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Flow Assurance Wax Study on the Ravn Production System_Approved

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Flow Assurance Wax Study on the Ravn Production System_Approved

  1. 1. Flow Assurance Wax Study on the Ravn Field Production System Wood Group Kenny: Hooman Haghighi, Jamie Littler, Fujiang Zhu, Temitope Solanke Wintershall: Leif H. Blidegn, Amir Mofidi
  2. 2. 2 - www.woodgroup.com Agenda • Introduction to the risks of wax deposition • Ravn Field Overview • Fluid Tuning • Results of Infield and Export Lines • Conclusions and recommendations
  3. 3. Wax deposition risks Wax is long-chain simple hydrocarbons. Can be stable in the solid phase at various temperatures. Wax deposition can cause: • Reduction in flow area • Change in wall friction • Blockage of the pipeline These pictures are from public domain.
  4. 4. Wax management Maintain the system temperature above the wax appearance and/or fluid pour point. • Insulation • Displacement with stabilized crude, diesel or condensate • Active heating of the pipeline Physical removal of wax • Periodic scraping of the wax layer via pigging operations. • Heating Chemical treatment Blideng et al. (2011), Running-in a new Platform, 22nd International Oil Field Chemistry Symposium
  5. 5. Ravn system schematic Ravn F3-FB Platform 4” Oil Export Line (119 km) 72m 5°C 0.34 m/s - 10°C 54 m/s 66m API° 38.2 Viscosity at 60°F (cP) 24.1 WAT (°C) 27.5 N-Paraffin Content (wt%) 2.625 Pour Point (°C) -51 Fluid Properties Export T = 55°C Ambient Conditions FWHT= 60°C 8” Infield Line (18 km) A6A Platform
  6. 6. Wax deposition (MolecularDiffusion) • Molecular Diffusion is the dominant wax deposition mechanism • Radial diffusion of dissolved wax molecules in the oil • Concentration gradient between dissolved wax in the turbulent core and the wax in solution at the pipe wall • Dissolved wax diffuses towards the wall where it precipitates Turbulent Core
  7. 7. • There are a few lab techniques available for wax measurements: • Viscosimetry • Cold finger • Differential Scanning Calorimetry (DSC) • Cross Polarization Microscopy • Filter Plugging • Fourier Transform Infrared Spectroscopy (FTIR) Wax Appearance Temperature (WAT): The temperature below which the paraffin's start to precipitate as wax crystals is defined as crude cloud point or WAT. Pour Point: The temperature at which oil sample movement stops is defined as the crude oil pour point. Wax testing These pictures are from public domain.
  8. 8. Wax properties Lab Data WAT [oC] 27.5 WDT [oC] 55 Wax Paraffinic content [wt%] NOTE1 2.625 Cold Finger Test Note 1: C17+ Static Cold Finger Set-up • Hayduk Minhas correlation was used to calculate the diffusion Coefficient (9.78E-08 cm2/s) • Wax Inhibitor from lab test was shown to reduce the deposition rate by 40-80% (40% has been assumed as a conservative approach in this study)
  9. 9. Viscosity tuning Shear Rate Calculation-Catcher Shear rate= 10 S-1 Temp= 15 C μ= 22 cp ID= 0.1016 m 4" ρ= 834.9702 kg/m3 QLT= 1950 bpd 0.003588 m3/s u= 0.442594 m/s Velocity Re= 1706.661 Re<2300 Laminar Re>4000 Turbulent Laminar τ= 1.0 N/m2 s= 0.045805 S-1 Turbulent s= 34.84989 S-1 Laminar note: use the lab data with shear rate =10 S-1 • A shear-thinning behaviour of the fluid has been observed at low temperature. • The shear rate has been identified to represent the actual flowing condition (for each flow rate) and viscosity has been tuned based on the selected shear rate. 0 10 20 30 40 50 60 70 80 90 100 0 5 10 15 20 25 30 35 40 45 50 DynamicViscositymPa.s[cP] Temperature °C Steady State Dynamic Viscosity Profiles For Ravn Oil 1 s-1 10 s-1 100 s-1 non-Newtonian Behaviour at Low temperature
  10. 10. Fluid modelling Effect of pressure & light end component on WAT Note:The dynamic changes in the fluid composition(e.g. Gas Oil Ratio) in the pipeline and the effecton WAT has been consideredin thermo-hydraulicsimulation.Howeverthe model has not taken into account the compositionchange due to wax drop-out(conservative). 0 50 100 150 200 250 0 10 20 30 40 50 60 70 80 90 Pressure[bara] Temperature / Wax Appearance Temperature [oC] Wintershall Ravn Field Fluid Modelling in-Field Line Wax Appearance Temperature (WAT) as a Function of GOR and Pressure Bubble Line (Case 1+GL - GOR = 1900 Scf/bbl) WAT (Case 1+GL - GOR = 1900 Scf/bbl) Bubble Line (Case 2 - GOR = 533 Scf/bbl) WAT (Case 2 - GOR = 533 Scf/bbl) Bubble Line (Case 3+GL - GOR = 986Scf/bbl) WAT (Case 3+GL - GOR = 986 Scf/bbl)
  11. 11. Risk of wax deposition (in-fieldline, early life) -20 -10 0 10 20 30 40 50 60 70 0 2 4 6 8 10 12 14 16 18 20 Temperature[C] Length [km] Wintershall Ravan Field Wax Deposition Simulation In-field Line Temperature and Wax Depostion Profiles (Case 1+ GL) Fluid T WAT T Ambient Wintershall Ravn Field
  12. 12. Wax deposition thickness (in-field,early life) 0 2 4 6 8 10 12 14 0 2 4 6 8 10 12 14 16 18 20 WaxThickness[mm] Length [km] Wintershall Ravn Field Wax Deposition Simulation In-field Line Wax Depostion Profiles (Case 1+ GL) 30 days 20 days 10 days Risk of Wax Depostion at the Topsides Risk of Wax Depostion at the Subsea
  13. 13. 0 1 2 3 4 5 6 7 8 9 10 0 5 10 15 20 25 30 TotalWaxDeposited[m3] Time [days] Wintershall Ravn Field Wax Deposition Simulation In-field Line - No Inhibitor Volume of Wax Deposition Case 1 Case 2 Case 3 Case 4 Total wax deposition (withoutinhibitor) Total growthrate of wax (in-fieldpipeline) is <0.4 m3/d without inhibitor Oil Gas Water [m3 /d] [m3 /d] [m3 /d] [m3 /d] 1 310 29448 0 + GL 2 620 58896 0 0 3 620 58896 0 50000 4 369 39979 52 100000 Case Production Rates Gas Lift
  14. 14. 0 1 2 3 4 5 6 7 8 0 5 10 15 20 25 30 TotalWaxDeposited[m3] Time [days] Wintershall Ravn Field Wax Deposition Simulation In-field Line - With Inhibitor Volume of Wax Deposition Case 1 Case 2 Case 3 Case 4 Total wax deposition (withinhibitor) Total growthrate of wax (in-fieldpipeline)is <0.3 m3/d with inhibitor Oil Gas Water [m3 /d] [m3 /d] [m3 /d] [m3 /d] 1 310 29448 0 + GL 2 620 58896 0 0 3 620 58896 0 50000 4 369 39979 52 100000 Case Production Rates Gas Lift
  15. 15. Summary of the results (in-fieldline) • WAT is lower at higher pressure for the live fluid • The effect of pressure is more pronounced for the fluids with a higher GOR (i.e. Gas Oil Ratio) • As soon as the fluid reaches ambient temperature, no wax deposition would occur (No heat flux to drive the wax deposition – cold slurry flow). • The first location for wax to deposit depends on the flow rates, GOR, phase fractions, etc. • After 30 days of operation <4mm and <13mm of (max) wax thickness can be expected at seabed and topside conditions respectively without inhibitor. • The recommended frequency of pigging operation is every month (based on maximum 4mm of wax deposition in the system) without inhibitor and every 45 days with inhibitor injection (40% efficiency).
  16. 16. Risk of wax deposition (ExportLine) -20 -10 0 10 20 30 40 50 60 0 20 40 60 80 100 120 140 Temperature[C] Distance (km) Wintershall Ravn Field Wax Deposition Simulation - Export Line Temperature Profile and Risk of Wax Deposition Ambient T WAT 1950bpd Fluid T 2500bpd Fluid T 3500bpd Fluid T 4500bpd Fluid T Risk of Wax Depostion at the Subsea Risk of Wax Depostion at the Topsides
  17. 17. Wax deposition thickness (Exportline) 0 2 4 6 8 10 12 14 0 20 40 60 80 100 120 140 WaxThickness[mm] Length [km] Wintershall Ravn Field Wax Deposition Simulation Export Line Wax Depostion Profiles (2500 bpd) 5 days 10 days 15 days 20 days 30 days Risk of Wax Depostion at the Topsides Risk of Wax Depostion at the Subsea
  18. 18. Pressure drop vs. max wax thickness (ExportLine) 0 5 10 15 20 25 30 35 0 2 4 6 8 10 12 14 PressureDrop[bara] Max Wax Thickness [mm] Wintershall Ravn Field Wax Deposition Simulation Export Line Pressure Drop vs. Max. Deposition Thickness 1950 bpd 2500 bpd 3500 bpd 4500 bpd
  19. 19. Self insulation on wax deposition (ExportLine) Results are forthe topsides (i.e.the highest deposition thickness and rates) 0 2 4 6 8 10 12 14 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 0 5 10 15 20 25 30 MaxWaxThickness[mm] MaxDepositionRate[mm/day] Time [days] Wintershall Ravn Field Wax Deposition Simulation In-Field Line Effect of Wax Thickness on Depostion Rates Case 1 - Deposition Rate Case 2 - Deposition Rate Case 3 - Deposition Rate Case 4 - Deposition Rate Case 1 - Max Wax Thickness Case 2 - Max Wax Thickness Case 3 - Max Wax Thickness Case 4 - Max Wax Thickness
  20. 20. Summary of the results (Exportline) • Higher flow rate leads to longer section of the export line subject to wax deposition risk. • Lower rate of deposition by time due to the wax self-isolation effect. • The maximum wax thicknesses identified for the 4 cases are comparable, however the total wax deposited is more at higher flow rates. • After 22 days and 31 days of operation <4mm of (max) wax thickness can be expected at seabed condition without and with inhibitor (40% efficiency), respectively. • Pigging of 4” >100 km export line is challenging and is currently under further evaluation. • Alternative wax mitigation strategy like wax dispersant, gas condensates has been considered.
  21. 21. Other Flow Assurance challenges • Slugging in the in-field line at the early life and during the start-up and turn-down operations has been observed. The following mitigation methods has been considered: o Increased back pressure (for start-up and turn-down operations) o Gas lift injection (if required) • Liquid handling capacity at the topsides (~27 m3) o High level pigging, start-up and ramp-up philosophies have been developed
  22. 22. Special thanks to: Questions?

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