Gas Hydrates in Low Water Content Gases: Experimental Measurements and ModellingUsing the CPA EoS

Antonin Chapoy, Hooman Haghighi, Rod Burgass and Bahman TohidiHydrafact Ltd. & Centre for Gas Hydrate ResearchInstitute of Petroleum Engineering

Heriot-Watt University Edinburgh EH14 4AS, UK

Ecole des Mines de Paris - Paris, France, Thursday, September 3th, 2009

Outline

• Introduction / Scope of work

• Experimental

– Materials

– Experimental setup

– Procedures

– Validation

• Thermodynamic Modelling

• Results - Discussions

• Remarks and Conclusions

Background• Natural gases are normally saturated

with water at reservoir conditions

• Reducing the water content of gas streams is commonly used as a means of preventing gas hydrate (gas lift..)

• However, severe hydrate blockages have occurred in pipelines transporting so-called dry gas

• Capability to accurately predict the water content is therefore essential to plan potential flow assurance issues associated with condensed water

• Lack of experimental data, especially for gas mixtures

What are gas hydrates ?What are gas hydrates ?

• Gas hydrates or clathrate

hydrates are:

– Ice-like crystalline

compounds

– Composed of water + gas

(e.g. methane, CO2)

– Formed under low

temperatures and elevated

pressures

– Stable well above the ice-

point of water Methane hydrate: the

burning snowball

Hydrate StructuresHydrate Structures

3

16

2

2 Methane, ethane, carbon dioxide….

Propane, iso-butane, natural gas….

Methane + neohexane, methane + mch….

6

8

1 Structure H

Structure 2

Structure 1

51268

51264

51262

512

435663

+

P T and

suitable guests

Flow Assurance- Hydrates: The problems

• Hydrate blockages are major

flow assurance problems in

offshore and deep water

operations

• Economic and safety hazard

• Challenges

– Long tiebacks

– High pipeline residence times

– Low T / high fluid P Gas hydrates removed from

a subsea transfer line

(Courtesy of Petrobras)

Avoiding Hydrate Problems - Current practice

• Increasing the system temperature- Insulation- Heating

• Reducing the system pressure

• Injection of thermodynamic inhibitors- Methanol, ethylene glycol, ethanol

• Using Low Dosage Hydrate Inhibitors- Kinetic Inhibitors (KHI)- Anti-Aggglomerants (AA)

•• Water removal (Water removal (dehydratationdehydratation))

• Combinations of the above

• New Approach: Cold Flow

P

No Hydrates

HydratesWellhead

conditions

P

No Hydrates

HydratesWellhead

conditions

P

No Hydrates

HydratesWellhead

conditions

P

No Hydrates

HydratesWellhead

conditions

Experimental

• Materials

– Methane (99.995%) from BOC

– Ethane, Propane, nButane, CO2, N2: 99.9%+ from BOC

– Distilled water

• Systems

– Made gravimetrically and checked by GC

0.5--CO2

2--N2

-3-nC4 H10

1.56-C3 H8

25-C2 H6

9486100CH4

System 3 (sII)System 2 (sII)System 1 (sI)Component

Water Content Measurements

• Experimental setup

P Transducer

T Probe

Cooling Jacket

2-way valve

2-way valve

Equilibrium Cell

Mixing Ball

Piston

Pivot

Cooling Fluid in/out

P Transducer

T Probe

Cooling Jacket

2-way valve

2-way valve

Equilibrium Cell

Mixing Ball

Piston

Pivot

Cooling Fluid in/out Main Characteristics:

Titanium piston vessel

Pmax: 70 MPa

Tmin: 193 K

Tmax: 323 K

T ±0.1K

P ±0.003 MPa

Water Content Measurements

• Schematic of the SpectraSensorsTM SS2000

TDLAS set-up

P Transducer

Sample in

Sample out

Mirror

Detector

Laser

P Transducer

Sample in

Sample out

Mirror

Detector

Laser

NLSI

I o

××=

ln

Main Characteristics:

Beer law

Standard error TDLAS

set-up is the greater of 4

ppm or 2% of the

reading.

Thermodynamic Modelling

• For VLE or VHE, we have:

• CPA EoS:

• For Hydrate: Solid solution theory of van der Waals and Platteeuw

( )∑∑ −

∂

∂+−

+−

−=

i

i

A

A

i

i

mmmm

Xxg

V

RT

bVV

Ta

bV

RTP 1

)ln(1

2

1

)(

)(

ρρ

SRK part Association part

LV ff = or HV ff =

∆−=

−

RTff

Hw

w

H

w

ββ µ

exp ∑ ∑

+=−=∆ −

m j

jmjmHww

Hw fCvRT 1lnµµµ ββ

where

Thermodynamic Modelling

• BIPs between water and gases adjusted using gas solubility data:

• Example: methane solubility in water

exp,

,exp,

1

1

i

caliin

x

xx

NFOB

−= ∑

0

0.001

0.002

0.003

0.004

0 10 20 30 40 50

P / MPa

CH

4 S

olu

bili

ty /

mo

le f

ract.

Culberson et al. (1951)Duffy et al. (1961)Yokoyama et al. (1988)Wang et al. (1995)Yang et al. (2001)Kim et al. (2003)Chapoy et al. (2004)

298.15 K

0

0.001

0.002

0.003

0.004

0.005

0 10 20 30 40 50 60 70

P / MPa

CH

4 S

olu

bili

ty /

mole

fra

ct.

Culberson et al. (1951)

Amirjafari and Campbell (1972)

344.26 K

Validations of the model

• Predictions of water content – System CH4 - Water

0

50

100

150

200

250

300

350

400

0.0001 0.001 0.01 0.1

Water content / mole fraction

P /

MP

a

Model (VLE)

Model (HSZ)

Althaus (1999)

Kosyakov and Ivchenko (1982)

Chapoy et. al (2003)

Rigby and Prausnitz (1968)

Yokoyama (1988)

Yarym-Agaev et. al (1985)

Rigby and Prausnitz (1968)

Water Content Measurements

• Validations – Water content measurements in

methane in equilibrium with liquid water

0

2

4

6

8

10

12

14

100 1000 10000

yw / ppm

P/

MP

a

288.55 K

282.65 K

278.25 K

273.15 K

Model

HSZ

Experimental Results – Model Predictions

• System 1: Methane at 3.44 MPa

10

100

1000

250 255 260 265 270 275 280 285 290

T/ K

yw /

pp

m

This work

data from Song et al. (2004)

data from Aoyagi et al. (1979)

Model Predictions

AAD = 6.1 %

Experimental Results – Model Predictions

• System 1: Methane at 6.89 MPa

10

100

1000

250 255 260 265 270 275 280 285 290

T/ K

yw /

ppm

This work

data from Song et al. (2004)

data from Aoyagi et al. (1979)

Model Predictions

AAD = 1.9 %

Experimental Results – System 2

• Experimental conditions

0

5

10

15

20

25

30

35

40

140 160 180 200 220 240 260 280 300

T/ K

P/

MP

a

Experimental Results – System 2

• Results

1

10

100

1000

250 255 260 265 270 275 280 285

T/ K

yw/

ppm

5 MPa10 MPa40 MPaModel

AAD = 2.8 %

Experimental Results – System 3

• Experimental conditions

0

5

10

15

20

25

30

35

40

150 170 190 210 230 250 270 290

T/ K

P/

MP

a

Experimental Results – System 3

• Results

1

10

100

1000

250 255 260 265 270 275 280 285

T/ K

yw/

pp

m

5 MPa

10 MPa

40 MPa

Model

AAD = 5.1 %

Experimental Results

• Correlation (GPA conference 2006)

with

TdcB

TbaA

BPAPw

+=+=

+= )exp( 2φ

))(

exp(RT

PPv

P

Py

sat

wLw

w

satw

w

−=

φ

0.0423158

-14.4573

-0.002303

0.55955

-0.0914835

15.896128

-0.02518

7.3906

0.027453

-18.76311

-0.0003318

0.17965

a

b

c

d

Hydrate - sIIHydrate - sILiquid Cst.

Conclusions - Perspectives

• New setup to measure water content in

gases down to 1 ppm

• New experimental data up to 40 MPa

for synthetic gases

• Good agreement between model

prediction in experimental results

• Future works: real North Sea natural gases, effect of compositions (i.e. CO2

content), water content in rich CO2

systems

Acknowledgements

• This work was part of a Joint Industry Project funded

by Clariant Oil Services, Petrobras, StatoilHydro, TOTAL, and the UK BERR, whose support is gratefully

acknowledged

Thank youThank you

for your attentionfor your attention