Porosity and fluid velocities in the upper continental crust (2 to 4 km) inferred from injection tests at the Soultz-sous-Forêts geothermal site
Résumé
A heat exchanger was created at the French Hot Dry Rock geothermal site using a doublet of boreholes at a depth of 3600–3800 m. After hydraulic stimulation, this exchanger was tested during a long-term circulation trial from July to November 1997. Tracer tests were carried out during the circulation trial.
An analysis of the tracers indicates a connected porous volume in the range of 0.5 to 2.5 × 106 m3, with a connected porosity of the order of 0.9 to 2.3%. These values are one order of magnitude higher than values obtained by extrapolation from petrophysical investigation of the boreholes. This result is interpreted as being due to the connection of the major faults to a dense network of 2nd-order structures.
Fluid circulation velocities inferred from the tracer test are of the order of 0.25 to 0.36 m/h. Velocities derived from observation of the migration of fresh fluids after injection into the granite during stimulation tests are of the same order of magnitude (0.02 to 0.17 m/h), yielding hydraulic conductivity estimates of 6.6 10−8 to 2.3 10−6 m/s−1. Both the pressure gradient during the circulation test and the density gradient are related to freshwater injection, and can be considered as possibly equivalent to temperature and hydraulic gradients in the Rhine graben, at least during some specific geological events such as the Alpine uplift phases. Thus, the test shows that fluid migration can occur in the upper continental crust exhibiting “reservoir” properties, and these should be taken into account in large-scale modelling. These properties are in good agreement with a proposed scenario of evolution involving active fluid migration during the Eocene (Alpine uplift phase). It also supports the hypothesis of seawater-derived saline fluids stored in old fractured shield rocks, which implies deep seawater circulation in the continental crust.