME/API Gas-Lift Workshop1 Using Chokes in Unloading Gas-Lift Valves by Ken Decker, Decker Technology Cleon Dunham, Oilfield Automation Consulting Burney Waring, Shell EP Technology

Feb. 6, ASME/API Gas-Lift Workshop2 Using Chokes in Unloading Gas-Lift Valves Outline Rationale for using chokesRationale for using chokes Reason for a gas-lift valve/choke modelReason for a gas-lift valve/choke model Development of the modelDevelopment of the model Implementation of the modelImplementation of the model Improving performance of unloading gas-lift valves using chokes – a comparisonImproving performance of unloading gas-lift valves using chokes – a comparison Outstanding issuesOutstanding issues Summary of benefitsSummary of benefits ConclusionsConclusions

Feb. 6, ASME/API Gas-Lift Workshop3 Why Use Chokes in Unloading Gas-Lift Valves? Rationale for this practice Shell has used chokes in unloading gas-lift valves for many yearsShell has used chokes in unloading gas-lift valves for many years Primary reasons are: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/seatAnother potential advantage – reduced risk of erosion to the valve ball/seat –Much of pressure drop is across the choke

Feb. 6, ASME/API Gas-Lift Workshop4 Decision to Develop a Model for Choked Gas-Lift Valves Historical perspective Historically, Shell “knew” advantages of using chokesHistorically, Shell “knew” advantages of using chokes But, design was limited to Thornhill-CraverBut, design was limited to Thornhill-Craver Advent of API RP 11V2 This provided opportunity to model gas-lift valve behaviorThis provided opportunity to model gas-lift valve behavior Shell asked Decker Technology to develop a model for choked gas-lift valves based on API experienceShell asked Decker Technology to develop a model for choked gas-lift valves based on API experience

Feb. 6, ASME/API Gas-Lift Workshop5 Objectives of a Model of Choked Gas-Lift Valves Objectives are to have a: Rigorous tool for designing choked unloading gas-lift valvesRigorous tool for designing choked unloading gas-lift valves Rigorous method of analyzing existing wells that use choked valvesRigorous 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 combinationWay to develop accurate models of choked valves without having to test every valve port/choke size combination

Feb. 6, ASME/API Gas-Lift Workshop6 Development of the Model for Choked Gas-Lift Valves Developing the model In simple termsIn simple terms –Develop an API RP 11V2 model of the valve –Develop a Thornhill-Craver model of the choke –Mathematically add them together Tricky partTricky 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 resultsAccuracy is within 15% of test results

Feb. 6, ASME/API Gas-Lift Workshop7 Implementation of the Gas-Lift Valve/Choke Model Shell has installed the model in WinGLUE For use in gas-lift designFor use in gas-lift design –To size gas-lift valve ports & chokes –To predict performance during unloading For use in gas-lift analysisFor 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)

Feb. 6, ASME/API Gas-Lift Workshop8 Choked Unchoked 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 Valve is open when tubing pressure is above about 900 psi. It "snaps" closed when closing pressure is reached. Comparison Between Choked and Unchoked Valves

Feb. 6, ASME/API Gas-Lift Workshop9 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 Note that choked valve remains open over entire range and actually transmits much more gas. It "snaps" closed when closing pressure is reached. Choked Unchoked Comparison Between Choked and Unchoked Valves

Feb. 6, ASME/API Gas-Lift Workshop10 Pressures Acting on an Unchoked Valve Unchoked Valve

Feb. 6, ASME/API Gas-Lift Workshop11 Pressures Acting on a Choked Valve Choked Valve

Feb. 6, ASME/API Gas-Lift Workshop12 Outstanding Issues Outstanding issues to be resolved Model was developed for 1" IPO valvesModel 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 unloadingChokes 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 valvesThe model may help to reduce cost of testing and modeling new gas-lift valves –This needs to be verified

Feb. 6, ASME/API Gas-Lift Workshop13 Summary of Benefits Known and Suspected Benefits Injection rate during unloading is limited to desired amountInjection rate during unloading is limited to desired amount Throttling during unloading is eliminatedThrottling 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 sizeBecause rates can be “tuned” with chokes, each field need only stock one port size Likelihood that risk of erosion is reducedLikelihood that risk of erosion is reduced

Feb. 6, ASME/API Gas-Lift Workshop14 Conclusions Use of Chokes in Unloading Gas-Lift Valves Strongly Supports Objectives of Continuous Gas-Lift To avoid both over and under injection during unloadingTo avoid both over and under injection during unloading To successfully reach bottom and stay thereTo successfully reach bottom and stay there To prevent valve damage due to erosionTo prevent valve damage due to erosion To minimize costs of maintaining gas-lift valve inventoriesTo minimize costs of maintaining gas-lift valve inventories

Feb. 6, ASME/API Gas-Lift Workshop15 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.