Numerical study of deformation behavior of seabed in methane hydrate production

1. Numerical study of deformation behavior of seabed in methane hydrate production Nagasaki University Geo-environmental Lab Kentaro Fukuda Yujing Jiang Yoshihiko Tanabashi

2. Background of Research Methane Hydrate (MH) Now Petroleum and Natural gas are main energy resources. Limit of the quantity of resources Future The development of new energy resources Methane Hydrate (MH)

3. Background of Research MH A material that Methane’s molecule is surrounded in the crystallization of the caged Water’s molecule forms Equilibrium conditions: Low temperature and High pressure Distribution area ① Sediment of seabed ② Eternal frozen ground area Triangle：Methane’s molecule　Ball：Water’s molecule Crystal structure of MH

4. Background of Research Gathered MH(White ice) The gathered MH in Niigata offing Distribution area 1. The Nankai Trough 2. Kuril Islands 3. Sea of Okhotsk 4. Tataru Trough 5. Okushiri submarine ridge 6. West Tsugaru basin The confirmation of existence of MH in sea area around Japan The amount of the resource : About 7.4 trillions cubic meters Equivalent to about 100 years of the amount of annual natural gas consumption inJapan(1999) Possibility of supplying energy for long term in Japan

5. Problem of Research As the results of MH decomposition… ・Change of environment of seabed in production area ・Buckling of the well caused by deformation of seabed Need of MH development considering the geo-environment

6. Purpose of Research Evaluation of deformation behavior of seabed in MH production Thermal stimulus method・Decompression method Influence on deformation of seabed caused by changes of temperature and pressure temperature and pressure Evaluation of deformation behavior Thermal-Stress-Fluid coupled analysis

7. Brief of Analysis Three dimensional　　　finite difference method Sediment in MH production Under conditions three phases (gas・liquid・solid) Evaluation of deformation behavior ・Energy conservation law Thermal ・Heat conduction equation ・Stress balance equation Stress ・Constitutive equation of Mohr Coulomb ・Infiltration equation Fluid ・Mass conservation law

8. Numerical Model Well Upper layer 200m MH layer 10m Lower layer 90m 200m 200m Water depth : 1000m, equivalent to seabed Water pressure : 10MPa

9. Properties of Materials

10. Judgments of Decomposition Initial pressure 12MPa Thermal stimulus method Decompression method Pressure at MH decomposition 4MPa Initial temperature 278K Temperature at MH decomposition 288K Phase equilibrium area Initial condition ● Decomposition area

11. Cases of Analysis(Thermal stimulus method) Initial temperature 5℃(278K)

12. Changes of temperature 30days later 30days later 30days later 20days later 20days later 20days later 10days later 10days later 10days later 4months later 6 months later 5months later 2months later 5months later 3months later 3months later 4months later 4months later 6 months later 6 months later 3months later 5months later 2months later 2months later 5 10 15 20 25 30 35 40 45 50℃ Case1-1 Case1-2 Case1-3 Set temperature 30℃ Set temperature 50℃ Set temperature 30℃ Water pressure inside well 0MPa Water pressure inside well 0MPa Water pressure inside well 4MPa

13. Settlement of seabed Distance from center of well (m) Settlement of seabed (mm) Case1-2 Case1-1 Influence of water pressure inside well is large Set temperature : 30℃ (Case1-1, 1-2) Water pressure inside well : 0MPa(Case1-1), 4MPa(Case1-2)

14. Temporal changes of settlement of seabed Distance from center of well (m) Settlement of seabed (mm) 10days later 20days later 30days later Change depending upon time Case1-1 Set temperature 30℃ Water pressure inside well 0MPa

15. Cases of Analysis(Decompression method) Initial pressure 12MPa

16. Settlement of seabed Distance from center of well (m) Settlement of seabed (mm) Case2-2 Case2-1 Influence of water pressure inside well is large Set pressure : 1MPa (Case2-1, 2-2) Water pressure inside well : 0MPa(Case2-1), 4MPa(Case2-2)

17. Temporal changes of settlement of seabed Distance from center of well (m) 10days later 20days later Settlement of seabed (mm) 30days later Change depending upon time Case2-1 Set pressure 1MPa Water pressure inside well 0MPa

18. Conclusions Design of analysis tool to judge the decomposition of MH Revealability of influence of seabed settlement in MH production ・Influence of water pressure inside well is larger than set temperature. (Thermal stimulus method) ・Influence of water pressure inside well is larger than set pressure. (Decompression method) ・Settlement of seabed depends upon time. (Both methods)

19. Future problems ① Verification of Thermal-Stress-Fluid method ② Establishment of the method to get the parameters needed for the analysis ③ Evaluation of influence on well of MH decomposition ④ Evaluation of interinfluence in plural wells

20. Thank you for your attention! End