Bouillante geothermal fluid: mixing and water/rock interaction processes at 250°C

In the geothermal Bouillante area, electricity is presently produced from well BO-2 only. Following a successful stimulation operation in 1998, a second well BO-4 will be soon connected to the power plant. This paper presents relevant geochemical data obtained for both wells during 1998 and 2000. For some dissolved species, results show discrepancies between surface and down hole samples. In this case, additional information from thermal springs can be very useful to reconstruct the chemical composition of the deep fluid. Despite an high water-rock ratio, the deep NaCl brine (20 g/l) composed of about 58 % seawater and 42 % freshwater reacts with volcanic rocks at about 250°C reaching chemical equilibrium with a mineralogical assemblage as shown by its composition and by saturation calculations. data on separated brine, steam and non condensable gas were collected from BO-4. The main aim of the 3 subsequent tests (from October 3 to 10, 1998; from July 1 to 14, 1999; from February to May 2000) was to monitor any potential change in the fluid characteristics due to the stimulation and production. At the end of the tests, more than 65,000 m of total fluid were discharged from BO-4. Sampling of separated fluids at separator and weir-box, on site and Gas/Steam Ratios (GSR) measurements carried out for the geochemical monitoring are described in Sanjuan et al. (1999; 2000). During the 1 and 3 test (short term tests), only 1 sampling of fluids from weir-box and separator was performed for complete chemical and isotopic analyses. During the 2 other tests, 5 fluid samples were collected at separator and 12 brine samples at weir box for detailed analyses. 9 GSR measurements and 7 chemical gas analyses were performed for BO4 against 2 measurements and 1 analysis for BO-2. Three down hole fluid samples were collected from BO-4 between 330 and 640 m (August 1, 1998; July 1 and 2, 1999) and one from BO2, at 320 m (July 7, 1999), using a KUSTER sampler. Most of on site measurements and sample preparations were performed immediately after sampling. Several submarine thermal springs (outflow temperatures up to 95°C) were sampled in October 1998 from the Bouillante Bay. Chemical and isotopic analyses similar to those performed for BO-2 and BO-4 fluid samples were carried out. All the analyses were performed using standard water analytical techniques such as titration, ion electrode, potentiometry, colorimetry, atomic absorption spectrophotometry, ion and HPLC chromatography, inductively coupled plasma-mass spectrometry (ICP-MS), mass spectrometry. The precision of major and trace species is better than ± 5 % and 15 % respectively. The precision of δD and δO determination is ± 0.8 ‰ and ± 0.1 ‰. Ion balance values lower than 5 % suggest a good quality and coherence of the major species analyses.