Flow Assurance Wax Study on the Ravn Production System_Approved

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 - 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

Wax deposition risksWax 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.

Wax managementMaintain 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 treatmentBlideng et al. (2011), Running-in a new Platform, 22nd

International Oil Field Chemistry Symposium

Ravn system schematic

Ravn

F3-FB Platform

4” Oil Export Line (119 km)

72 m 5°C

0.34 m/s

- 10°C54 m/s

66 m

API° 38.2Viscosity at 60°F (cP) 24.1WAT (°C) 27.5N-Paraffin Content (wt%) 2.625Pour Point (°C) -51

Fluid Properties

Export T = 55°C

Ambient Conditions

FWHT= 60°C8” Infield Line (18 km)

A6A Platform

Wax deposition (Molecular Diffusion)

• 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

• 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.

Wax propertiesLab 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)

Viscosity tuning

Shear Rate Calculation-CatcherShear rate= 10 S-1

Temp= 15 Cμ= 22 cp

ID= 0.1016 m 4"ρ= 834.9702 kg/m3

QLT= 1950 bpd0.003588 m3/s

u= 0.442594 m/sVelocity

Re= 1706.661 Re<2300 LaminarRe>4000 Turbulent

Laminarτ= 1.0 N/m2s= 0.045805 S-1 Turbulents= 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.

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Dyna

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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

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 effect on WAT has been considered in thermo-hydraulic simulation. However the model has not taken into account the composition change due to wax drop-out (conservative).

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Temperature / Wax Appearance Temperature [oC]

Wintershall Ravn FieldFluid Modelling

in-Field LineWax 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)

Risk of wax deposition (in-field line, early life)

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Wintershall Ravan FieldWax Deposition Simulation

In-field LineTemperature and Wax Depostion Profiles (Case 1+ GL)

Fluid T

WAT

T Ambient

Wintershall Ravn Field

Wax deposition thickness (in-field, early life)

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Wintershall Ravn FieldWax Deposition Simulation

In-field LineWax 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

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Time [days]

Wintershall Ravn FieldWax Deposition Simulation In-field Line - No InhibitorVolume of Wax Deposition

Case 1

Case 2

Case 3

Case 4

Total wax deposition (without inhibitor)

Total growth rate of wax (in-field pipeline) 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 RatesGas Lift

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In-field Line - With InhibitorVolume of Wax Deposition

Case 1

Case 2

Case 3

Case 4

Total wax deposition (with inhibitor)

Total growth rate of wax (in-field pipeline) 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 RatesGas Lift

Summary of the results (in-field line)• 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).

Risk of wax deposition (Export Line)

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Wintershall Ravn Field Wax Deposition Simulation - Export Line

Temperature Profile and Risk of Wax Deposition Ambient TWAT1950bpd Fluid T2500bpd Fluid T3500bpd Fluid T4500bpd Fluid T



Wax deposition thickness (Export line)

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Wax

Thi

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Length [km]


Export LineWax Depostion Profiles (2500 bpd)

5 days

10 days

15 days

20 days

30 days



Pressure drop vs. max wax thickness (Export Line)

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Max Wax Thickness [mm]


Export LinePressure Drop vs. Max. Deposition Thickness

1950 bpd

2500 bpd

3500 bpd

4500 bpd

Self insulation on wax deposition (Export Line)

Results are for the topsides (i.e. the highest deposition thickness and rates)

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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 RateCase 1 - Max Wax Thickness Case 2 - Max Wax Thickness Case 3 - Max Wax Thickness Case 4 - Max Wax Thickness

Summary of the results (Export line)• 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.

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

Special thanks to:

Questions?

Flow Assurance Wax Study on the Ravn Production System_Approved

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