fac 2007 04 wax.pdf

1April 2006 Pg 1

Wax / Paraffin

FLOW ASSURANCE & OPERABILITY

APR-2006 2

Flow Assurance and Operability

Definitions (2 20) Impacts on Facilities and Operations (21 25) Prediction and Design Strategies (26 50) Control and Remediation (51 69)

Wax / Paraffin

2APR-2006 3


DEFINITIONSWhat is wax? Wide range of high molecular weight

paraffins (saturated hydrocarbons) Straight chain, branched, and cyclic

paraffins Slightly soluble in oil Solidifies from oil primarily due to a

decrease in temperature .

Wax / Paraffin

APR-2006 4


What is wax?

Wax / Paraffin

CH3CH2

CH2CH2

CH2CH2

CH2CH2

CH2CH2

CH2CH2

CH2CH3

CH3CH2

CH2CH2

CH2CH2

CHCH2

CH2CH2

CHCH3

CH2CH2

CH2CH3

CH2 CH3

CH3CH2

CH2CH2

CHCH2

CHCH2

CH2CH2

CH2CH3

CH2CH

CH2CH3

CH2

STRAIGHT CHAIN OR NORMAL PARAFFIN

BRANCHED OR ISO-PARAFFIN

CYCLIC PARAFFIN OR NAPHTHENES

3APR-2006 5


What is wax?

Wax / Paraffin

ALKANES

-200

-100

0

100

200

300

400

0 2 4 6 8 10 12 14 16 18 20

Carbon Number

Tem

pera

ture

, C

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Melting Point

Boiling Point

Specific Gravity

APR-2006 6


What is wax? Paraffin and wax are used interchangably As wax solidifies from oil, there are three major concerns:

Wax deposition on tubing and pipe walls Increases in viscosity due to wax particles suspended in the oil Gelling of the oil during shutdown

There are a number of parameters and terms used to describe wax behavior

Wax / Paraffin

4APR-2006 7


Definition of Wax Terms Cloud point or wax appearance temperature Wax deposition Pour point Gel strength or yield stress Viscosity Wax Content Re-melt temperature Dissolution temperature

Wax / Paraffin

APR-2006 8


Cloud Point Temperature at which the first wax crystals form At the cloud point, only a small fraction of wax molecules

crystallize as the temperature continues to drop more crystallizes

Combine with thermal-hydraulic modeling to determine where wax can deposit

Also known as Wax Appearance Temperature (WAT)

Wax / Paraffin

5APR-2006 9


Cloud Point In a live oil, cloud point is lowest at the bubble point

Light molecules, which act as solvent for waxes, are at their highest concentration in the oil at the bubble point

As the pressure increases above the bubble point, the cloud point increases

In a dead oil, cloud point is lowest at atmospheric pressure

Higher pressure favors crystal formation

Typically measure dead oil cloud point at atmospheric pressure

Wax / Paraffin

APR-2006 10

Flow Assurance and OperabilityWax / Paraffin

0

25

50

75

100

125

150

175

200

20 25 30 35 40 45 50 55 60Temperature (C)

Pres

sure

(bar

a)

LIVE OIL BUBBLE POINT

LIVE OIL CLOUD POINT

DEAD OIL CLOUD POINT

6APR-2006 11


Wax Deposition When the pipewall temperature drops below the cloud

point, wax can deposit on the pipewall Most of wax does not deposit Relatively slow process Occurs during normal flow Very little deposition during

shutdown, i.e. no flow

Wax / Paraffin

APR-2006 12


Pour Point Lowest temperature at which an oil can be poured under gravity It is not the temperature at which oil stops in a flowing pipeline Below the pour point, wax crystals form a matrix structure or a gel in

static (not flowing) oil A yield force is required to break the gel and start the fluids flowing The pour point is defined by the

IP 15 or ASTM D97 methods Similar to cloud point, live oil

pour point < dead oil pour point

Wax / Paraffin

7APR-2006 13


Gel Strength Also called yield stress The force required to break the gel developed below the

pour point Thermal and flow history can affect the gel strength It determines the pump pressure required to restart flow Gel strength will vary proportionally with the pipeline

length

Wax / Paraffin

APR-2006 14


Viscosity Is strongly dependent upon

temperature Above the cloud point the viscosity

of oil is only a function of temperature Newtonian behavior.

Below the cloud point, wax crystals suspended in the oil affect the viscosity, and thus viscosity is a function of temperature and of shear rate non-Newtonian behavior.

Shear rate = (4*Q)/(*r3), where Q = volumetric flow rate

Wax / Paraffin

8APR-2006 15


Non-Newtonain Viscosity

10

100

1000

10000

100000

0 10 20 30 40 50Temperature (C)

Visc

osity

(cP)

1030507090110

Shear Rate (1/s)

Cloud Point

APR-2006 16


Wax Content The weight percent of wax in an oil sample (depends

upon test procedure) It is not a predictive parameter High wax content oils (over 10%) are likely to be viscous

and to have gelling and deposition problems Low wax content oils are less likely to have gelling and

viscosity problems but can still have wax deposition problems

Note: waxy oils (typically API > 30) are not heavy oils (API < 22)

Wax / Paraffin

9APR-2006 17


Wax Content

0

2

4

6

8

10

12

-40 -20 0 20 40 60Temperature (C)

Wax

Con

tent

(wt %

)

Cloud Point

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0

10

20

30

40

50

60

0 1 2 3

WAX CONTENT

TEM

PER

ATU

RE

LOW MEDIUM HIGH

NO WAXPROBLEMS

LIKELY

WAX DEPOSITIONIS POSSIBLE,

NO WAX GELLING WAX DEPOSITIONAND WAX GELLING

ARE POSSIBLE

CLOUD POINT

POUR POINT

MINIMUM AMBIENTTEMPERATURE

10

APR-2006 19


Re-Melt Temperature Temperature at which a sample of solid wax melts Soft waxes have melting temperatures in the range of 120

to 140F (49 to 60C) Harder waxes can have melting temperatures as high as

190F (88C) Defines temperature at which the pipewall needs to be

heated to remove (melt) solid wax deposits

Wax / Paraffin

APR-2006 20


Dissolution Temperature Temperature at which wax crystals re-dissolve in the oil Typically greater than the cloud point but less than re-melt

temperature Used for ensuring that all wax is re-dissolved in the oil

during operation or hot oiling

Wax / Paraffin

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APR-2006 21


IMPACT ON FACILITIES AND OPERATIONS Deposition High Viscosities Gelling

Wax / Paraffin

APR-2006 22


Wax Deposition Wax can deposit on tubulars and pipewalls

The build-up of wax reduces the internal diameter The wax deposit increases the surface roughness of the pipewall Diameter reduction and increased surface roughness reduces

flow capacity May eventually lead to blockage Potential to have a stuck pig

Wax can plug instrumentation lines

Wax / Paraffin

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APR-2006 23


High Viscosities Below the cloud point, viscosities can increase

significantly Wax crystals form a slurry in the bulk oil The viscosity depends on the flow rate (or shear rate)

Viscosity is higher at low shear rates (non-Newtonian flow)

At low flow rates the viscosity may be too high to maintain flow

Wax / Paraffin

APR-2006 24


Pipeline Transporting High Wax Content Oil

0

200

400

600

800

1000

1200

1400

1600

1800

0 10000 20000 30000 40000 50000Oil Flow Rate (BPD)

Pipe

line

Pres

sure

Dro

p (p

si)

High Viscosities

13

APR-2006 25


Gelled Oil If a pipeline is shutdown and cools below the pour point, a

gel structure will form An initial yield force (i.e. pressure) must be applied to

break the gel structure and to start the fluids flowing.

The restart pressure may be greater than the pipeline and/or pump design pressure

Wax / Paraffin

APR-2006 26


PREDICTION AND DESIGN STRATEGIES1. Sample exploration/appraisal well (bottom hole sample

preferred)2. Perform laboratory analyses

Measure cloud point, pour point, wax content. Measure viscosities above and below cloud point.

3. Review production scenarios and oil/water/gas production rates over the life of the field

4. Perform steady state thermal-hydraulic modeling for system Consider all production scenarios Consider whole field life Determine operating temperatures and pressures

Wax / Paraffin

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PREDICTION AND DESIGN STRATEGIES5. Identify conditions and locations for wax deposition

(compare operating temperature to cloud point). Perform wax deposition lab tests if necessary Perform wax deposition modeling

6. Identify viscosity concerns Look at pressure drop over the range of flow rates

7. Determine if the pour point is above the ambient temperature

Measure gel strength if necessary Determine capability to restart gelled pipeline

Wax / Paraffin

APR-2006 28


PREDICTION AND DESIGN STRATEGIES8. Examine design/operating options to manage wax

Insulation or heat Chemical injection Pigging Other

9. Develop overall wax control and remediation strategies10. Monitoring

Wax / Paraffin

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PREDICTION AND DESIGN STRATEGIES Sampling for Wax Representative samples are needed (as always). Sample should be taken at a location above the cloud point. If initial evaluation shows a significant waxing potential, then

large sample quantities may be needed. Sample types

Bottomhole preferred but limited volume Separator large volumes which may be needed for deposition

and/or gelling tests

Sample handling Prevent loss of wax in sample containers

Wax / Paraffin

APR-2006 30


PREDICTION AND DESIGN STRATEGIES Lab Analyses for Wax Viscosity Cloud point Compositional analysis Wax deposition tests Pour point Gel strength

Wax / Paraffin

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Lab Analyses for Wax Viscosity Measure viscosity from above the cloud point to the

coldest operating temperature at a constant shear rate Measure at other shear rates anticipated in the field Viscometers considered appropriate are:

Concentric Cylinder Viscometer Cone and Plate Viscometer Model Pipelines

Wax / Paraffin

APR-2006 32


Lab Analyses for Wax Cloud Point Difficult to measure trying to detect first crystals A number of measurement techniques are available No industry standard method Very dependent on sample quality and handling Variations in measured cloud points can be 20F (11C)

or more for a given crude It is recommended that two different measurement

techniques be used

Wax / Paraffin

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APR-2006 33


Lab Analyses for Wax Cloud Point Direct Methods

Crossed Polar Microscopy Filtration Cold Finger Deposition

Indirect Methods Differential Scanning Calorimetry (DSC) Viscometry

Other Methods Infrared Detection Light Scattering NMR - Nuclear Magnetic Resonance

Wax / Paraffin

APR-2006 34


Lab Analyses for Wax Compositional Analysis A detailed compositional analysis of the oil

Required for modeling of wax deposition A quantitative compositional analysis out to C80+ (or higher) is

required

High temperature gas chromatography

Wax / Paraffin

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Lab Analyses for Wax Wax Deposition Tests Wax deposition models are most reliable if combined with

deposition rate measurements from the lab There is limited validation to real systems

Wax deposition is usually measured in the lab using flow loops or cold fingers

Output of tests is diffusion coefficient Testing of chemical inhibitors

Wax / Paraffin

APR-2006 36


Lab Analyses for Wax Wax Deposition Tests

Wax / Paraffin

Ref. Leontaritis, SPE, 2003.

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Lab Analyses for Wax Pour Point Lowest temperature at which an oil is observed to flow,

when cooled under conditions prescribed by test method ASTM D 97. The pour point is 3C (5F) above the temperature at which the oil in a test vessel shows no movement when the container is held horizontally for five seconds.

Wax / Paraffin

APR-2006 38


Lab Analyses for Wax Gel Strength Gel strength is usually measured using model pipelines or

controlled strain rheometers Consider the effects of

shear and thermal history Both test methods can be

used to test pour point depressants

Wax / Paraffin

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APR-2006 39


PREDICTION AND DESIGN STRATEGIES Predictive Modeling for Wax Wax deposition modeling Restart pressures

Wax / Paraffin

APR-2006 40


Wax Deposition Modeling Models are available for predicting deposition rates Ideally combined with deposition rate measurements from

the lab There is limited validation to real systems Deposition modeling errors can be large as much as a

factor of 3 Used to predict pigging frequency

Wax / Paraffin

21

APR-2006 41


Wax Deposition Modeling There is some uncertainty about the deposition

mechanism Molecular diffusion primary mechanism Shear dispersion Other?

Wax deposits grow from the pipe/tube wall Deposits can be sheared off (sloughing) Flow regime can influence deposits

Wax / Paraffin

APR-2006 42


Wax Deposition Modeling

Wax / Paraffin

PIP

EWA

LL

HEATLOSS

LAMINARBOUNDARY

TURBULENTBULK FLOW

VELOCITYPROFILE

WAX CONCENTRATION GRADIENT

DIFFUSION OF DISSOLVED WAX

22

APR-2006 43


Wax Deposition Modeling Diffusion Equation (Ficks Law)

Ji = A Di (ci/z) = A Di [(ci/T) (T/z)]

where Ji = diffusion rate of component iA = areaDi = diffusion coefficient for component i

Wax / Paraffin

Concentration gradient Temperature gradient

Solubility of component i as a function of temperature

APR-2006 44


Wax Deposition Modeling Wax deposition requires outward heat flux The wax deposition rate is proportional to the heat flux Deposition depends upon the oil viscosity, D 1/ Other

Shear effects Less deposit at high shear Softer deposit at low shear

Oil porosity of deposits (20 90%)

Wax / Paraffin

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APR-2006 45



Wax / Paraffin

Laminar Turbulent Annular Flow Stratified Slug

APR-2006 46



Wax / Paraffin

Distance

Low T

urbule

nce

Highly

Turbu

lent

Lami

nar

Deposition Rate Profile

Dep

osit

Thic

knes

s

24

APR-2006 47



Wax / Paraffin

Wax Layer Thickness Profile - Oil Export Line50K BOPD, 3025 psi

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0 20000 40000 60000 80000 100000 120000 140000Distance From Inlet (ft)

Wax

Lay

er T

hick

ness

(ft)

After 1-monthAfter 7-month After 3-month After 5-month

1 mon.

3 mon.

5 mon.

7 mon.

APR-2006 48



Wax / Paraffin

Wax Volume Accumulation - 16.124-in Export Line50K BOPD, 3025 psia

0

10

20

30

40

50

60

0 2 4 6 8 10 12 14Period (Month)

Wax

Vol

(bbl

)

0

0.2

0.4

0.6

0.8

1

1.2

Max

Wax

Thi

ckne

ss (i

n)

Wax Vol (bbl)Max Wax Thickness (in)

25

APR-2006 49


Wax Deposition Modeling Pigging Frequency

Estimated from deposition modeling Wax thickness (2 4 mm) Total wax volume (depends upon facility capacity)

Wax / Paraffin

APR-2006 50


Restart Pressure Prediction Based on measured Yield Stress (Gel Strength) Conservative prediction versus actual pressure seen in

the field There are more sophisticated models under development

Wax / Paraffin

)()()(4)(

m Diameterbar Stress Yield m Length bar Pressure Restart =

26

APR-2006 51


WAX CONTROL AND REMEDIATION Mechanical removal

Pigs

Thermal management Insulation Active heating

Chemicals Deposition inhibitors (minimize deposition) Pour point depressants (reduce pour point) Flow improvers (reduce viscosity)

Other

Wax / Paraffin

APR-2006 52


Pigging Most commonly used method for removing wax deposits Pigging can be successful and cost effective if regularly scheduled Pigs can become stuck in wax deposits when too much has

accumulated Pigs do not remove the entire wax deposit, but leave a smooth layer The pig should be specifically designed for paraffin removal Pigging frequencies should be optimized based

upon field observation A regularly scheduled pigging program should be

initiated upon commissioning, optimized based upon field experience, and followed religiously.

Wax / Paraffin

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APR-2006 53


Other Mechanical Removal Wireline Cutting

Wireline cutting of wax deposits in wellbores is a common Need to consider the frequency of wireline workovers and the

downtime For subsea wells, the cost may be cost prohibitive

TFL (Through Flowline Pigging) TFL pumps a cutting tool down the flowline and into the wellbore

to cut wax deposits. TFL is not a common practice It can remediate subsea flowlines and wellbores BP Pompano

Wax / Paraffin

APR-2006 54


Thermal Management Maintain fluid temperature above

cloud point Prevent deposition Prevent viscosity concerns

Minimize heat flux Minimize deposition

Provide cooldown time to pour point Provide operators with time to respond to shutdown

Wax / Paraffin

28

APR-2006 55


Thermal Management Wellbore Insulation

Vacuum insulated tubing Gelled packer fluid

(e.g. Insugel by BJ Services)

Active heating Electrical heating Hot fluid circulation Not commonly applied

Wax / Paraffin

APR-2006 56


Thermal Management Flowline Insulation

Pipe-in-pipe Syntactic / other foam external insulation Buried pipe

Active heating Hot fluid circulation in bundle Electrically heated with insulation

Wax / Paraffin

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APR-2006 57


Paraffin Treating Chemicals Deposition Inhibitors Paraffin inhibitors can reduce wax deposition rates Inhibitors cannot completely prevent wax deposition Can reduce deposition rate by a factor of 5 Typically combined with pigging Inhibitors can create a soft deposit that is easily pigged or

sloughed off There is no universal wax deposition inhibitor Significant OPEX

Wax / Paraffin

APR-2006 58


Paraffin Treating Chemicals Deposition Inhibitors Wax inhibitors may be

wax crystal modifiers co-crystallize with the wax to prevent wax crystal from forming on the pipewall

dispersants coat wax crystals to keep them from agglomerating surfactants water wet the wax crystals so that the do not adhere

to each other or the pipe wall combination of all three

Wax / Paraffin

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APR-2006 59


Paraffin Treating Chemicals Deposition Inhibitors Wax deposition inhibitors should be selected using the

same procedures for measuring wax deposition rates. Inhibitor effectiveness can be temperature dependent Inhibitor may need to be changed after initial startup or

other time in the field life

Wax / Paraffin

APR-2006 60


Paraffin Treating Chemicals Pour Point Depressants Pour point depressants can reduce pour point, viscosity,

and/or yield stress Pour point depressants should be selected using the

same test procedures as used for measuring pour points, yield stress and viscosities

Flow improvers used to reduce viscosity

Wax / Paraffin

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APR-2006 61


Paraffin Treating Chemicals In general, all types of paraffin treating chemicals should

be applied at a location with a temperature greater than the cloud point (at least 10F or 5C above cloud point)

These chemicals are very crude oil specific no guarantee a suitable chemical can be found

Cannot predict inhibitor effectiveness lab testing is required

Must work with chemical vendors Injection rates as high as 100-1000 ppm are common

Wax / Paraffin

APR-2006 62


Other Methods of Deposition Prevention Do nothing if you have confidence that deposition rates

will not be significant over the lifetime of the well, flowline, or pipeline

Coatings yet to be effectively demonstrated Solvents not effective except at high temperatures or at

high solvent ratios Magnets some believe they can be effective, not

effective in controlled lab experiments

Wax / Paraffin

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APR-2006 63


Managing Gelled Restart Pipeline sizing

Pipelines are generally sized based upon maximum flow, but maximum pressure drop may be at low flow rates

Reduce the diameter and increase the flow velocity to have higher temperatures and lower viscosities, e.g. use two smaller pipelines rather than a single larger line

Chemical options Continuously inject pour point depressant

Wax / Paraffin

APR-2006 64


Managing Gelled Restart Insulation

Longer to cool during a shutdown

Active heating heat above pour point after shutdown Electrical heating Hot fluid circulation

Dilution Reduce viscosity and/or pour point by blending in diesel or a

lower wax content oil

Displacement During shutdown displace the oil with a lower wax content oil,

diesel, or water

Wax / Paraffin

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APR-2006 65


Managing Gelled Restart Core annular flow

Maintain a water layer at the pipewall: a very viscous oil flows as a solid core inside the water (has been done for heavy oils)

Oil-in-Water emulsions Form an oil-in-water emulsion with lower viscosity requires a

surfactant (has been done for heavy oils)

Other Maintain a low flow to prevent gelling Pulse oil to mitigate gelling

Wax / Paraffin

APR-2006 66


Remediation of Thick Wax Deposits Aggressive Pigs Progressive Pigging Wireline Cutting (wells) Coiled Tubing

Wells Coiled tubing has been used to jet out deposits in short sections

of flowlines

Wax / Paraffin

34

APR-2006 67


Remediation of Thick Wax Deposits Coiled Tubing

Wax / Paraffin

Superior Energy Services

APR-2006 68


Remediation of Thick Wax Deposits Heat

Hot oil and hot water circulation can effectively melt deposits depending upon insulation and length

In-situ heat generation method (SGN) developed by Petrobras

Solvents Waxes are relatively insoluble in most solvents except at higher

temperatures A solvent may break down or soften the wax deposit so that it can

be more easily flushed or pigged out

Wax / Paraffin

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APR-2006 69


Remediation of a Gelled Pipeline If a oil in a pipeline cools below pour point and gels, and if

there is not sufficient pressure to break the gel and restart the pipeline, it may be difficult to recover.

Some potential solutions: Apply maximum pipeline pressure and wait Additional access points along the line can be used to apply

additional pressure Wait till summer, warmer temperatures Coiled tubing combined with heated solvent

Wax / Paraffin

APR-2006 70


Example Wax Management Guidelines

Wax / Paraffin

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APR-2006 71


Example wax control guidelines for subsea production system (low to medium wax content no gelling issues)

Operate the well at sufficiently high production rates to avoid deposition in the wellbore and tree

Consider using insulated tubing to maximize flowing wellhead temperatures

Note: very important not to have deposition in wellbore, too expensive to mobilize rig to scrape tubing

Remove wax from flowlines by pigging and pig frequently enough to ensure that the pig does not stick

Utilize insulation and chemicals to reduce pigging frequency

Wax / Paraffin

APR-2006 72


Example wax control guidelines for high wax content production system

Insulate wells and flowlines to minimize deposition and viscosity

Insulate to provide cooldown time to pour point Give operators time to restart or prepare system for long-term

shutdown

Inject pour point depressant to reduce pour point below minimum ambient temperature or to reduce gel strength

Provide restart capability Design pressure > restart pressure, or Displace flowlines with inert fluid following shutdown, before fluids

cool below pour point

Wax / Paraffin

37

APR-2006 73


Viscosity

Wax / Paraffin

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