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

The Bouillante geothermal field provides about 8% of the annual electricity needs of the French West Indies island of Guadeloupe. It has been the subject of several studies covering various disciplines. A hydrogeological conceptual model of the field is proposed. The reservoir consists of two perpendicular sets of fractures and faults, related to major regional tectonic structures, which have been clogged near surface by self sealing. The heat exchanges of the reservoir with the outside are reduced, through thermal and 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 60% seawater and 40% freshwater and has reached a chemical equilibrium with a mineralogical assemblage at 250–260°C. Many arguments (equilibrium state, absence of tritium, low in/out fluxes, large reservoir volume) suggest a long residence time (>100 years) of the geothermal fluid. Three factors explain the existence and location of the 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.