Conceptual model of the Roseau Valley geothermal system, Dominica, Lesser Antilles

Abstract

The Roseau Valley (Wotten Waven) geothermal system is a prolific high-temperature resource situated in the southern highland regions of the volcanic island of Dominica in the Lesser Antilles. Three intermediate diameter (6-1/8” bottomhole) wells and four standard sized wells (8.5” bottomhole) have been drilled at the prospect area between 2012 and 2023.
The results of these wells and other geoscientific investigations reveal a broad, ~400 m-thick, 240-255°C outflow reservoir that is channeled through fractured andesitic volcanics and directed to the southwest from a likely upflow situated adjacent to the ~25 ka Morne Microtrin dacite dome. In contrast to some of the other documented seawater-influenced resources of the Lesser Antilles (e.g., Bouillante, Nevis, Garibaldi Hill), the reservoir at Roseau Valley is a lower salinity, meteoricwater-derived, 0.5 wt% TDS Na-Cl fluid, with 0.2-0.5 wt% dissolved NCGs. High permeability (productivity indices of 60-240 kg/s/bar) is documented in the field, with key structural features being the West-striking Trafalgar and Papillote Faults. Image log data provide further context for fracture orientations associated with these structures, and also give insight into the maximum principal horizontal stress direction in the field area through the analyses of drilling induced tensile fractures. These analyses help place the permeability controls at Roseau Valley in the broader context of the intra-arc stress regime in the Lesser Antilles. Ormat will be constructing a 10 MW-net power plant at
Roseau Valley through 2024-2025, which is expected to supply approximately 50% of the island’s peak electricity demand. Expansion of this plant in the near future will enable the island to achieve its goal of 100% renewable energy before 2030.