Geochemical results of a single-well hydraulic injection test in an experimental hot dry rock geothermal reservoir, Soultz-sous-Foreˆts, Alsace, France

As part of the investigation of the Hot Dry Rock (HDR) potential of the Soultz test site, 25 300 m3 of fresh water were injected into borehole GPKI in granite between depths 2850 and 3400 m. The returned fluids were chemically monitored during the 57 h of production that followed the 21 h of shut-in time, and there is evidence of mixing and water-rock interactions. Although the chemical composition of the two sampled indigenous fluids (1817 m and 3485 m depth) which circulate at various levels in the granite is very similar, participation of both fluids in mixing with injected water has been revealed from the slight chemical differences. This test also suggests evidence for the presence of a third fluid. The large volume of injected water induced greater cooling of the reservoir than in previous tests, and a temperature effect on water-rock interactions has been demonstrated. There is release of Mg, and of SiO2 and F to a lesser extent, whereas SO4 release is greater because it is partly related to anhydrite dissolution. Mixing of the injected water with the highly saline indigenous fluid conceals part of the water-rock interactions because there is no evidence of the release of major elements (K, Na). However, trace elements are useful for determining some dissolution reactions, for example, relatively high concentrations of F and relatively low87Sr/86Sr ratios. The presence of the brine also induces some chemical reactions as it increases calcite and anhydrite solubility. Lithium and B in the indigenous fluid are taken up during the test, and adsorption reactions on solid surfaces are proposed as an explanation for this depletion. Chemical monitoring during this single-well hydraulic test has proved relevant for determining which parts of the processes (dissolution and precipitation) are liable to modify the physical properties of the reservoir during fluid circulation for energy extraction, particularly in the vicinity of the injection borehole.