GEOCHEMICAL RESULTS OF A SINGLE-WELL HYDRAULIC INJECTION TEST IN AN EXPERIMENTAL HOT DRY ROCK GEOTHERMAL RESERVOIR, SOLUTZ-SOUS-FORETS, ALSACE, FRANCE

As part of the investigation of the Hot Dry Rock (HDR) potential of th e Soultz test site, 25 300 m(3) of fresh water were injected into bore hole GPK1 in granite between depths 2850 and 3400 m. The returned flui ds were chemically monitored during the 57 h of production that follow ed the 21 h of shut-in time, and there is evidence of mixing and water -rock interactions. Although the chemical composition of the two sampl ed indigenous fluids (1817 m and 3485 m depth) which circulate at vari ous levels in the granite is very similar, participation of both fluid s in mixing with injected water has been revealed from the slight chem ical differences. This test also suggests evidence for the presence of a third fluid. The large volume of injected water induced greater coo ling 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 grea ter because it is partly related to anhydrite dissolution. Mixing of t he injected water with the highly saline indigenous fluid conceals par t of the water-rock interactions because there is no evidence of the r elease of major elements (K, Na). However, trace elements are useful f or determining some dissolution reactions, for example, relatively hig h concentrations of F and relatively low Sr-87/Sr-86 ratios. The prese nce of the brine also induces some chemical reactions as it increases calcite and anhydrite solubility. Lithium and B in the indigenous flui d are taken up during the test, and adsorption reactions on solid surf aces are proposed as an explanation for this depletion. Chemical monit oring during this single-well hydraulic test has proved relevant for d etermining 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. (C) 1997 Elsevier Science Ltd. All rights reserved.