37 th Gas-Lift Workshop Houston, Texas, USA February 3 7, 2014 A Combined Experimental & Numerical Research Program to Develop a Computer Simulator for Intermittent Gas-lift Bordalo, S. (1), Barreto, M. (2), Pestana, T. (1), Ochoa Lara, I. (1) (1) UNICAMP, (2) PETROBRAS Feb. 3-7, 2014 2014 Gas-Lift Workshop 1
Slide 3 Motivation Hundreds of petroleum wells, mainly in the northeast region of Brazil, are equipped with Intermittent Gas-Lift systems (IGL), due to the high number of mature fields with low reservoir static pressure. Desire for a rational tool to evaluate the IGL s performance and to compare artificial lift methods
Slide 4 Objective To develop a computer simulator, employing a model based on the fundamental equations of fluid dynamics. 1991 Liao: laid out most of the general equations. 1997 Santos: applied the equations to 4 IGL methods (conventional IGL, IGL with plunger, IGL with chamber, Pig-lift). 1997 Bordalo et al.: established the foundation for the IGL simulator. 2004 Carvalho: wrote the first full simulator (with variants for the conventional IGL, IGL with plunger, inverted IGL).
Slide 5 Intermittent Gas Lift IGL Cycle
Slide 6 Mathematical Modeling Pestana s work: Based on the simulator developed by Carvalho (2004) Introduction of specific formulations for topics such as: the throttling flow regime of the gas-lift valve the behavior of the pressure upstream of the motor valve the behavior of the bottom-hole pressure when the standing valve closes the gas velocity during the decompression stage the two-phase flow in the production line
Slide 7 Mathematical Modeling Pressure Upstream of the Motor Valve: Mass Conservation applied to the gas injection line Weymouth s Correlation Pressure Upstream of the Motor Valve is estimated Pressure Upstream of the Motor Valve Thornhil-Craver s equation New gas flow is estimated
Slide 8 Mathematical Modeling Gas-Lift Valve Throttling flow Force balance determines the valve stem s position. Stem s position determines the equivalent port size (Hepguler model, 1993). Flow is estimated using the equivalent port size (Thornhill-Craver s equation).
Slide 9 Mathematical Modeling Bottom-Hole Pressure (BHP) Depends on the state of the standing valve (open / close) Valve closes immediately when the injection gas raises the pressure downstream of the standing valve to a value higher than what prevails upstream of the valve. While closed, as suggested by Brown (1984), the BHP remains increasing as if the reservoir fluid is accumulating in a virtual hydrostatic column.
Slide 10 Mathematical Modeling The well is divided into several subsystems (the control volumes) and, for each of these systems, mass and momentum balance equations are applied, as well as specific correlations for fluid properties (gas compressibility), flow through valves and friction factors. Subsystems: Casing Gas Core Liquid Slug Liquid Film Liquid Load Nonlinear system of 23 equations: 7 OTDE + 16 Algebraic
Gas Slug Gas Gas Gas Slug Gas Gas Slug Gas Slug Slide 11 Extension to other Variants Production Line Production Line Motor Valve Motor Valve Motor Valve Production Line Plunger Gas Gas-Lift Valve Gas-Lift Valve Packer Packer Packer Reservoir Reservoir Reservoir Gas-Lift with Plunger Gas-Lift with Chamber Inverted Gas-Lift
Slide 12 Numerical Solution Nonlinear System ODE + Algebraic? Solution
Slide 13 Numerical Solution Nonlinear System ODE + Algebraic Crank-Nicolson Method System of Nonlinear Algebraic Equations For each time-step Newton-Raphson Method Solution based on a Convergence Criteria LU Decomposition System of Linear Equations
Slide 14 Mathematical Modeling Bottom-Hole Pressure (P bh ) and Tubing Pressure (P t ) P t - glv P bh
Slide 15 Computational Code / Graphical User Interface Codes were implemented using FORTRAN 90 Each gas-lift method was implemented independently and has it s own executable code. The Graphical User Interface was developed using Python and the PySide library (freeware). Numerical Output Volume of produced liquid Fallback Volume of Injected Gas Graphical Output Bottom-Hole Pressure Wellhead Pressure Tubing & Casing Pressure
Graphical User Interface Slide 16
Graphical User Interface Slide 17
Graphical User Interface Slide 18
Bottom Hole Pressure (MPa) Slide 19 Graphical User Interface Graphical Output 5.5 5 4.5 4 3.5 3 2.5 2 1.5 Bottom-hole Pressure 0 500 1000 1500 2000 2500 3000 Time (s)
Tubing Pressure (MPa) Slide 20 Graphical User Interface Graphical Output 7 Tubing Pressure 6 5 4 3 2 0 500 1000 1500 2000 2500 3000 Time (s)
Wellhead Pressure (MPa) Slide 21 Graphical User Interface Graphical Output 1.4 1.3 1.2 1.1 1 0.9 0.8 0.7 0.6 Wellhead Pressure 0 500 1000 1500 2000 2500 3000 Time (s)
Casing Pressure (MPa) Slide 22 Graphical User Interface Graphical Output 7 6.8 6.6 6.4 6.2 6 5.8 5.6 Casing Pressure 0 500 1000 1500 2000 2500 3000 Time (s)
Slide 23 Intermittent Gas-Lift & Zadson pneumatic pump The gas injection occurs at regular time intervals synchronized with the feeding of oil from the reservoir to the well. Similar to the IGL, the ZPP lifts of oil through cycles of compression and decompression of gas.
Slide 24 Diagram of physical simulator for IGL and ZPP S-5 M-11M-12 M-13 M-10 M-9 M-8 S-6 T-1 2' S-4 1.5' Columns of Production 2' 1.5' 1' T-3 1.5' VE-1 VE-2 VE-3 F-1 F-2 F-3 S-3 F-4 C-1 T-4 Injection of Compressed Gas 2' 2' M-5 SV-4 SV-3 M-4 V-7 SV-2 M-3 M-2 M-6 S-2 1.5' V-6 V-5 SV-1 M- 7 I-17 T-2 Globe valve Ball valve Solenoid valve Check valve Pressure Transducers and Manometers Pressure regulator Y Filter Centrifugal pump V-4 V-3 V-2 V-1 Compressor 2' Compressed air vessel (Horizontal) V-8 M-1 Compressed air vessel (Vertical) 2' S-1 V-IP P-2 Reservoir P-1
Slide 25 Elements of the physical simulator for IGL and ZPP Motor valve Gas-Lift valve
Slide 26 Cycles Stability for GLI Operation map for the IGL in which stable cycles are found. Tests were performed with different timings ( tc, tinj ) The parameters used were: Pinj = 1.5 bar, Pto = 0.29 bar, and the gas-lift valve was calibrated with Pd = 0.5 bar and R = 0.71
Slide 27 Stable cycles Stability is identified by Synchronizing of gas-lift valve with motor valve Stability of fallback over the cycles
Slide 28 Short cycle time Instability is presented by the mismatch of the gas-lift valve and motor valve
Slide 29 Long cycle time Tubing pressure at the moment of the injection increases with the number of cycles; greater fallback at each cycle; gas cannot adequately lift the liquid load fed into the tubing.
Slide 30 Influence of dome pressure on stability Increasing the dome pressure shifts the stable area to the right Higher cycle times are required to acchieve stable cycles
Slide 31 Dynamic behavior of ZPP for first mode Behavior of key pressures, featuring a ZPP cycle in Mode 1. During the second pressurization, part of the accumulated gas in the intermediate annular is transferred to the internal annular, passing through the V2 valve. The moment when the liquid reaches the surface is identified by increasing wellhead pressure (Pwh).
Slide 32 Dynamic behavior of ZPP for second mode Feeding and first pressurization happens similarly to Mode 1. This mode is used when injection pressure in the first mode doesn t provide an efficient lifting, but it demands higher gas volumes.
Slide 33 Dynamic behavior of ZPP for third mode The principle of lifting in the third mode is similar to a Sucker Rod Pump (SRP), in which the tubing is totally filled and the volume fed into the tubing is produced at the surface Figure shows the behavior of the Pam and Pwf vs time, as well as the actuation of V1 and V3 valves following compression and decompression
Conclusions a working computer simulator was developed for the dynamics of IGL systems, using a rational mechanistic approach. results are qualitatively consistent. a GUI was created to operate the simulator. the IGLsim may be improved using lab or field data. the IGLsim may be expanded with new features. a laboratory apparatus was built for the study of the Intermittent Gas-lift and Zadson Pneumatic Pump. a map was draw as a function of tc and tinj indicating the region where the cycles are stable. increasing dome pressure requires higher cycle times for stable cycles the dynamics of the stages of a ZPP cycle can be observed, for all the three modes of operation, by monitoring key pressures. produced volumes and fallback can be determined.
The authors wish to acknowledge the support of Petrobras and the Gas-lift Lab of the Dept. of Petroleum Engineering at UNICAMP. pestana.tiago@gmail.com orlando.ochoa.lara@gmail.com bordalo@dep.fem.unicamp.br barretofilho@petrobras.com.br
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