Depth- and pressure-dependent permeability in the upper continental crust: data from the Urach 3 geothermal borehole, southwest Germany

Permeability is a critical geologic parameter because migrating fluids play a fundamental role in heat and mass transfer. Because of their assumed low hydraulic conductivities, crystalline basement rocks are being considered for nuclear waste repositories. A compilation of existing data and a description and interpretation of new hydraulic test data in the crystalline basement of the 4.5-km-deep Urach 3 borehole (southwest Germany) are presented, together with an interpretation of all test campaigns for Urach 3. The tests showed that within the crystalline basement, permeability is decreasing with depth. In the Urach borehole many high-pressure tests with well-head pressures of more than 600 bar were carried out. During hydraulic tests with well-head pressures above 176 bar, permeability of the crystalline basement increased dramatically, showing the elastic reaction of the rock as a result of pressure; during tests with well-head pressures below 176 bar, there was no significant elastic reaction of the rock. The pressure-dependent change of transmissivity was described in terms of a set of power laws relating injection rate to transmissivity and fracture width. Probably due to the lack of shear stress, no significant increase in permeability remained after any high-pressure tests.