Wax control

Wax control
Arild Stokkenes
Leading advisor Multiphase Fluid Control
Outline
• Flow Assurance in Statoil
• What is wax and what problems may it cause?
• How to control wax deposition
• How to monitor wax deposition
• Case example: How to not control wax deposition
2/10/2014
Flow Assurance
Process
Separator
Slug catcher
Main deliverables/competence:
Thermohydralic multiphase analysis
System design
Hydrate- and wax control philosophies
Slug control
Operational support
Multiphase metering
Scale control
Asphaltene control
Wax control
Chemical Injection
Package
Fluid properties
Rheology
Hydrate control
Emulsion control
Corrosion control
Multiphase equipment:
Wellbore hydraulics
Transient pipeline
thermohydraulics
• Multiphase meter
• Multiphase pump
“Flow assurance” = safe, uninterrupted and simultaneous
transport of gas, oil and water from reservoirs to processing facilities.
2/10/2014
Fluid control – the problems
Gas hydrates
Asphaltenes
Wax
Kristin-NJ/DR Wye
- wax deposition and temperature profile after 600 h
70
50
0.003
40
30
0.002
20
0.001
10
0
0
0
20
40
60
Pipeline length [km]
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Wax
deposition
60
0.004
80
100
Temperature [°C]
Wax deposition [m]
0.005
Fluid
temperature
The future ….
Arctic / harsh
environment
Longer distance
Deeper
water
More difficult
fluids
2/10/2014
Increased field
complexity
Outline
• Flow Assurance in Statoil
• What is wax and what problems may it cause?
• How to control wax deposition
• How to monitor wax deposition
• Case example: How to not control wax deposition
2/10/2014
What is wax?
Wax consistency
range
Soft wax
2/10/2014
Hard wax
n-alkane
What is wax?
• 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
2/10/2014
wax
crystal
Wax-forming components in crude oils
C10+
Mainly n-alkanes
Non-wax
Wax
C7
C8
C9
Lab. analysis Pseudo-components
• subtype of the saturates (non-polar
compunds without double bonds)
• Mainly alkanes of > C18
• Can be linear, branched or cyclic
9 - 2/10/2014
Simple questions – difficult to answer !
Steady-state
Wax deposition
Shut-down/restart
Gelling
• 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 (wax
inhibitor)?
Key parameters:
2/10/2014
• 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)?
Wax appearance temperature (WAT)
Wax content
Pour Point
Wax precipitation and wax depositon
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.
3 inch
2 inch
Cold finger device
2/10/2014
11
Flow loop
Wax precipitation curve
Norne crude at 1 bar
8
Wt% solid wax
7
6
5
4
3
2
1
0
-20
-10
0
10
20
Temperature (°C)
2/10/2014
30
40
50
Wax diffusion towards cold surface
1. The cold wall removes wax molecules from the oil
2. Give rise to a diffusion of wax molecules toward the wall
n   wax DM
n
wax
DM
dC / dr
dC / dT
dT / dr
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13 -
dC
dC dT
  wax DM
dr
dT dr
mass flux of dissolved wax molecules towards the pipe wall
density of solid wax
molecular diffusion coefficient of dissolved wax molecules
concentration gradient of dissolved wax in the laminar sub-layer
solubility of wax components as a function of the temperature
radial temperature gradient close to the wall
Wax deposition by molecular diffusion
Laminar boundary
layer
Turbulent core
Temperature gradient
Heat loss
WAT
dT/dr
dC/dr = dC/dT * dT/dr
Dissolved
Pipe wall
wax
2/10/2014
14 -
Wax concentration gradient
dC/dr
Velocity profile
Wax depositon_Process
1. Transport
to pipe wall
2. Inital wax
layer formation
sites
3. Growth
or
4. Aging
Thickness
thin gel
Roughness
Hardness
Diffusion
Dispersion
Fluid-solid
interaction
Crystal growth
Trapping of oil
Time
Shear/hydrodynamics
Diffusion/Counter diffusion
Wax deposition process shown by Rønningsen
Rønningsen HP, 6th Int. Conference on Phase Behaviour and
Fouling, Keynote speech, 2005
15
2/10/2014
What happens in the pipeline?
7
40
Wax after 7 days
35
30
5
25
Wax after 2 days
4
20
3
15
Wax after 1 day
2
Temperature
1
5
0
0
10
20
30
40
50
60
70
Length (km)
2/10/2014
10
80
90
0
100 110 120
Temperature (C)
wax thickness (mm)
6
Wax deposition modelling in Statoil
 Commercial tools like OLGA, PVTsim
 Wax precipitation curve tuning developed internally
 Mutivariate analysis
3
Wax content [wt%]
2.5
Cold flow cool
down section
2
1.5
Subsea separation and
multiphase pumps
1
Water injection
pumps
0.5
0
0
10
20
30
40
Wax thickness (mm)
Data
Power and control
Before tuning
distribution unit
After tuning
Predicted value
Measured value
50
o
Temperature [ C]
Wax precipitation curve tuning
17
2/10/2014
Multivariate analysis validation
The wax build-up can be reproduced
30
…….. but is hard to predict !
•
The pressure build-up can also be reproduced by
proper tuning of the roughness effect of the wax
deposit, i.e.
– Wax roughness factor
Rough. 0.5 - Diff. 7 - Shear C3 0.7
Rough. 1.0 - Diff. 2 - Shear tuning
Rough. 0.5 - Diff. 6
3
200 m wax
20
280 m3 wax
15
10
The wax deposition profile can be reproduced by
various combinations of model parameters:
– Diffusion coefficient
– Wax porosity
– Shear stripping
•
wax thickness (mm)
The wax build-up profile in a pipeline can be
reproduced using the OLGA (RRR) model.
210 m3 wax
5
0
0
10
20
30
40
50
60
70
80
90
100
110
120
Length (km)
220
Field Pressure
210
Heimdal Export Pressure (bara)
•
25
Rough. 0.5 - Diff. 6
200
190
180
170
160
150
140
130
120
110
100
•
Different ongoing JIP and internalt research
ongoing for improving the models
2/10/2014
18
0
50
100
150
200
250
Time (days)
300
350
400
Outline
• Flow Assurance in Statoil
• What is wax and what problems may it cause?
• How to control wax deposition
• How to monitor wax deposition
• Case example: How to not control wax deposition
2/10/2014
Wax deposition
challenges
• Stuck pigs
• HSE
• Inspection tools
• Plugged pipelines
2/10/2014
The most famous wax illustration !
• 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 pig
Ref. SPE 77573 (2002)
2/10/2014
Methods for controlling wax deposition
 Pipeline insulation
 External insulation coating on single pipes
 Pipe-in-pipe systems
 Pigging
 Chemicals
 Inhibitors
 Dispersants
 Dissolvers
PPD treated oil; this work
 Heat
 Bundles
 Electric heating
 Hot oil flushing
2/10/2014
PP-Solid
PP-Solid
PP-Syntactic
PP-Solid
PP-Adhesive
FBE
PP-Foam
1.
2.
3.
Wax control strategies
Single phase oil/condensate pipelines:
–
Wax control normally by regular pigging
Medium length multiphase oil and gas condensate pipelines:
–
–
Normally insulated (or heated)
Prevents wax deposition and hydrate formation
Long-distance multiphase pipelines:
a) Low-wax gas condensates (Snøhvit):
• Wax deposition will normally not be an issue
b) Oils and waxy gas condensates:
• No general, proven way to control wax deposition…
• Wax-repellent surface coatings?
2/10/2014
Outline
• Flow Assurance in Statoil
• What is wax and what problems may it cause?
• How to control wax deposition
• How to monitor wax deposition
• Case example: How to not control wax deposition
2/10/2014
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
25 2/10/2014
Outline
• Flow Assurance in Statoil
• What is wax and what problems may it cause?
• How to control wax deposition
• How to monitor wax deposition
• Case example: How to not control wax deposition
2/10/2014
Heimdal – Brae condensate export pipeline
Huldra
Vale
Vale
Skirne
Heimdal
Statpipe
Brae
2/10/2014
Introduction of Vale fluids in 2002
Before 2002, no wax and no pigging performed. Then Vale field started up with
high wax content.
Normalized pressure drop, bar
80
WAT (°C)
WAX in STO (wt%)
70
Volume rate (Sm3/d)
60
50
Heimdal
Vale
3,2
4,2
24,6
7,3
-22,3
0,5
13,1
4,9
1000
700
300
2000
Start-up waxy cond.
40
30
20
10
0
19.4.01
5.11.01
24.5.02
10.12.02
28.6.03
14.1.04
1.8.04
- Build up of line differential pressure was insignificant until 2004
28
2/10/2014
Huldra Mixture
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 29
014
Heimdal – Brae wax characteristics
• 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.
2/10/2014
30 -
2010: Aggressive pigging!
Why change strategy?
1. The pipeline NEEDS to become wax free due to inspection requirements
2. Progressive approach with foam pigs does not work
Two Alternatives for consideration:
1.
Hydraulically Activated Power Pig (HAPP)
•
Limited experience
•
Assumed best for downstream facilities
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
2/10/2014
HAPP pigging operation January 2012
Markland tests before and after
Pig stopped 15.01.12 at 8357 m
2/10/2014
Estimated wax removed by HAPP = 80 m3
Remaining wax in pipeline = approx 350 m3
2013 – High Friction Jet Pig
Brae
Heimdal
- 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
2/10/2014
500m zone
Finally SUCCESS
~10 m3 wax left in the pipeline (+/- 50%)
Reduced from ~350 m3
Wax layer of ~1mm
Reduced from up to 20mm
Learning
• A main learning:
Consequences of
changed operating conditions (e.g.
new fluid composition) have to be
carefully evaluated and wax control
philosophy updated accordingly.
– New tie-backs or reservoirs
– Retrograde gas condensates may
become significantly leaner as
reservoir pressure declines
• An original wax problem may in
fact disappear !
2/10/2014
34
WAT (°C)
WAX in STO (wt%)
Volume rate (Sm3/d)
Heimdal
Vale
Huldra Mixture
3,2
4,2
24,6
7,3
-22,3
0,5
13,1
4,9
1000
700
300
2000
Year
Mole% C1
Mole%
C18+
Bottomhole
pressure
(bar)
Condensateto-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
549
418
346
290
226
189
166
146
132
122
22
16
11
6
-2
<0
<0
<<0
<<0
<<0
65
Thank you
2/10/2014

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