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

Size: px
Start display at page:

Download "Gas Hydrates in Low Water Content Gases: Experimental Measurements and Modelling Using the CPA EoS"

Transcription

1 Gas Hydrates in Low Water Content Gases: Experimental Measurements and Modelling Using the CPA EoS Antonin Chapoy, Hooman Haghighi, Rod Burgass and Bahman Tohidi Hydrafact Ltd. & Centre for Gas Hydrate Research Institute of Petroleum Engineering Heriot-Watt University Edinburgh EH14 4AS, UK Ecole des Mines de Paris - Paris, France, Thursday, September 3th, 2009

2 Outline Introduction / Scope of work Experimental Materials Experimental setup Procedures Validation Thermodynamic Modelling Results - Discussions Remarks and Conclusions

3 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

4 What are gas hydrates? Gas hydrates or clathrate hydrates are: Ice-like crystalline compounds Composed of water + gas (e.g. methane, CO 2 ) Formed under low temperatures and elevated pressures Stable well above the icepoint of water Methane hydrate: the burning snowball

5 Hydrate Structures 2 Methane, ethane, carbon dioxide Structure 1 + P T and suitable guests Structure 2 Propane, iso-butane, natural gas. 1 Structure H Methane + neohexane, methane + mch

6 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)

7 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 (dehydratation( dehydratation) Combinations of the above New Approach: Cold Flow P Hydrates Wellhead conditions No Hydrates

8 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 Component System 1 (si) System 2 (sii) System 3 (sii) CH C 2 H C 3 H nc 4 H N CO

9 Water Content Measurements Experimental setup Cooling Fluid in/out Main Characteristics: Pivot P Transducer 2-way valve Cooling Jacket Equilibrium Cell Mixing Ball Piston T Probe Titanium piston vessel P max : 70 MPa T min : 193 K T max : 323 K T ±0.1K P ±0.003 MPa 2-way valve

10 Water Content Measurements Schematic of the SpectraSensorsTM SS2000 TDLAS set-up Laser Sample out Detector P Transducer Main Characteristics: Beer law I o ln = S L N I Sample in Mirror Standard error TDLAS set-up is the greater of 4 ppm or 2% of the reading.

11 Thermodynamic Modelling V L f = f or For VLE or VHE, we have: CPA EoS: V f = f P RT a( T ) 1 RT ln( g) = 1+ ρ i Vm b Vm( Vm + b) 2 Vm ρ 1 i Ai H x ( A X ) i SRK part Association part For Hydrate: Solid solution theory of van der Waals and Platteeuw f H w = f β w RT H β µ w where µ β H = µ µ = RT v ln 1 + exp w w w m mj j m j β H C f

12 Thermodynamic Modelling BIPs between water and gases adjusted using gas solubility data: FOB n 1 = N 1 x i,exp x x i,exp i, cal CH 4 Solubility / mole fract. Example: methane solubility in water K 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) P / MPa CH4 Solubility / mole fract K Culberson et al. (1951) Amirjafari and Campbell (1972) P / MPa

13 Validations of the model Predictions of water content System CH 4 - Water P / MPa Model (VLE) Model (HSZ) Althaus (1999) Water content / mole fraction Kosyakov and Ivchenko (1982) Chapoy et. al (2003) Rigby and Prausnitz (1968) Yokoyama (1988) Yarym-Agaev et. al (1985) Rigby and Prausnitz (1968)

14 Water Content Measurements Validations Water content measurements in methane in equilibrium with liquid water 14 P/ MPa K K K K Model HSZ y w / ppm

15 Experimental Results Model Predictions System 1: Methane at 3.44 MPa 1000 This work y w /ppm 100 data from Song et al. (2004) data from Aoyagi et al. (1979) Model Predictions AAD = 6.1 % T/ K

16 Experimental Results Model Predictions System 1: Methane at 6.89 MPa y w /ppm This work data from Song et al. (2004) data from Aoyagi et al. (1979) Model Predictions AAD = 1.9 % T/ K

17 Experimental Results System 2 Experimental conditions P/ MPa T/ K

18 Experimental Results System 2 Results MPa 10 MPa 40 MPa Model y w / ppm 10 AAD = 2.8 % T/ K

19 Experimental Results System 3 Experimental conditions P/ MPa T/ K

20 Experimental Results System 3 Results MPa 10 MPa 40 MPa Model y w / ppm 10 AAD = 5.1 % T/ K

21 Experimental Results Correlation (GPA conference 2006) y w sat Pw = φ P w v exp( L w ( P P RT sat w ) ) with φ w A 2 = exp( AP + BP ) = a + b T B = c + d T Cst. Liquid Hydrate - si Hydrate - sii a b c d

22 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

23 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

24 Thank you for your attention