Hydrogeological model of a high energy geothermal field ( Bouillante area , Guadeloupe , French West Indies )

The Bouillante geothermal field presently provides about 8% of the annual electricity needs of the French West Indies island of Guadeloupe. It has been the subject of a large number of studies covering various disciplines. These results enable the proposal of a hydrogeological conceptual model of the field. The reservoir consists of two perpendicular sets of fractures and faults, related to major regional tectonic structures. The capacitive and transmissive functions of the field are assured by these deep faults and fractures, which have been clogged near surface by self sealing and clay fill phenomena. The heat exchanges of the reservoir with the outside are thus reduced, through thermal and moreover hydraulic blanketing, to conductive transfers. Convection cells are active within the reservoir, ensuring its thermal and geochemical homogeneity. Heat exchange with the magmatic chamber is only conductive. The Na-Cl geothermal fluid in the reservoir is composed of about 60% sea water and 40% fresh water of meteoric origin and has reached a chemical equilibrium with a mineralogical assemblage at 250-260°C. This equilibrium state, the absence of tritium in solution, the low ratio between water in and out fluxes and the large reservoir volume (estimated at more than 30 millions m 3 using tracer tests) suggest a relatively long (>> 100 years) residence time of the geothermal fluid in the reservoir. Three main factors, all essential, explain the existence and location of the Bouillante geothermal field: a heat source (cooling hypovolcanic intrusion), a network of permeable fractures at the origin of the geothermal aquifer, and an impermeable surface cover, limiting the loss of energy and ensuring the durability of the field.