Natural Gas Hydrates

1. Shell – Tsinghua Chair Professorship Natural Gas Hydrates Jakob de Swaan Arons Professor Royal Dutch Shell Chair Chemical Engineering Department Tsinghua University, Beijing, China 19th September 2006

2. Royal Dutch Shell Shell Transport and Trading Company (British) and Royal Dutch Petroleum Company (Dutch)

3. Contents • What are gas hydrates • Models, thermodynamics and phasebehavior • Applications • Conclusions

4. What is a Gashydrate?

5. Gashydrates Crystalline structures of water with cavities of molecular size, containing (hosting) molecules of compounds (guest) with boiling points mainly below and sometimes above room temperature.

6. Guest molecule: Hydrate bonding H20 molecule CH4, C2H6, i-C4H10, CO2, N2, O2, CHF3 Water Molecule What are gashydrates?

7. Ice structure: more stable 512 cavity T = 273 K H H C H H Hydrogen bonding H2O molecules Interaction between guest and H2O molecules stabilizes the structure Interaction and Stability

8. Structure II Structure I 136 water molecules 46 water molecules Structure H 34 water molecules Various Gashydrate Structures Uit: E.D. Sloan Jr., Hydrate Engineering, Bloys, B. (ed.), SPE Monograph Series, 21,Richardson, Texas, V.S., 2000

9. O O 4 Å 4 Å N2 O2 CH4 512 [sI] 512 [sII, sH] 5 Å 5 Å CO2 435663 [sH] 51262 [sI] C2H6 CF4 O 6 Å 6 Å C3H8 51264 [sII] 7 Å 7 Å O 51268 [sH] CH3 8 Å 8 Å Compare size of molecule and cavity

10. Gashydrate flame

11. Importance • Nuisance • Blessing ? • Separations • Scientific

12. hydrate hydrate vapour vapour oil & water oil & water Hydrate crystals Hydrate plug vapour oil & water Formation of a hydrate plug

13. Gas hydrate Gas They resemble ice, we find them in Nature.

14. Blake Ridge Barents Zee Zee van Okhotsk Noorse Zee Hydrate Ridge Prudhoe Bay McKenzie Delta Other 3.780 Locates Gas Hydrate: Zeebodem Permafrost Hydrate 10.000 Fossil 5.000 K.A. Kvenvolden, A Primer on the Geological Occurrence of Gas Hydrate, in: Gas Hydrates – Relevance to World MarginStability and Climate Change, Henriet, J.-P., Mienert, J. (eds.),Geol. Soc. Special Publ., 137, Geological Society, Londen, GB,p. 9-30, 1998 Natural gas hydrate reservoirs

15. 0 0 stable gas hydrate 200 200 400 400 diepte permafrost Phase boundary 600 600 hydrate- mische gradient fasen begrenzing Water Depth [m] Depth van sediment [m] 800 800 1000 1000 stable gas hydrate basis gas hydrate water 1200 1200 sediment geothermal gradient 1400 1400 geothermal gradient basis gas hydrate 1600 1600 -20 -10 0 10 20 30 -20 -10 0 10 20 30 Temperature [°C] Temperature [°C] Stability and natural conditions Oceaan Permafrost

16. Industrial question Dutch natural gas may contain up to 14% N2. Could hydrates act as a good separation agent?

17. Scientific importance As we will see later, gas hydrates offer an extremely interesting example of a large family of so called inclusion compounds made up of host- and guest- molecules. • Water • Urea • Hydroquinone

18. Question What has the subject of Natural Gas Hydrates (NGH) to do with a course in Advanced Chemical Engineering Thermodynamics?

19. Answer In dealing with NGH we can demonstrate the power and beauty of Applied, Molecular and Statistical Thermodynamics.

20. Classical thermodynamics Presents broad relationships between macroscopic properties but it is not concerned with quantitative prediction of these properties. Example John M. Prausnitz

21. Statistical thermodynamics Seeks to establish relationships between macroscopic properties and intermolecular forces and other molecular properties. Example John M. Prausnitz

22. Molecular thermodynamics Seeks to overcome some of the limitations of both classical and statistical thermodynamics. It is an engineering science, based on classical thermodynamics but relying on molecular physics and statistical thermodynamics ……. . In application it is rarely exact and has an empirical flavour. John M. Prausnitz

23. Van der Waals – Platteeuw model for gashydrates These former colleagues at Shell Research International developed a wonderful model, back in the 1950-ies, that since then has seen many small modifications but still “stands as a rock”. Johan van der Waals and Joost Platteeuw Adv. Chem. Phys. 2, 1-57 [1959]

24. Assumptions for the model • Guest molecules don’t affect the cavity structure • At most one guest molecule/cavity • No interactions between guest molecules • Guest molecule can rotate freely in cavity • Lennard- Jones type potential for interaction between guest and cavity

25. s 0 e a kB 2 0 2 Interaction potential guest and cavity u(r) < R >

26. Intermolecular potential (1) a = radius guest R = radius cavity r = variable distance from center cavity = r value for which potential is 0 = potential at maximum attraction

27. Intermolecular potential (2) The most successful potential has been proposed by the Japanese scientist Kihara. Its parameters have been optimized from experimental data on hydrate phase equilibria.

28. The “ Langmuirconstant” This constant can be expressed for guest k in cavity of type i by

29. Guest k in cavity i expresses the fraction of cavity type i occupied by guest k. In case of CH4 the fugacity is approximately the total pressure P

30. Cavity occupancy and host water “Langmuir” “Raoult” In case of water CH4 : the higher the gas pressure, the higher the cavity occupancy, the more stable the hydrate vi= number of cavities type i

31. Analogies The equations for the thermodynamic potential or fugacity of solutes (guests) and solvent (host) show a remarkable resemblance with those for adsorption (Langmuir) and solvency (Raoult).

32. Phase diagram of water P A A B L D S C B D V T

33. How gashydrate may “ take over” from ice below the melting point …… empty …… ice …… filled increase CH4-pressure

34. How gashydrate may form from liquid water above the melting point ……empty …… empty …… ice …… water …… filled Increasing CH4- pressure

35. Freezing point depression (FPD) …… l …… l ice …… ice l add FPD- agent Methanol Ethylene glycol Salt?

36. Hydrate inhibition Just like an FPD- agent is effective in suppression of ice formation it may suppress hydrate formation …… empty H …… …… liquid …… …… ice …… filled H …… liquid A costly “ affair”

37. Hydrate promoters (1) Certain molecules, like tetrahydrofuran (THF), may promote hydrate formation by assisting in filling the vacancies.

38. Hydrate Promoters (2) These, usually non-volatile, promoters may produce the hydrate structure in which the gas molecules can be included although their pressure is too low to achieve this by themselves. (e.g. H2)

39. Solution? It is my impression that these days industry employs inhibitors that don't suppress hydrate formation but suppress hydrate crystal growth producing some kind of “hydrate milk” that does not block pipeline operation. “If you can’t beat the enemy, join them……”

40. Prediction (1) In the oil-and gas industry one likes to know when hydrate formation can be expected, especially at temperatures above 0°C. Possible phases Hydrate H Liquid W aqueous Liquid non-aqueous Vapour V

41. Prediction (2) Components of natural gas: C1 C2 C3 …… N2 CO2 …… For example: where is the location of the HLwV-equilibrium curve? k = 1,2,……N Models required for the various phases

42. Prediction (3) These days the large oil- and gas companies make use of powerful software to allow them to predict not only all possible hydrate phase diagrams but also the effect of inhibitors (by including for example methanol in the calculation programme).

43. I-H-V H-Lw-V H-Lw-L CH4 red blue C2H6 green C3H8 i-C4H10 magenta Equilibrium conditions for different hydrocarbons Pressure Temperature [K]

44. I-H-V H-Lw-V H-Lw-L H-L-V N2 red blue CO2 H2S magenta Equilibrium conditions for some other gases Pressure Temperature [K]

45. H2O + CH4 + NaCl of H2O + CH4 + MeOH Concentration log Pressure H2O + CH4 H - Lw - V H2O + CH4 + cyclic organic component Temparature Influence salts, organic compounds

46. gas recycle CO2or air Separation drink H2O sea water decomposition Hydrate formation brine Application: desalination

47. Threat CH4 is a much more serious contributant to greenhouse effect than CO2. So with the Earth warming up, natural gas hydrates may start dissociating and we may face a “runaway” greenhouse effect. Also: Leaking pipelines in former Soviet-Union

48. Living on NGH ? US Geological Survey [1998]

49. May I introduce myself ?

50. Acknowledgment I wish to acknowledge the contributions of my former Ph. D. student Miranda Mooijer- Van den Heuvel, who is now with Shell Global Solutions International. She graduated on a thorough study of how certain compounds can promote hydrate formation.