Identification and characterization of preferential flow paths play an important role in underground engineering, such as enhanced geothermal systems (EGS), naturally fractured reservoirs (NFR) and deep underground waste disposals. Observations from boreholes and galleries were combined to create a baseline geological model and to constrain the main flow paths as part of the in-situ stimulation and circulation (ISC) experiment at the Grimsel Test Site (GTS), Switzerland. Geophysical and borehole imaging methods combined with core logging were used to identify potentially open and closed fractures at the borehole scale. Additionally, large-scale shear zones were correlated throughout the entire rock volume. Various single-hole and cross-hole hydraulic packer tests were conducted to estimate the connectivity and conductivity of the identified dominant fractures. The fracture flow conditions were further constrained by solute tracer tests. Based on the geological model, the rock mass in the experimental volume is dissected by three NE-SW-striking/SE-dipping ductile (S1) and two E-W-striking/S-dipping brittle-ductile (S3) shear zones. There exists a brittle fracture zone between two adjacent S3 shear zones where the southern boundary between the fracture zone and S3shear zone is the main conductive flow path. This main flow path also interacts with intersected brittle fractures in the areas where the fracture frequency increases.

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