H. Traineau, B. Herbrich, E. Lasne, D. Tournaye

1. Geothermal Development in the Volcanic Caribbean Islands and EGS Perspectives H. Traineau, B. Herbrich, E. Lasne, D. Tournaye CFG Services, 3 av. Cl. Guillemin 45064-Orléans cedex 2 – France A. Genter, B. Sanjuan BRGM, 3 av. Cl. Guillemin, 45060-Orléans cedex 2 - France

2. Map of the Caribbean volcanic islands (red color) and their active or recent volcanoes. Subduction of the Atlantic Plate below the Caribbean Plate

3. Energy in the Caribbean • More than 90% of electricity comes from fossils fuels; • Low contribution of Renewable energy sources : • Electricity needs increase by 4-5% every year. • Biomass • Hydro • Solar • Wind • Geothermal

5. Geothermal resources might contribute to power generation in some islands instead of fossil fuels Bouillante, the case history in Caribbean Dominica, the most promising geothermal potential

6. The Bouillante geothermal field, Guadeloupe Urban area Steep topography area Sea proximity

7. The story of the Bouillante geothermal exploitation

8. The Bouillante geothermal field - Several recent eruptive centers (<1 MA) around the Bouillante Bay, with the possible occurrence of shallow magmatic intrusion under cooling; - Several main normal faults which promote permeability and fluid circulations at depth and represent potential fractured reservoirs; - Only a part of the reservoir is now exploited through BO-4 vertical production well, BO-5 and BO-6 deviated and inclined production wells which intersect the Cocagne Fault.

9. Simplified model of fluid circulations in the Bouillante geothermal reservoir • Simplified cross-section along a West-East profile within the Bouillante geothermal field showing the assumed model of fluid circulation with seawater and rainfall recharge, mixing (60% seawater + 40% rainfall) and heating to 250-270°C.

10. Selected temperature profiles in the Bouillante wells. 240°C at shallow level (well BO-2) Reservoir temperature around 250°C

11. Cross-section along a NW-SE profile showing well trajectories, location and quality of permeable zones in relation with faults deduced from surface geology. - Only the Cocagne Fault appears to be highly permeable. - Plateau Fault shows low permeability. - Descoudes Fault has no permeability possibly due to carbonates scaling

12. Main characteristics of the 7 deep wells drilled at Bouillante High temperature conditions at shallow depth (< 1000 m deep) but strong anisotropy in permeability related to fracture network and possibly to scaling (carbonates, silica).

13. Main characteristic of the 7 deep wells drilled at Bouillante High temperature conditions at shallow depth (< 1000 m deep) but strong anisotropy in permeability related to fracture network and possibly to scaling (carbonates, silica). Extension of EGS methods might be addressed first to accurate location of faults within the high temperature, fracture-permeable reservoir.

14. Stimulation of the low productive well BO-4 A limited, low cost experiment with cold seawater injected into the well BO-4 has been conducted from August 10 - 27, 1998 to enhance the secondary permeability through thermal cracking. Productivity increases by 50% and fluid discharge is more stable.

15. Stimulation of the low productive well BO-4 A limited, low cost experiment with cold seawater injected into the well in steps has been conducted from August 10 - 27, 1998 to enhance the secondary permeability through thermal cracking. Productivity increases by 50% and fluid discharge is more stable. Extension of EGS methods could be very useful to improve permeability of low producers.

16. Aerial view of the geothermal plant located within the city Bouillante 1 Unit (4,5 MWe) Bouillante 2 Unit (11 MWe) The location of the plant within the city induced strong environmental constraints (no noise, no steam plume, …).

17. Aerial view of the well pad and the pipe route. Steep topography and urban area reduce land availability for well pads, pipe routes, etc…, and consequently may hinder best locations for production and reinjection zones.

18. Bouillante Sea proximity also reduces surface availability for well siting and might require to reduce distances between production and reinjection zones.

19. Bouillante Sea proximity also reduces surface availability for well siting and might require to reduce distances between production and reinjection zones. Extension of EGS methods could be very useful to improve and secure reservoir monitoring and exploitation (tracer tests, …)

20. Dominica Island Valley of Desolation Two promising high enthalpy geothermal fields with abundant surface manifestations, related to very recent volcanic activity. Wotten Waven Soufriere

21. Dominica Island Development of geothermal resources in Dominica and reservoir exploitation will face similar problems as experienced in Guadeloupe (remote location, steep topography, national park, …) Valley of Desolation Micotrin lava dome Wotten Waven

22. Conceptual modelof the Wotten Waven geothermal reservoir(s) with inferred location of deep chloride reservoirs and fluid flow pattern.

23. Rose diagram of fractures recorded on the field Permeability within the basement is provided by fault networks. Faults will be the main targets for well siting.

24. CONCLUSIONS • Volcanic islands in the Caribbean have proved or expected high enthalpy geothermal resources which could be used for electricity production. • Due to their location, Bouillante and other fields (will) have to face problems related to : • Permeability anisotropy within the geothermal reservoir; wells have to intersect main faults in order to be good producers; • Steep topography and limited land availability for well pad, pipe routes,…; • Environmental constraints due to urban areas or national park; • Remote location (extra costs for survey, drilling, logging, stimulation and work over operations, plant maintenance,…); • Limited size of the exploitation and limited financial capacity of geothermal operators;

25. EGS PERSPECTIVES • Some problems could be addressed by EGS methods to promote development of their geothermal resources : • Accurate location of fault network; • Permeability improvement through well stimulation; • Reservoir monitoring (tracer tests, logging); • Management of close production and reinjection zone; • ….