IMAGE-D7.03: Final report on chemical geothermometers and tracers

In the framework of the IMAGE project, the main objective of the task 7.3 was to develop auxiliary chemical geothermometers and tracer tests adapted to environments of crystalline basements and sedimentary basins in order to consolidate the temperature estimation of the geothermal reservoir from surface exploration and better know the fluid circulation in this type of reservoir, respectively. To begin, two exhausted literature reviews were carried out. 

The first one was focussed on the development of potential auxiliary geothermometers, using the chemical composition of deep fluids discharged from crystalline basements or sedimentary basins and collected from wells, in order to better estimate the temperature of the geothermal reservoir in this type of environments. In addition to classical geothermometers (Na-K, Na-K-Ca, K-Mg, silica, etc.), auxiliary geothermometers like Na-Li, Na-Rb, Na-Cs, K-Sr, K-Fe, K-Mn, K-F and K-W were found for dilute deep geothermal waters from more than 60 granite areas, in Europe (Michard, 1990). For saline geothermal waters discharged from sedimentary basins, only the Na-Li and Mg-Li thermometric relationships were found in the literature. From chemical compositions of 20 deep geothermal and oil brines found in the literature, especially from the Rhine Graben and Salton Sea areas, three new relationships Na-Rb, Na-Cs and K-Sr, different from those applied to dilute geothermal waters, were determined in a temperature range of 70-320°C and will be very useful for the exploration of geothermal systems where this type of fluids will be circulating (Sanjuan et al., 2016b). Other less reliable thermometric relationships (K-Fe, K-Mn, K-F) were also found. 

The second literature review was associated with the development of tracing tests in environments of crystalline basement and has benefited from the long BRGM experience in the Soultz geothermal site. After a review of the tracers recommended in the literature following the fixed objectives, examples of applications in different crystalline sites were given. The most recent inter-well tracer tests conducted in June 2013 at the Habanero EGS site, central Australia, and in 2014, in the Rittershoffen site, Alsace, near Soultz-sous-Forêts, in granite environments, were also integrated in this review. Unfortunately, after on-site works, no tracer test could be conducted in the PVGT-LT1 borehole, only borehole available in the Litomerice area, Czech Republic, and in the Thônex-1 borehole, near Geneva, in Switzerland, especially because of the very low fluid flow-rate existing in these boreholes. Consequently, we decided to focus our on-site works on the development of auxiliary chemical geothermometers at low-temperatures. 

After the literature review, the chemical compositions of deep fluids discharged from existing boreholes drilled up down the crystalline basement, in the Litomerice area, Northwest Bohemia, in Czech Republic, were investigated. Only the fluid chemical compositions from 5 deep boreholes were found and among these boreholes, only one (PVGT-LT1 borehole) could be sampled by GEOMEDIA, in May 2015, and by BRGM, in November 2015. Another fluid sample was collected by BRGM from the Pravridlo thermal spring, at Teplice, near the Litomerice city. The poor concordance between estimated and measured fluid temperatures for these boreholes is probably due to the low values of this parameter, which do not allow reaching a full chemical equilibrium between the fluids and the surroundings rocks. At temperatures ≤ 50°C, only the chalcedony, K-Fe and K-Mn thermometric relationships seem give relatively good estimations of temperatures. At temperatures > 50°C, a significant part of the auxiliary geothermometers determined by Michard (1990) gives concordant temperature values for the dilute geothermal waters collected from the Litomerice PVGT-LT1 borehole (55 ± 20°C) and from the Pravridlo spring (100 ± 25°C), as well as from the Thônex-1 (70 ± 20°C) and Lavey-les-Bains (105 ± 25°C) boreholes, in Switzerland. However, additional chemical and isotope analyses must be performed by BRGM in the Thônex-1 fluid, in the next months, in order to confirm and be sure this fluid is representative of the deep reservoir. Even if the auxiliary geothermometers are very useful tools, they must be always used together and in the framework of a global geochemical data interpretation because if it is not the case, they may induce important errors in the estimations of the reservoir temperatures in geothermal exploration. A scientific paper is envisaged in 2017 within the WP8 framework.