Assessing the role of fractures on the permeability of the Permo-Triassic sandstones at the Soultz-sous-Forêts (France) geothermal site

Quantification of the systemic permeability of geothermal reservoirs is essential to the assessment of their economic feasibility. Here we investigate experimentally the role of fractures on the permeability of a 400 m-thick unit of Permo-Triassic sandstone from the EPS-1 exploration borehole in the Upper Rhine Graben near Soultz-sous-Forêts (France). The permeability of initially intact sandstone samples was measured before and after the introduction of a through-going tensile fracture. While the permeability of the fracture-free samples varied over five orders of magnitude between 1 × 10−19 and 1 × 10−14 m2, the presence of through-going fractures increased sample permeability to between 8 × 10−14 and 4 × 10−12 m2. Using the fracture aperture of open fractures provided by borehole televiewer data, we model the equivalent permeability down the borehole to be between 7 × 10−18 and 3 × 10−13 m2, which is in agreement with values of hydraulic conductivity determined using borehole tests. Overall, these equivalent permeability values are not sufficiently high to sustain hydrothermal convection at Soultz-sous-Forêts, highlighting the need for continued anthropogenic stimulation.