FLOW ASSURANCE IN PIPELINES

Jón Steinar GuðmundssonTPG4140 NaturgassNovember 7, 2011

– Flow assurance, recent concept– Asphaltene, Paraffin wax, Gas hydrate, Inorganic solids– Temperature in pipelines (drops quickly with distance)– Hydrate in pipelines (happens when T<20 C)– Summary

A: Drilling Unit, B: Production and Injection Wells, C: Process (Separation and 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, Hydrocarbon condensate, Condensed water, Formation water, Liquid slugging

Flow Assurance

Flow assurance is a concept used to describe the phenomena of precipitation and deposition of solids (and multiphase flow) in flowlines and pipelines. Flow assurance offers technical solutions at reasonable costs and without risk to installations, operators and the environment.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

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 light components have bubble out

• 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

Temperature in Pipelines

LMTDTUAq

)TT(Cmq 21p

TTTT

TTTTTLMTD

2

1

21

ln

)()(

TTTT

TTTLMTD

2

1

21

ln

TTTT

TTLdUTTCm p

2

1

2121

ln

)()()(

L

mCdUTTTTp

exp)( 12

)(LdA

T = Constant = Sea Temperature

Temperature and Distance

Temperature in Pipelines

L

mCdUTTTTp

exp)( 12

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

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

Calculation ExampleWhat is temperature at 20 km?

m=67 kg/sCp=3500 J/kg.KU=2 W/m2.Kd=0.370 mT=5 CT1=86 C

CT 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)

110120130140150160170180190200210

0 200 400 600 800

Distance KP (km)

Pres

sure

(bar

g)

05101520253035404550

Tem

pera

ture

(°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 increases viscosity.

Paraffin WaxCloud point (WAT) and pour point

Wax Build-UpWith time and distance

xkkdtdx

21

)exp(1 22

1 tkkkx

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 moist reservoir gas) and natural gas are present at p & T above equilibrium line (typically 20 C and 100 bara).

•Water molecules are stabilized by small gas molecules such that hydrates form (physical process, not chemical reaction).

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

•Typically, 50 % antifreeze (in liquid phase) required to prevent hydrate formation. Expensive.

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 with water 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.

Structure II Gas Hydrate

OHX 213624

Dissociation Pressure

Hydrate Equilibrium

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

Pa]

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

Hammerschmidt’s Equation

)1( xx

MKT

Hydrate Equilibrium Midgard Field Gas

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

Summary

– More than natural gas flow 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 incontact at low temperature and high pressure, as in subsea production of oil and gas.

– Hammerschmidt’s equation can be used to estimated the mass fraction of antifreeze required to prevent hydrate formation.