Wax control
Arild Stokkenes
Outline
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Flow Assurance in Statoil
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What is wax and what problems may it cause?
•
How to control wax deposition
•
How to monitor wax deposition
2/10/2014
Flow Assurance
Wellbore hydraulics Transient pipeline thermohydraulics Chemical Injection Package Fluid properties Rheology Multiphase equipment: • Multiphase meter Process Separator Slug catcher
Scale control Asphaltene control
Wax control Hydrate control
Emulsion control Corrosion control
• Multiphase pump
Main deliverables/competence:
Thermohydralic multiphase analysis
System design
Hydrate- and wax control philosophies
Slug control
Operational support
Multiphase metering“Flow assurance” = safe, uninterrupted and simultaneous
Fluid control – the problems
Asphaltenes
Kristin-NJ/DR Wye
- wax deposition and temperature profile after 600 h
0 0.001 0.002 0.003 0.004 0.005 0 20 40 60 80 100 Pipeline length [km] W a x de po s it ion [ m ] 0 10 20 30 40 50 60 70 T e m p e ra tu re [° C ] Wax deposition Fluid temperature Wax Gas hydrates
The future ….
2/10/2014 Longer distance More difficult fluids Deeper water Increased field complexity Arctic / harsh environmentOutline
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Flow Assurance in Statoil
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What is wax and what problems may it cause?
•
How to control wax deposition
•
How to monitor wax deposition
What is wax?
2/10/2014
Soft wax Hard wax
Wax consistency range
What is wax?
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Natural constituents of crude oils and most gas condensates•
Typical wax content 1-15 wt%•
Mostly long chain n-alkanes•
Solubility strongly dependent on temperature•
Operational consequences:–
Gelling–
Deposition n-alkane wax crystal2/10/2014
Wax-forming components in crude oils
Non-wax Mainly n-alkanes Wax C7 C8 C9 C10+
Lab. analysis
Pseudo-components
• subtype of the saturates (non-polar compunds without double bonds) • Mainly alkanes of > C18
• Can be linear, branched or cyclic
Simple questions – difficult to answer !
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Will wax accumulate on the pipe wall when the oil flows?•
If so, where and how fast?•
How often do we have to pig the line?•
Is chemical assistance needed (waxinhibitor)?
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When we shut down a pipeline, do we have enough power (pressure) to make it flow again?•
How long will it take to reach normal flow rate?•
Is chemical assistance needed (pour point depressant)?Steady-state Wax deposition
Shut-down/restart Gelling
Key parameters:
Wax appearance temperature (WAT)
Wax content
Pour Point
11
Wax precipitation and wax depositon
3 inch 2 inch
Flow loop Cold finger device
Wax precipitation is defined as the formation of solid particles out of the liquid, directly related to thermodynamic properties.
Wax deposition is describing the formation and growth of the precipitated solid on a surface, related to flow and transport process.
Wax precipitation curve
Norne crude at 1 bar
0 1 2 3 4 5 6 7 8 -20 -10 0 10 20 30 40 50 Temperature (°C) W t% s o li d w a x
13 - 2/10/2014
Wax diffusion towards cold surface
dr
dT
dT
dC
D
dr
dC
D
n
wax M
wax Mn mass flux of dissolved wax molecules towards the pipe wall
wax density of solid wax
DM molecular diffusion coefficient of dissolved wax molecules
dC / dr concentration gradient of dissolved wax in the laminar sub-layer dC / dT solubility of wax components as a function of the temperature dT / dr radial temperature gradient close to the wall
1. The cold wall removes wax molecules from the oil
Wax deposition by molecular diffusion
Wax concentration gradient Dissolved wax WAT Temperature gradient Velocity profile Pip e w al l Heat loss Turbulent core Laminar boundary layer
dT/dr
dC/dr
dC/dr = dC/dT * dT/dr
15
Wax depositon_Process
2/10/2014
Wax deposition process shown by Rønningsen
Rønningsen HP, 6th Int. Conference on Phase Behaviour and
Fouling, Keynote speech, 2005
1. Transport to pipe wall 2. Inital wax layer formation 3. Growth 4. Aging Thickness Roughness Hardness sites Fluid-solid interaction Crystal growth Trapping of oil Time Shear/hydrodynamics Diffusion/Counter diffusion Diffusion Dispersion or thin gel
What happens in the pipeline?
0 1 2 3 4 5 6 7 0 10 20 30 40 50 60 70 80 90 100 110 120 Length (km) w a x th ickn ess (mm) 0 5 10 15 20 25 30 35 40 T emp er atu re (C )Wax after 1 day
Wax after 2 days
Wax after 7 days
Wax deposition modelling in Statoil
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Power and control distribution unit Cold flow cool
down section
Subsea separation and multiphase pumps
Water injection pumps
Commercial tools like OLGA, PVTsim
Wax precipitation curve tuning developed internally
Mutivariate analysis
Temperature [oC] Wax content [wt%] 0 10 20 30 40 50 0 0.5 1 1.5 2 2.5 3 Data Before tuning After tuningWax precipitation curve tuning
Predicted value Measured value W a x th ic kne ss (m m )
Multivariate analysis validation
The wax build-up can be reproduced
100 110 120 130 140 150 160 170 180 190 200 210 220 0 50 100 150 200 250 300 350 400 Time (days) He imdal Ex po rt Pre ss ure (bar a) Field Pressure Rough. 0.5 - Diff. 6 0 5 10 15 20 25 30 0 10 20 30 40 50 60 70 80 90 100 110 120 Length (km) wax th ick nes s (mm )Rough. 0.5 - Diff. 7 - Shear C3 0.7 Rough. 1.0 - Diff. 2 - Shear tuning Rough. 0.5 - Diff. 6
280 m3 wax 210 m3 wax 200 m3 wax
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The wax build-up profile in a pipeline can be reproduced using the OLGA (RRR) model. …….. but is hard to predict !•
The wax deposition profile can be reproduced by various combinations of model parameters:–
Diffusion coefficient–
Wax porosity–
Shear stripping•
The pressure build-up can also be reproduced by proper tuning of the roughness effect of the wax deposit, i.e.–
Wax roughness factor•
Different ongoing JIP and internalt research ongoing for improving the modelsOutline
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Flow Assurance in Statoil
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What is wax and what problems may it cause?
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How to control wax deposition
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How to monitor wax deposition
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Case example: How to not control wax deposition
Wax deposition
challenges
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Stuck pigs•
HSE•
Inspection tools•
Plugged pipelinesThe most famous wax illustration !
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Pipeline between Snorre B and Statfjord B platforms (N. Sea)•
3 m3 of accumulated wax ahead of pig•
Nearly stuck non-bypass pig in riser•
Now the line is pigged regularly with optimized bypass pigRef. SPE 77573 (2002)
Methods for controlling wax deposition
Pipeline insulation
External insulation coating on single pipes
Pipe-in-pipe systems
Pigging
Chemicals
Inhibitors
Dispersants
Dissolvers
Heat
Bundles
Electric heating
Hot oil flushingFBE PP-Adhesive PP-Solid PP-Syntactic PP-Solid PP-Foam PP-Solid FBE PP-Adhesive PP-Solid PP-Syntactic PP-Solid PP-Foam PP-Solid
PPD treated oil; this work PPD treated oil; this work
Wax control strategies
1.
Single phase oil/condensate pipelines:–
Wax control normally by regular pigging2.
Medium length multiphase oil and gas condensate pipelines:–
Normally insulated (or heated)–
Prevents wax deposition and hydrate formation3.
Long-distance multiphase pipelines:a)
Low-wax gas condensates (Snøhvit):•
Wax deposition will normally not be an issueb)
Oils and waxy gas condensates:•
No general, proven way to control wax deposition…•
Wax-repellent surface coatings?Outline
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Flow Assurance in Statoil
•
What is wax and what problems may it cause?
•
How to control wax deposition
•
How to monitor wax deposition
Methods for monitoring of wax deposition
Method Features
Pressure drop Kind of proven
Gives no deposit profile
Pressure pulse Proven for single phase lines
Gives axial deposit distribution
Distributed temperature sensing with fiberoptics Proven for temperature measurements
Potential for deposit detection (utilize insulation effect) Local measurement
Heat pulse monitoring Not fully qualified (WO 2009/051495)
Deposit detection by response to heat pulse (utilize insuation effect)
Local measurement
2/10/2014 25
Outline
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Flow Assurance in Statoil
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What is wax and what problems may it cause?
•
How to control wax deposition
•
How to monitor wax deposition
2/10/2014 Vale Skirne Huldra Vale Skirne Brae Statpipe Vale Skirne Vale Heimdal Vale Huldra Vale Brae Statpipe ValeVale Heimdal
Introduction of Vale fluids in 2002
- Build up of line differential pressure was insignificant until 2004
Before 2002, no wax and no pigging performed. Then Vale field started up with high wax content.
0 10 20 30 40 50 60 70 80 19.4.01 5.11.01 24.5.02 10.12.02 28.6.03 14.1.04 1.8.04 No rm al iz e d p re ssu re d rop , b ar
Start-up waxy cond.
Heimdal Vale Huldra Mixture
WAT (°C) 3,2 24,6 -22,3 13,1
WAX in STO (wt%) 4,2 7,3 0,5 4,9
2004 - 2008
- Foam pigging program
- Stuck pigs
2008
- Fill and soak operation - Chemical dissolvant
- Very good effect in laboratory - Only minor effect in field
2008 - 2010
- Foam pigging - Stuck pigs 2/10/2 014 29Heimdal – Brae wax characteristics
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Heimdal – Brae wax consists mainly of high molecular weight paraffins that are hard to dissolve.•
Supported by indications of high melting temperature (60 °C +).•
Wax removal must be based on a combination of dissolution and ”break-down” of the wax deposit.2010: Aggressive pigging!
Two Alternatives for consideration:
1. Hydraulically Activated Power Pig (HAPP) • Limited experience
• Assumed best for downstream facilities Why change strategy?
1. The pipeline NEEDS to become wax free due to inspection requirements 2. Progressive approach with foam pigs does not work
2. High Friction Jetting Pig (HFJP) • Well proven technology • New application
Overall risk was evaluated together with our downstream partners, and the HAPP was chosen
HAPP pigging operation January 2012
Markland tests before and after
Estimated wax removed by HAPP = 80 m3 Remaining wax in pipeline = approx 350 m3 Pig stopped 15.01.12 at 8357 m
2013 – High Friction Jet Pig
2/10/2014 50 0m z on e Heimdal Brae- Launch 1 off pig from Heimdal using condensate - Pig to be tracked through topsides down to riser hang-off
- Pigging speed: ca 0.4 m/s
Finally SUCCESS
~10 m3 wax left in the pipeline (+/- 50%) Reduced from ~350 m3
Wax layer of ~1mm
Learning
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A main learning: Consequences ofchanged operating conditions (e.g. new fluid composition) have to be carefully evaluated and wax control philosophy updated accordingly.
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New tie-backs or reservoirs–
Retrograde gas condensates may become significantly leaner as reservoir pressure declines•
An original wax problem may in fact disappear !Year Mole% C1 Mole% C18+ Bottomhole pressure (bar) Condensate-to-gas ratio Sm3/MSm3 Simulated WAT (PVTsim) (deg C) 2 3 4 5 6 7 8 9 10 11 76,91 77,91 78,39 78,76 79,72 79,89 79,53 79,45 79,30 78,65 0,928 0,406 0,280 0,173 0,098 0,036 0,017 0,009 0,007 0,004 485 65 549 418 346 290 226 189 166 146 132 122 22 16 11 6 -2 <0 <0 <<0 <<0 <<0 Heimdal Vale Huldra Mixture
WAT (°C) 3,2 24,6 -22,3 13,1
WAX in STO (wt%) 4,2 7,3 0,5 4,9
2/10/2014
