Preliminary geochemical and thermodynamic assessment of the geothermal resources, Sulphur Springs area, St Lucia, W.I.

Abstract

A high-temperature geothermal resource has been located on the island of St Lucia (W.I.), which was tapped by means of a well drilled in 1988 to a total depth of 1410 m, in the Sulphur Springs area. The well encountered mainly dacitic agglomerates and lava flows and a permeable zone below ≈1340 m, with a maximum temperature of ≈290°C. Well productivity decreased from an initial value close to 62 t/h to about 33 t/h under well-head pressures of 1.5 MPa (15 bar a) after 255 h of production. Initial reservoir static pressure was 7.5 MPa (75 bar a). A decline in reservoir pressure of about 15% was observed at the end of the production test. During the first two days of exploitation the well initially produced a two-phase fluid with high steam fraction, which then developed into superheated steam with a high content of non-condensable gas exceeding 100 1/kg at standard conditions and a computed P(CO 2) of 1 MPa (10 bar). The average steam fraction at reservoir conditions, computed from gas composition after 120 h of production is ∼0.3. High HCl concentrations of about 300 ppm are present in the condensate steam indicating the presence of a high concentrated boiling brine to the point of halite saturation. All data support the hypthesis of a hydrothermal hot-water system prior to drilling, that underwent a very rapid drawdown with production.