Flow Assurance Solids

8/11/2019 Flow Assurance Solids

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Relevance of Natural Gas Hydrate

• Gas kick in offshore drilling!!!• Deposits in oil & gas pipelines!!

• Storage and transport of gas!• Cold flow in subsea pipelines?

• Gas resource (big claims)??• Global warming (hyd. melting)???



A: Drilling Unit, B: Production and Injection Wells, C: Process (Separationand Compression etc.), D: Storage, E: Off-Loading, F: Living Quarters,

G: Riser Base, H: Template, I: Flare, J: Flowlines and Pipelines.



Flowlines and PipelinesNatural Gas Production

Natural gas, Sour gases, Hydrocarboncondensate, Condensed water, Formationwater, Liquid slugging



Flow Assurance

Flow assurance is a concept used to

describe the phenomena of precipitationand deposition of solids (and multiphase

flow, not discussed here) in flowlines and

pipelines. Flow assurance offers technical

solutions at reasonable costs without risk

to installations, operators and theenvironment.

Precipitation is not the same as deposition…



Flow Assurance Solids

• Asphaltene (pressure changes)

– Heavy, polar molecules, amorphous solid

• Paraffin wax (pipeline cooling)

– Normal paraffin C20 to C40• Gas hydrate (pipeline cooling)

– Methane, ethane, propane and butane

• Inorganic scale (fluid mixing…)

– Carbonates and sulphates



Hydrocarbon Solids

A: Phase envelope, B: Gas hydrate, C: Paraffin wax, D: Asphaltene, E: Multiphase flow



Siljuberg 2012 (from Rønningsen 2006)





Asphaltene• Precipitates from crude oil when reservoir

pressure falls during production• Crude oil density reduces when reservoir

pressure falls, causing precipitation

• Crude oil density increases again when lightcomponents have bubbled out (associated gas)

• Precipitation envelope, light crude main problem

• Deposition prevented by additives (wells and

flowlines) to hinder agglomeration of particles



Asphaltene Precipitation

[MPa]

[kg/m3]



Temperature in Pipelines




LMTDTUAq

)TT(Cmq 21 p

T T

T T

T T T T

T LMTD

2

1

21

ln

)()(

T T

T T

T T T LMTD

2

1

21

ln

T T

T T

T T Ld U T T C m p

2

1

2121

ln

)()()(

L

mC

d U T T T T

p

exp)( 12

)( Ld A

T = Constant = Sea Temperature



Temperature and Distance




L

mC

d U T T T T

p

exp)( 12

Insulated pipeline on seafloor: 1 < U (W/m2.K) < 2

Non-insulated pipeline on seafloor: 15 < U (W/m2.K) < 25



Calculation Example

What is temperature at 20 km?

m=67 kg/s

Cp=3500 J/kg.KU=2 W/m2.K

d=0.370 m

T=5 C

T1=86 C

C T 711020350067

370.01416.32exp)586(5 32



Temperature and Distance

Booster compressor duty: 15.5 MW (most likely roughness)

Åsgard Transport (69.4 vs. 76.9 MSm³/d)

110

120

130

140

150

160

170

180

190

200

210

0 200 400 600 800

Distance KP (km)

P r e s s u r e ( b a r g )

0

5

10

15

20

25

30

35

40

45

50

T e m p e r a t u r e ( ° C )

Pressure Booster_press Temperature Booster_temp

Aamodt (2006)



Wax Appearance Temperature

Crude oil and condensate WAT (=cloud point) typically at 30-40 [C]. Pour

point typically 15 [C] below cloud point. Wax crystals in oil increase viscosity.



Botne 2012



Paraffin WaxCloud point (WAT) and pour point



Wax Build-UpWith time and distance

xk k dt dx

21

)exp(1 2

2

1 t k k

k x



Botne 2012



Botne 2012



Botne 2012





Water Vapour at 10 (Top), 20

Middle) and 30 (Bottom) MPa

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

20000

0 20 40 60 80 100 120 140

T [C]

c [ m g

/ S m 3 ]



Gas Hydrate•Major obstacle to production of oil and gas through

subsea pipelines (due to cooling). Blocks pipelines.

•Form when liquid water (condensed out from moistreservoir gas) and natural gas are present at “wrong” side

of equilibrium line (typically 20 C and 100 bara).

•Water molecules are stabilized by small gas moleculessuch that hydrates form (physical process, not chemical

reaction).

•Antifreeze chemical used/injected to lower the T at whichhydrates form (lower “freezing” point of hydrate).

•Typically, 50 % antifreeze (in liquid phase) required to

prevent hydrate formation. Expensive, very expensive.



Equilibrium & Flow Assurance

Carroll 2003

L

mC

d Uexp)TT(TT

p

u1u2

Cooling w. distance



A: Gas reservoir,B: Oil reservoir,

C: Aquifer,

D: Cap rock,

E: Sealing fault.

A/B: Gas-oil-contact.

B/C: Oil-water-contact.

Gas in A saturated withwater vapour (condenses

out at surface).

Oil formation B contains

formation water (saline).



Gas Molecules Trapped in Cages12-sided, 14-sided and 16-sided polyhedra

Small non-polar molecules, methane, ethane, propane and butane form gas

hydrate. Carbon dioxide, hydrogen sulphide and nitrogen also form hydrate.



Gas Inside Ice Crystal Cages

Carroll 2003

Skalle 2009



Structure II Gas Hydrate

O H X 213624



Dissociation Pressure



Hydrate Equilibrium (Dissociation Pressure)



Dissociation Pressure Gas Hydrate

0

5000

10000

15000

20000

25000

30000

35000

40000

45000

0 5 10 15 20 25 30 35

T [C]

p

[ k P a ]

Lower line natural gas mixture; upper line with CO2 and N2



Christiansen 2012





Hammerschmidt’s Equation

)1( x

x

M

K T



Hydrate Equilibrium Midgard Field Gas

Lunde (2005): Design av flerfasesystemer for olje og gass, Tekna





Natural Gas Resource?Hydrate Zone Limited by Subsurface Temperature

Senger 2009



Subsurface Gas Hydrate



Mary Boatman, unknown reference



Krey et al. 2009



Global Warming & Gas Resource

William Dillon, USGS



Gas Kick in Drilling

Skalle 2009

Deepwater Horizon, GoM, Teknisk

Ukeblad, May 6, 2010



NTNU Cold Flow



Sintef Cold Flow



Sintef Cold Flow

Sintef 2010



Summary

– More than natural gas flows in gas flowlines

– Asphaltene problem in oil production. Paraffin wax problem in

crude oil and condensate. Gas hydrate problem in oil and gas

production. Inorganic solids when saline water.

– Temperature drop equation does not include the Joule-

Thomson effect (small in large diameter pipelines). U values

based on experience.

– Hydrates form when liquid water and natural gas are in

contact at low temperature and high pressure, as in subsea

production of oil and gas.

– Hammerschmidt’s and similar equations can be used to

estimated the mass fraction of antifreeze required to preventhydrate formation. Hysys gives dissociation pressure.

– Hydrates important in drilling operations and environmental

considerations (global warming).

Flow Assurance Solids

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