flow assurance: gas hydrates and wax · flow assurance: gas hydrates and wax – june 2004 steering...

Flow Assurance: Gas Hydrates and Wax – June 2004 Steering Committee Meeting

Flow Assurance:Gas Hydrates and Wax

December 2002-November 2005 Programme


Objectives• Hydrate formation in low water content gases

• Hydrate inhibition, inhibitor loss and/or salt precipitation in methanol, glycol, and salt systems

• Hydrate stability zone of oil systems at high pressure conditions

• Gas hydrates in water-oil emulsions

• Wax phase boundary, and effect of wax on hydrates (and vice versa)


Hydrates in low water content gases• Water content in gas systems

– Experimental measurements (gas, water, ice, hydrates)– Reliability of the measurements

– Extension to other conditions– Reliability of the assumptions

• Hydrate phase equilibria in low water content gasesProgress in the last six months• Experimental

– A comprehensive literature review of all water content data– Gas solubility data on C2 and N2 in water from 274 K (EIERO)

• Theoretical– Tuning BIPs for C2 and N2 using the new gas solubility data– Semi-empirical correlation for predicting water content


Hydrate inhibition in methanol, glycol, and salt systems• Salts and organic inhibitor systems

– Hydrate inhibition– Salting-out– Inhibitor distribution

Progress in the last six months• Effect of water/gas ratio on methanol inhibition data• Freezing & boiling points and solubility data for EtOH/KCl• Methane hydrate data on EtOH/NaCl and EtOH/KCl• NG hydrate data on MeOH/NaCl• Thermodynamic modelling of EtOH/NaCl and EtOH/KCl• Extension of the correlation to EtOH/NaCl


Hydrate stability zone of oil systems at high pressure conditions• Deepwater operations and long tiebacks• Limited available data• Measurement challenges

– Visual techniques– P vs T techniques

Progress in the last six months• Oil with aqueous 16 and 30 mass% ethanol and 30

mass% ethanol + 5 mass% NaCl (1500 bar)• Developing an isochoric technique for measuring Pb

• Tuning the thermodynamic model using low temperature Pb data


Wax-hydrate combinations• Wax and hydrates in subsea pipelines• Wax phase boundary determination

– Experimental (WAT vs WDT, step-heating)– Thermodynamic modelling

Progress in the last six months• Experimental

– WAT measurement on base and two live synthetic mixtures– Hydrate dissociation point measurements on the above system– Effect of hydrates on wax phase boundary– Wax phase boundary measurement on a heavy oil

• Modelling– Developing a new multiple solid semi-empirical correlation for

wax

FLOW ASSURANCE: GAS HYDRATES AND WAX 2002-2005 PROGRAMME

Experimental work ,Physical Property Measurements

December 2003-May 2004

Rod BurgassFlow Assurance: Gas Hydrate and Wax – June 2004 Steering Committee Meeting

EXPERIMENTAL WORK: PHYSICAL PROPERTY MEASUREMENTS

• Boiling point measurements

• Freezing point depression

• Salt solubility

Flow Assurance: Gas Hydrate and Wax – June 2004 Steering Committee Meeting

Schematic of boiling point apparatus constructed in-house

Heating Mantle

Condenser

Thermocouple

Cottrell Pump

Boiling point elevation for aqueous solutions of sodium chloride

373

375

377

379

381

383

0 5 10 15 20 25 30Sodium chloride concentration/mass%

T/K

This workICT

Boiling point data for aqueous solutions of glycerol

373

378

383

388

393

398

403

0 10 20 30 40 50 60 70 80 90Glycerol concentration/mass%

T/K

This workICT

Boiling point data for aqueous solutions of ethanol and potassium chloride

Mass% ethanol

±0.1

Mass%

potassium chloride ±0.1

Mass% water

±0.1

Boiling point °C ±0.2

2.3 2.3 95.4 96.5 9.4 9.4 81.2 87.3 7.6 7.8 84.6 90.2 5.1 5.1 89.8 92.8

12.4 12.5 75.1 86.1 16.1 4.8 79.1 86.0

5.0 15.0 80.0 91.7

Freezing point method schematic of sample temperature probe

Aluminium Tube

Test Sample

PRT

PTFE Sleeve

Example of freezing point measurement data for aqueous solution of sodium chloride

T di

ffere

nce

betw

een

prob

es/K

Freezing point

3.6

3.3

3.0

2.7

2.4-4.0 -3.6 -3.2 -2.8 -2.4 -2.0 -1.6 -1.2

Sample Temperature/K

Freezing point measurements for aqueous solutions of sodium chloride

255

258

261

264

267

270

273

0 5 10 15 20 25Sodium chloride concentration/mass%

T/K

This work

CRC Handbook

Freezing point measurements for aqueous solutions of ethylene glycol

238

243

248

253

258

263

268

273

0 10 20 30 40 50Ethylene glycol concentration/mass%

T/K CRC Handbook

This work

Freezing point data for aqueous solutions of ethanol and potassium chloride

Mass% ethanol

±0.1

Mass%

potassium chloride ±0.1

Mass% water

±0.1

Freezing point

°C ±0.2

2.7 2.7 94.6 -2.8 5.1 5.1 89.8 -5.4 7.5 7.5 85.0 -8.6

10.2 10.3 79.5 -13.0 19.6 5.5 74.9 -17.0

5.2 15.2 79.6 -12.5

Schematic of solubility measurement apparatus

PRT

Coolingjacket

SamplecontainerStirrer

Septum

Solubility in mass% of KCl* in aqueous solutions of ethanol at different temperatures

Mass% Ethanol in salt-free solution

Temperature ºC

10 20 30

10 18.36 13.47 9.69 25 21.25 16.52 12.63 50 25.13 20.90 16.66 75 - 24.79 -

* mass% KCl=mass KCl/(Mass Water+Mass EtOH+KCl)

Plot of solubility data for KCl in aqueous solutions of ethanol at different temperatures

8

12

16

20

24

28

0 20 40 60 80 100T/C

K C

l mas

s%

10 mass% ethanol

20 mass% ethanol

30 mass% ethanol

Experimental Work:

HYDRATE MEASUREMENTSMethane / NG with Salts and Organic Inhibitors

Ross AndersonRahim (Amir) Masoudi

Hydrates: Outline

• Dissociation point measurements: Mixed salt-organic inhibitor systems (480 bar)– Methane with ethanol / NaCl– Methane with ethanol / KCl– Natural gas with methanol / NaCl

• Importance of aqueous solution to gas ratios (480 bar)– 10 mass% methanol aqueous solution– Gas rich and aqueous solution rich systems: comparison

of dissociation conditions

Hydrates: Experimental Methods

• Salts / organic inhibitors with methane / natural gas– Rig-1– Isochoric step-heating techniques

• Experimental methods detailed in March 2003 Progress Report

Hydrates: Rig-1 Set-up

T2

T1

Hydrates: Experimental Methods

3.5

4.0

4.5

5.0

5.5

6.0

250 255 260 265 270 275 280 285

T/K

P/M

Pa

Cooling cycleHeating cycle (non-equilibrium points)Heating cycle (equilibrium points)Dissociation point

C1 - 15 mass% K2CO3

3.5

4.0

4.5

5.0

5.5

6.0

250 255 260 265 270 275 280 285

T/K

P/M

Pa


C1 - 15 mass% K2CO3

3.5

4.0

4.5

5.0

5.5

6.0

250 255 260 265 270 275 280 285

T/K

P/M

Pa


C1 - 15 mass% K2CO3

3.5

4.0

4.5

5.0

5.5

6.0

250 255 260 265 270 275 280 285

T/K

P/M

Pa


C1 - 15 mass% K2CO3

3.5

4.0

4.5

5.0

5.5

6.0

250 255 260 265 270 275 280 285

T/K

P/M

Pa


C1 - 15 mass% K2CO3

Hydrates: Methane with Ethanol / NaCl

10

100

1000

-20 -15 -10 -5 0 5 10 15 20 25 30

T / C

P / b

ar

10 mass% NaCl / 10 mass% Ethanol (aqueous)7 mass% NaCl / 30 mass% Ethanol (aqueous)C1, Distilled water

C1, Distilled water data:Deaton and Frost (1946)Mcleod and Campbell (1961)Jhaveri and Robinson (1965)

Hydrates: Methane with Ethanol / KCl

10

100

1000

-15 -10 -5 0 5 10 15 20 25 30

T / C

P / b

ar

10% KCl / 10% Ethanol (90.2% aqueous phase)7% KCl / 20% Ethanol (82.6% aqueous phase)C1, Distilled water

C1, Distilled water data:Deaton and Frost (1946)Mcleod and Campbell (1961)Jhaveri and Robinson (1965)(Values in mass%)

Hydrates: NG with NaCl / Methanol

4.991.12

86.36

5.431.49 0.18 0.31 0.06 0.07

0

20

40

60

80

100

N2 CO2 C1 C2 C3 iC4 nC4 iC5 nC5 +C6+

Component

Mol

e %

NG-1

Hydrates: NG with NaCl / Methanol

10

100

1000

0 2 4 6 8 10 12 14 16 18 20

T / C

P / b

ar

NG-2, 10 mass% NaCl / 10 mass% MethanolNG-2, Distilled water, This workNG-1, Distilled water, In-house data

Composition:71.54 mass% aqueous phase28.46 mass% natural gas

Next test: 7 mass% NaCl / 20 mass% methanol

Hydrates: Effect of Gas to Solution Ratios

WaterMethanolMethane

Gas rich:

40.11 mole% H2O

2.51 mole% CH4O

57.39 mole% CH4

Aqueous solution rich:

93.55 mole% H2O

5.84 mole% CH4O

0.61 mole% CH4Volumetric solution:gas = 1:42.3

Volumetric solution:gas = 1:0.1


10

100

1000

-10 -5 0 5 10 15 20 25 30

T / C

P / b

ar

Gas-rich, This workAq. solution-rich, This workNg and Robinson (1985)C1, Distilled water

C1, Distilled water data:Deaton and Frost (1946)Mcleod and Campbell (1961)Jhaveri and Robinson (1965)

10 mass% MeOH (aq)


3.0

3.5

4.0

4.5

5.0

5.5

6.0

0 100 200 300 400 500

P / bar

∆T

/ C

Gas-rich, This workAq. solution-rich, This workNg and Robinson (1985)

10 mass% MeOH (aq)

Hydrates: Summary

• Dissociation conditions (to 480 bar) measured for: – Methane with ethanol / NaCl– Methane with ethanol / KCl– Natural gas with 10% methanol / 10% NaCl (further

concentration to be tested: 7% NaCl / 20% methanol)

• Proposed to investigate further mixed salt / organic inhibitors with NG

• Importance of gas to aqueous solution ratios (480 bar)– Gas to aqueous solution ratios do influence hydrate

dissociation conditions– Methanol loss to gas (methane) phase reduces inhibitor

concentration in aqueous phase– May become more significant at low water

concentrations

Flow Assurance: Gas Hydrate and Wax - June 2004 Steering Committee Meeting

Thermodynamic ModellingSalts and Organic Inhibitors

Rahim (Amir) Masoudi


Outline• Thermodynamic modelling of KCl-

EtOH, NaCl-EtOH, and NaCl-MeOH

• Validation of the model for gas hydrate

• Extension of the newly developed correlation for NaCl-EtOH

• Conclusions


Thermodynamic modelling: salt precipitation

Formation Water

Organic Inhibitors

Salt deposition

Temperature reductions as fluids are transported from the reservoir to the surface.Concentration of the brine increases as produced or injected gas strips water, leaving the salt behind.Reduction in CO2 concentration in the aqueous phase can result in the deposition of bicarbonate as carbonates.Incompatibility between formation water and sea waterAddition of hydrate organic inhibitors reduces salt Addition of hydrate organic inhibitors reduces salt solubility in the aqueous phase.solubility in the aqueous phase.


Effect of salt precipitation on hydrate formation and vice versa

Gas

Saline solution

Hydrate formation

Remaining aqueous phase becomes concentrated

Salt formation!!!

Salt formation

Less hydrate inhibition effect for aqueous phase

Hydrate formation!!!


Thermodynamic modelling

• The new thermodynamic approachSalt is treated as a pseudo component while its critical properties and acentric factor are optimised.Valderrama-Patel-Teja (VPT) EoSNon-Density Dependent (NDD) Mixing RulesSolid solution theory of van der Waals and Platteeuw

• Data requirements:Initial guess for Critical properties of salt (TC, PC, VC)Experimental data

Freezing point of salt aqueous solutionsBoiling point of salt aqueous solutionsSalt solubility


Thermodynamic modelling

• Binary Interaction Parameters (BIPs) OptimisationWater-SaltSalt-SaltSalt-Organic InhibitorHydrocarbon-Salt

• NaCl, KCl and CaCl2 as well as MEG have already been modelled.


Capabilities of the modelCapabilities of the model

• Salt precipitation

• Hydrate stability zone

• Maximum hydrate inhibition effect

• Gas solubility

• Freezing point prediction

• Boiling point prediction

• Vapour pressure prediction

• Composition of all present equilibrium phases


Modelling KCl and ethanolModelling KCl and ethanol

• Experimental and calculated freezing point temperature for ternary KCl/EtOH/water mixtures

KCl EtOH Experimental Calculated AD(mass%) (mass%) (oC ± 0.2) ( oC ) ( oC )

2.74 2.70 -2.80 -2.59 0.215.14 5.08 -5.40 -5.38 0.027.50 7.45 -8.60 -8.65 0.0515.17 5.20 -12.50 -12.50 0.0010.33 10.19 -13.00 -13.25 0.255.51 19.56 -17.00 -17.06 0.06

Freezing Point temperature


Modelling KCl and ethanolModelling KCl and ethanol

• Experimental and calculated boiling point temperature for ternary KCl/EtOH/water mixtures

KCl EtOH Experimental Calculated AD(mass%) (mass%) (oC ± 0.1) ( oC ) ( oC )

2.33 2.29 96.45 97.20 0.755.13 5.12 92.80 93.90 1.1015.00 4.95 91.70 91.75 0.057.82 7.64 90.15 90.96 0.819.39 9.43 87.30 88.98 1.6812.49 12.44 86.10 85.50 0.604.84 16.09 86.00 86.90 0.90

Boiling Point temperature


Modelling salt precipitationModelling salt precipitation

• Comparison of the available literature data for KCl solubility in EtOH aqueous solutions.

0

6

12

18

24

30

36

0 10 20 30 40 50

EtOH / mass%

KCl /

mas

s%

Table N 935, Stephens (1963)Table N 938, Stephens (1963)Table N 939, Stephens (1963)Table N 942, Stephens (1963)Exp. data, this workPrediction

T=25 oC



• Solubility of KCl in aqueous ethanol solutions as a function of temperature.

0

5

10

15

20

25

30

35

10 15 20 25 30 35 40 45 50 55 60 65 70 75

T / oC

KCl /

mas

s%

0 mass% EtOH, Stephens (1963)

10 mass% EtOH, this work



Predictions, this work



• Solubility of KCl in aqueous ethanol solutions as a function of ethanol concentration.

0

5

10

15

20

25

30

35

0 5 10 15 20 25 30

EtOH / mass%

KCl /

mas

s%

10 deg C25 deg C50 deg C75 deg CPredictions, this work

Exp. data: this work






• Conclusions


Methane hydrate phase boundaries in the presence of NaCl and EtOH aqueous solutions

10

1000

-23 -19 -15 -11 -7 -3 1 5 9 13 17 21 25

T / C

P / b

ar

Exp., distilled waterExp., 10 mass% NaCl + 10 mass% EtOHExp., 7 mass% NaCl + 30 mass% EtOHPredictions

C1-distilled water data:Deaton and Frost (1946)Mcleod and Campbell (1961)Jhaveri and Robinson (1965)


Methane hydrate phase boundaries in the presence of KCl and EtOH aqueous solutions

10

100

1000

-20 -15 -10 -5 0 5 10 15 20 25

T / C

P /

bar

C1- distilled water

10 Mass% KCl / 10 mass% EtOH

7 mass% KCl / 20 mass% EtOHPredictions

C1-distilled water data:Deaton and Frost (1946)Mcleod and Campbell (1961)Jhaveri and Robinson (1965)


NG hydrate phase boundaries in the presence of NaCland MeOH aqueous solutions

NG CompositionC1 87.32C2 5.67C3 1.68I-C4 0.23n-C4 0.4I-C5 0.1N2 3.24CO2 1.36

100

1

10

100

1000

-23 -18 -13 -8 -3 2 7 12 17 22 27

T / oC

P /

bar

distilled water10 mass% NaCl / 10 mass% MeOH7 mass% NaCl / 20 mass% MeOHPredictions






• Conclusions


Existing correlations• No general correlation for a combination

of salts and/or organic inhibitors

• Shortcomings:

Effect of the system pressure

Effect of the gas/oil composition

Effect of the type of the inhibitor


New CorrelationNew Correlation

- P: Pressure of the system (kPa)- W: Concentration in the solution (mass%)- P0: Dissociation pressure in the presence of

distilled water at 273.15 K (kPa)- Ci and D1: Constants

PDPWW

TPWW

WT

WPWW

T ISI

IS

IS

IS

SSI *

*021.0*

**

* 1

+

+∆+

+∆+

=∆

orST∆ ( )( )( )1)1000()ln( 06543

32

21 +−+++=∆ PCCPCWCWCWCT IIII


Methane hydrate phase boundaries in the presence of NaCl aqueous solutions

10

100

1000

-13 -8 -3 2 7

T / C

P /

bar

Exp., 11.8 mass% NaClExp., 21.5 mass% NaClNew CorrelationHammerschmidt CorrelationYousif & Young Correlation

Exp. data: de Roo et al. 1983


Methane hydrate phase boundaries in the presence of NaCl and KCl aqueous solutions

10

100

-11 -6 -1 4 9

T / C

P / b

ar

Exp., 3 mass % NaCl + 3 mass% KClExp., 5 mass% NaCl + 10 mass% KClExp., 5 mass% NaCl + 15 mass% KClNew CorrelationYousif & Young CorrelationPure water, HWHYD model

CH4 Hydrate

exp. data: Dholabhai, 1991


Methane hydrate phase boundaries in the presence of NaCl and EtOH aqueous solutions

10

100

1000

-18 -13 -8 -3 2 7 12 17 22 27

T / oC

P / b

ar

distilled water, HWHYD model10 mass% NaCl / 10 mass% EtOH7 mass% NaCl / 30 mass% EtOHNew correlation

Exp. data: this work


Conclusions• Modelling NaCl-EtOH and KCl-EtOH was successfully

implemented.

• Precipitation of KCl in EtOH aqueous solutions was modelled.

• Comparison with the independent experimental data, has demonstrated the reliability of the developed model.

• The results show that the model can be reliably applied to double hydrates as well as simple hydrates.

• Newly developed correlation capable of predicting hydrate inhibition effect of salts and/or organic inhibitors was extended to NaCl-EtOH systems.

FLOW ASSURANCE: GAS HYDRATES AND WAX 2002-2005 PROGRAMME

Experimental work Hydrate and Wax measurements

December 2003-May 2004

Rod BurgassFlow Assurance: Gas Hydrate and Wax – June 2004 Steering Committee Meeting

EXPERIMENTAL WORK: WAX AND HYDRATE MEASUREMENTS

• HYDRATE PHASE BOUNDARY MEASUREMENTS FOR AN OIL IN THE PRESENCE OF AQUEOUS SOLUTIONS OF ETHANOL AND ETHANOL/NaCl

• WAX AND HYDRATE MEASUREMENTS ON SYNTHETIC MIXTURES OF ALKANES

• WAX MEASUREMENT ON HEAVY OIL SAMPLE USING QUARTZ CRYSTAL MICROBALANCE (QCM)

Flow Assurance: Gas Hydrate and Wax – June 2004 Steering Committee Meeting

HYDRATE PHASE BOUNDARY MEASUREMENTS FOR AN OIL IN THE PRESENCE OF AQUEOUS

SOLUTIONS OF ETHANOL AND ETHANOL/NaCl

• APPARATUS AND METHOD

• OIL COMPOSITION

• HYDRATE STABILITY ZONE FOR OIL WITH - 16 MASS% ETHANOL- 30 MASS% ETHANOL- 30 MASS% ETHANOL 5 MASS% NaCl

• OIL BUBBLE POINT MEASUREMENT

Schematic of ultra high pressure cell (up to 2000Bar)

HIGH PRESSURE CELL

WATER JACKET

PRESSURE TRANSDUCER

CONSTANT TEMPERATURE BATH PRT

Composition of oil

Component Mass%

Mole% Component Mass% Mole%

CO2 0.35 0.67 C10s 4.04 2.42 N2 0.96 2.92 C11s 3.62 1.97 C1 9.71 51.52 C12s 2.95 1.48 C2 1.14 3.24 C13s 3.56 1.64 C3 0.46 0.89 C14s 2.88 1.24 iC4 0.07 0.11 C15s 2.94 1.18 nC4 0.18 0.26 C16s 2.51 0.94 iC5 1.18 1.39 C17s 1.82 0.64 nC5 1.63 1.92 C18s 2.17 0.73 C6s 2.45 2.42 C19s 2.30 0.73 C7s 4.12 3.50 C20s 1.48 0.45 C8s 5.47 4.08 C21+ 37.74 10.84 C9s 4.27 2.84

Summary of dissociation point data for oil with distilled water and in the presence of aqueous

solutions of ethanol and ethanol/NaCl

10.0

100.0

1000.0

10000.0

-10 -5 0 5 10 15 20 25 30 35T/C

P/ba

r

Distilled water16 mass% ethanol30 mass% ethanol30 mass% ethanol 5 mass% NaCl

Summary of dissociation point data for oil with distilled water and in the presence of aqueous

solutions of ethanol and ethanol/NaCl

0.0

400.0

800.0

1200.0

1600.0

-10 -5 0 5 10 15 20 25 30 35T/C

P/ba

r

Distilled water

16 mass% ethanol

30 mass% ethanol

30 mass% ethanol 5 mass% NaCl

Bubble point measurement using constant volume method

290

300

310

320

330

340

350

360

12 14 16 18 20 22 24 26T/C

P/ba

r

BUBBLE POINT 310bar @ 19C

WAX AND HYDRATE MEASUREMENTS ON SYNTHETIC MIXTURES OF ALKANES

• Apparatus and methods• Experimental WAT/WDT and hydrate

dissociation point measurements- Base fluid WAT at different pressures- Live fluid 45.1 mole% gas (LSF1), WAT at different

pressures and hydrate dissociation point measurements

- Live fluid 69.0 mole% gas (LSF2), WAT and WDT measurements and hydrate dissociation point measurements

Schematic of high pressure (52MPa) visual rig

Composition of base synthetic hydrocarbon mixture

Component

Mass%

Mole%

C10 94.61 97.52 C21 1.71 0.84 C22 1.25 0.59 C23 0.92 0.41 C24 0.67 0.29 C25 0.49 0.20 C26 0.36 0.14

Composition of Live Synthetic Hydrocarbon Mixture (LSF 1), 45.1 mole% gas. Bubble point measured as

119.7bar at 0ºC

Component

Mass%

Mole%

CO2 0.46 1.46 C1 7.13 39.42 N2 0.30 0.61 C2 0.87 2.56 C3 0.38 0.76 iC4 0.07 0.10 nC4 0.12 0.18 iC5 0.04 0.05 C10 85.76 53.49 C21 1.55 0.46 C22 1.14 0.32 C23 0.83 0.23 C24 0.61 0.16 C25 0.44 0.11 C26 0.32 0.08

Experimental wax data measured on base synthetic fluid and wax and hydrate data measured on live

synthetic fluid (LSF1) 45.1 mole% gas

0.00

100.00

200.00

300.00

400.00

0 2 4 6 8 10 12 14 16 18 20

T/C

P/ba

r

Base synthetic fluid

Live synthetic fluid

Hydrate dissociation points

Composition of Live Synthetic Hydrocarbon Mixture (LSF2), 69.0 mole% gas. Bubble point measured as

263bar at 15.3ºC

Component

Mass%

Mole% CO2 0.71 0.94

C1 16.61 60.24 N2 1.08 2.24 C2 2.02 3.91 C3 0.88 1.16 iC4 0.16 0.16 nC4 0.28 0.28 iC5 0.09 0.07 C10 73.97 30.24 C21 1.34 0.26 C22 0.98 0.18 C23 0.72 0.13 C24 0.52 0.09 C25 0.38 0.06 C26 0.28 0.04

WAT and WDT (measured by visual method) and hydrate dissociation points for LSF2 (69.0 mole%

gas)

0

100

200

300

400

-2 2 6 10 14 18 22 26

T/C

P/ba

r

WAT LSF2Hydrate DP LSF2

Base synthetic fluidWDT LSF2

Graph showing the effect of increasing gas content on the wax phase boundary for three synthetic fluids

0.00

100.00

200.00

300.00

400.00

-2 0 2 4 6 8 10

T/C

P/ba

r

Base synthetic fluid

WAT LSF1

WAT LSF2

Experimental wax and hydrate data measured on LSF 2

0

100

200

300

400

-2 2 6 10 14 18 22 26

T/C

P/ba

r

WAT LSF2

Hydrate DP LSF2

WAT LSF2 withhydrate

WAX MEASUREMENT ON HEAVY OIL SAMPLE USING QUARTZ CRYSTAL MICROBALANCE (QCM)

• Apparatus and method

• Test fluid

• Results of measurement

Schematic of QCM wax measurement rig

Example of WAT/WDT measurement for North Sea dead crude using QCM

-2600

-2100

-1600

-1100

-600

-100

28 33 38 43 48 53 58 63T/C

Cha

nge

in re

sona

nt fr

eque

ncy/

Hz

Cooling

Heating

WATWDT

Example of WDT measurement for heavy oil using QCM

-16500

-14500

-12500

-10500

-8500

-6500

-4500

-2500

-500

50 54 58 62 66 70 74 78 82 86 90 94

T/C

Cha

nge

in re

sona

nt fr

eque

ncy/

Hz

WDT ~80C

Flow Assurance: Gas Hydrates and Wax - June 2004 Steering Committee Meeting

Thermodynamic Modelling -Wax

Hongyan Ji


• Background– Two predictive wax models– The semi-empirical wax model and its required

parameters– The correlation for estimating solid composition

• Improvements made in this work– Improving the solid (s) correlation– Predicting WDTs for unsaturated fluids– Validations and Discussions– Conclusions

Outline


BackgroundTwo predictive wax models are being developed in this study:

• A compositional wax model - HWWAX– Used to predict wax phase boundary, wax amount and wax

composition.– Required input data: full composition of the studied fluid.– It has been validated using multi-component systems.

• A semi-empirical wax model– Used to predict wax phase boundary.– Required input data: limited composition of the studied fluid.


• Both liquid and solid phases are described using the activity coefficient equation.

• xk: the liquid composition of the heaviest component• Empirical parameters

– a & b: determined by fusion properties– γ : activity of components in the solid phase– s: solid composition

( ) 1527310201 .P.b

sx

lnaWDT

kSkk

kk

−−×+

+

=

γ (WDT in oC and P in bar)

Background: the semi-empirical wax model


Advantages of the semi-empirical wax model

• Input: xk• Output: WDT

• Input: WDT measured at atmospheric pressure

• Output: xk– The xk estimation is easier due to the

simplicity of the equation.

• The estimated xk can be used to calculate WDTs in the presence of light ends and/or at other pressures.

( ) 1527310201 .P.b

sx

lnaWDT

kSkk

kk

−−×+

+

=

γ


Background: the semi-empirical wax modelIn the previous meeting• a & b

– a: based on binary SLE data.– b: based on the fusion temperatures of pure

compounds.

• γ– A constant value was obtained by matching binary

SLE data.• s

– Assumed: the solid phase consists of two components.

– Developed: a two-solids (st) correlation using binary SLE data.


Background: the semi-empirical wax modelIn the previous meeting• The semi-empirical wax model in conjunction

with the two-solids (st) correlation was used to estimate wax phase boundaries for saturated fluids.

• In general, the predictions were acceptable.• However, the predictions for systems with

small amounts of wax-forming compounds were not satisfactory, possibly due to the limitation of the st correlation.


Tasks for this meeting

• To improve the solid (s) correlation to take into account multiple compounds coexisting in the solid phase.

• To examine the semi-empirical wax model for unsaturated fluids.


• A multiple solids (sm) correlation is needed.

• To develop the sm correlation, composition data of solidformed in multi-component systems are required.

• It is difficult to obtain these data directly from the experimental measurements.

• The HWWAX model is used to generate the required data.

• For generating the required data, the mixtures are designed to model simplified reservoir fluids.

Improving the solid (s) correlation


Compositions of a reservoir fluid

Experimental data of composition for single carbon numbers (SCN) and normal paraffins of a real reservoir fluid (Pan et al., 1997).

ySCN = 0.1835e-0.1058x

yn-paraffin = 0.0828e-0.1577x

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0 5 10 15 20 25 30 35 40 45 50

Carbon number

Mol

e fra

ctio

n

SCN

n-paraffins


• An exponential decay function expressed as is used to design the mixtures.

• Different q values between 0.1 and 0.95 are used.

• For each q value, different C20+ amounts are considered.

1−×= ii xqx

Improving the solid (s) correlation – cont.


• The HWWAX model was further examined before it was used for generating the data required in this study.

• A mixture containing 2.5 mole% C20+ with q of 0.7 was prepared in this laboratory.

• The WDT was measured by visual as 3 oC at 1 bar in this lab.

• The WDT independently predicted using the HWWAX model was 3.4 oC.



• Then the HWWAX model was used to calculate WDTs for a series of mixtures with different qand C20+ amounts.

• The solid composition of the heaviest component was plotted against q for different C20+ amounts.



C20+ ≥ 1 mole%

Solid compositions as functions of q

0.0

0.2

0.4

0.6

0.8

1.0

0 0.2 0.4 0.6 0.8 1

q

Cm

ax s

olid

mol

e fra

ctio

n

C20+ mole% = 1C20+ mole% = 2.5C20+ mole% = 5C20+ mole% = 10C20+ mole% = 20C20+ mole% = 40C20+ mole% = 80C20+ mole% = 100


C20+ ≥ 1 mole%

0.0

0.2

0.4

0.6

0.8

1.0

0 0.2 0.4 0.6 0.8 1

q[(C20+ mole %)-0.11]

Cm

ax s

olid

mol

e fra

ctio

n

C20+ mole% = 1C20+ mole% = 2.5C20+ mole% = 5C20+ mole% = 10C20+ mole% = 20C20+ mole% = 40C20+ mole% = 80C20+ mole% = 100Regressed data y = -4.2265x3 + 7.3014x2 -

2.4306x + 0.1803

Solid compositions as a function of q(C20+ mole%)-0.11


C20+ < 1 mole%

0.0

0.2

0.4

0.6

0.8

1.0

0 0.2 0.4 0.6 0.8 1

q[(C20+ mole %)-0.11]

Cm

ax s

olid

mol

e fra

ctio

n

C20+ mole% = 0.1C20+ mole% = 0.25C20+ mole% = 0.50C20+ mole% = 0.75C20+ mole% = 1Regressed data

y = -3.3789x3 + 6.5327x2 - 2.4675x + 0.2157

Solid compositions as a function of q(C20+ mole%)-0.11


• For C20+ ≥ 1 mole%

• For C20+ < 1 mole%

( )[ ] ( )[ ]( )[ ] 1803043062

3014722654110

20

211020

311020

.%moleCq.

%moleCq.%moleCq.s.

..k

+××−

××+××−=−

+

−+

−+

( )[ ] ( )[ ]( )[ ] 2157046752

5327637893110

20

211020

311020

.%moleCq.

%moleCq.%moleCq.s.

..k

+××−

××+××−=−

+

−+

−+

Multiple solids (sm) correlation


• It is difficult to consider the vapour phase using the semi-empirical model.

• The WDT for an unsaturated fluid is estimated using those for saturated fluids.– The WDT is calculated for the fluid without light ends

at atmospheric pressure.– The WDT is calculated for the fluid with full light ends

at the bubble point pressure.– The WDT for a unsaturated fluid is estimated by

interpolating the above two data using pressure.

Predicting WDTs for unsaturated fluids


Compositions (mole%) for Mixtures A, B and C (Daridonet al., 1996).

Component A B CC1 43.70 43.80 43.60C10 46.10 45.90 46.15C18 0.00 1.65 1.33C19 0.00 1.43 1.27C20 3.27 1.25 1.16C21 2.24 1.15 1.10C22 1.53 0.91 1.04C23 1.05 0.78 0.98C24 0.72 0.67 0.92C25 0.49 0.58 0.87C26 0.34 0.50 0.81C27 0.23 0.43 0.77C28 0.16 0.37 0.00C29 0.11 0.31 0.00C30 0.07 0.27 0.00C20+ 6.93 5.96 6.50

q 0.68 0.86 0.94


Validations and discussions

Experimental (Daridon et al., 1996) and predicted WDT data for Mixtures A and B (st: two-solids correlation; sm: multiple-solids correlation).

0

100

200

300

400

500

15 20 25 30 35 40

WDT/oC

P/ba

r

A: exp. dataA: predictions (st)A: predictions (sm)B: exp. dataB: predictions (st)B: predictions (sm)C: exp. dataC: predictions (st)C: predictions (sm)


Experimental (Srivastava et al, 2002)) and estimated WDT data for mixtures based on Distillate Fraction 4 (st: two-solids correlation; sm: multiple-solids correlation).


-5

5

15

25

35

45

0 10 20 30 40 50 60 70 80 90 100

n-paraffin concentration /mass%

WD

T/oC

Exp. data, mixtures consisiting of n-, iso-, & cyclo-paraffinsExp. data, fraction 4 distillated from crude oil Estimated data, empirical model with st correlationEstimated data, empirical model with sm correlation

Trendline of exp. data


Experimental (Srivastava et al, 2002)) and estimated WDT data for mixtures based on Distillate Fraction 5 (st: two-solids correlation; sm: multiple-solids correlation).


-5

5

15

25

35

45

55

0 10 20 30 40 50 60 70 80 90 100

n-paraffin concentration/mass%

WD

T/o C Exp. data, mixtures consisiting

of n-, iso-, & cyclo-paraffinsExp. data, mixtures consisitingof n-paraffins and aromaticsExp. data, fraction 5 distillatedfrom crude oil Estimated data: st

Estimated data: sm

Trendline of exp. data


Compositions (mole%) of fluids with 0, 45 and 69 mole% light ends (this lab).

mole% light end

0 45 69C1 39.424 60.24C2 2.560 3.91C3 0.758 1.16iC4 0.104 0.16nC4 0.181 0.28iC5 0.045 0.07N2 1.463 2.24

CO2 0.614 0.94C10 97.516 53.489 30.24C21 0.845 0.463 0.26C22 0.591 0.324 0.18C23 0.414 0.227 0.13C24 0.290 0.159 0.09C25 0.203 0.111 0.06C26 0.142 0.078 0.04

the amount of wax-forming compoundsC21+ 2.484 1.363 0.770

Bubble pointT/oC 0.0 15.3

Pb/bar 119.7 263.0


Measured and predicted wax phase boundaries for fluids with 0 and 45 mole% light ends (st: two-solids; sm: multiple-solids).


0

50

100

150

200

250

300

350

400

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28

T/oC

P/ba

r

exp. WATs: no light endsexp. WATs: 45 mole% light ends

HWWAX predictionsEmpirical model predictions (st)Empirical model predictions (sm)


Measured and predicted wax phase boundaries for the fluid with 69 mole% light ends (st: two-solids; sm: multiple-solids).


0

100

200

300

400

-2 0 2 4 6 8 10 12 14

T/oC

P/b

ar

Exp. WATs: 69 mole% light endsExp. WDTs: 69 mole% light endsHWWAX predictionsEmpirical model predictions (st)Empirical model predictions (sm)


DiscussionsFor the fluid containing 69 mole% light ends• The wax phase boundary predicted using the semi-

empirical model was not in line with the measurements.

• To analyse the reason, the solid compositions of C26was calculated– 0.59 using the HWWAX model.– 0.44 using the two-solids (st) correlation.– 0.57 using the multiple-solids (sm) correlation.

• The deviations may due to the activity coefficient equation used for describing the liquid phase.


• New correlations have been developed to take into account multiple components in the solid phase.

• The multiple-solids correlations have combined the effects of C20+ amount and the exponential decay coefficient on WDT.

• For fluids with approximately 45 mole% light ends, a good agreements was observed between the experimental data and the predictions using the semi-empirical wax model in conjunction with the multiple-solids correlation.

• It was also found that the accuracy of the semi-empirical wax model was limited for a fluid with 70 mole% light ends.

Conclusions


ThanksThanks


Water Content of Gases


• Collecting the existing VLE data

• Tuning the BIPs in the model for predicting the water content of gases at low temperature conditions

• Developing a semi – empirical approach

Outline


A Typical P-T Diagram for Water–Hydrocarbon System

T

log

(P)

H - LHC

H-V

HC Vapour Pressure

I-V

LW - V

Q 2

Q 1I-H-V Water Vapour Pressure

H-V

H - LHC

L W -H-V

L W -H-L HC

Ice Line


Typical Water Content of Methane at 100 bar

1

10

100

1000

-40 -30 -20 -10 0 10 20 30 40 50

T/ °C

mg/

Nm3

Chapoy et al. (2003)cGERG (2000)KSEPL WAGA (Supplied by Shell)GPA RR45 by interpolation (Supplied by Shell)Dhima et.al. (2000)Ugrozov (1996) + Olds et.al. (1942)Yarym - Agaev et al. (1985)Kosyakov et al. (1982)STFlash phase transitions (Supplied by Shell)

H-V region

Lw- V region


Which Data?

• Vapour-Liquid Equilibria– Gas solubility in the water rich– Water mole fraction in the gas phase

• Gas Solubility Data for Tuning the BIPs at low temperature conditions.


Model Description

• fwH = fw

V H-V Equilibrium

• fiLw= fi

V (i = 1, N) Lw-V Equilibrium

• fwI = fw

V I-V Equilibrium


Experimental and calculated C2solubility (mole fraction)

0

0.2

0.4

0.6

0.8

1

1.2

0 1 2 3 4 5

P /MPa

Eth

ane

Solu

bilit

y (x

10

3 )

Hydrate Phase Boundary

Ethane Vapour Pressure

T


Experimental and calculated N2solubility (mole fraction)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 2 4 6 8 10

P /MPa

Nitr

ogen

Sol

ubili

ty /

Mol

Fra

ctio

n x

10 3


Experimental and predicted Water Content of N2 (mole fraction)

0.00001

0.00010

0.00100

0.01000

0.10000

1.00000

0 2 4 6 8 10 12 14 16

P /MPa

Wat

er c

onte

nt /M

ol f

ract

ion


Semi – Empirical Approach

• fiLw= fi

V (i = 1, N) :VLE

• yw=

• yw=

• φw= exp(BP + CP 2)

• B = a + & C = c + Tb

Td

))(exp()1(

RTPPv

PPx sat

wLw

w

satwwg −−

φγ

))(exp(RT

PPvP

P satw

Lw

w

satw −

φ


A Comparison Between the Predictions of This Approachand Bukacek Correlation for Water Content (mole fraction) of Natural Gas

T /K P /MPa SGg Experimental

Water Content

Predicted Water

Content, This WorkAD %

Predicted Water

Content, Bukacek

(1955)

AD %

NG1

273.15

273.15

278.15

278.15

283.15

283.15

288.15

288.15

288.15

0.500

1.500

0.500

1.500

1.500

6.000

1.500

6.000

10.000

0.5654

0.5654

0.5654

0.5654

0.5654

0.5654

0.5654

0.5654

0.5654

1.169E-03

4.26E-04

1.68E-03

6.05E-04

8.42E-04

2.51E-04

1.16E-03

3.56E-04

2.50E-04

1.25E-03

4.39E-04

1.79E-03

6.25E-04

8.78E-04

2.68E-04

1.22E-03

3.69E-04

2.60E-04

6.93

3.05

6.55

3.31

4.28

6.77

5.17

3.65

4.00

1.24E-03

4.62E-04

1.82E-03

6.53E-04

9.11E-04

2.97E-04

1.25E-03

4.02E-04

2.88E-04

6.07

8.45

8.33

7.93

8.19

18.33

7.76

12.92

15.20


Conclusions

• A comprehensive literature survey was made on the existing water content data.

• BIPs between Ethane – Water and N2 – Watersystems were tuned using new gas solubility data.

• The results of water content predictions are comparable with the experimental data.


Conclusions (Continue)

• The semi – empirical approach was developed to predict the water content of gases in the Lw – V and I – V regions using more data. The results are in good agreement with experimental data.


HYDRATE STABILITY ZONE IN OIL SYSTEMS


Outline

• To develop the HWHYD model for predicting hydrate stability zone at high-pressure conditions.

• To investigate the effect of bubble point on prediction of hydrate phase boundary.


Various hydrate stability zones and phase envelope for a multi-component system.

Pres

sure

Temperature

•C

Q1

Bubble point line

Dew point line

I-H-V Lw-H-V

Lw-LHC-H-V

Lw-LHC-HLog scale Ice line

Pres

sure

Temperature

•C

Q1

Bubble point line

Dew point line

I-H-V Lw-H-V

Lw-LHC-H-V

Lw-LHC-HLog scale Ice line


Thermodynamic model

• Equality of fugacity:– Fluid Phases: VPT-EoS and NDD mixing rules– Hydrate Phase: van der Waals-Platteeuw theory

• Equation of State:– Using a new α function for water– Tuning EoS using bubble point data at low-

temperature conditions


Preliminary Results (Synthetic live oil )

10

100

1000

10000

5 10 15 20 25 30 35

T /C

P/ba

r

Experimental (HWU)

This Prediction

Bubble Point 1 K


Discussion

• Effect of Heavy Hydrate Formers– Single carbon numbers (SCN): No information on

single heavy hydrate formers is available

• Effect of Bubble Point Measurement – Bubble point measurement at low-temperature

conditions


Conclusions

• Low temperature bubble point data were used for the tuning the thermodynamic model.

• The results for hydrate formation conditions were relatively comparable with experimental data.

flow assurance: gas hydrates and wax · flow assurance: gas hydrates and wax – june 2004 steering...

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