Hydraulic Stimulation Experiments in a Decimeter-scale True Triaxial Compressive Apparatus

Geothermal energy is considered a sustainable energy source for the transition to a carbon-neutral economy. In Central Europe, sufficiently hot source rocks are buried deep underground and comprise tight crystalline basement formations. To extract their thermal energy, hydraulic stimulation is used to create efficient heat exchangers in the context of Enhanced Geothermal Systems (EGS). Successful geothermal reservoir initiation requires a broad understanding of the hydro-mechanical coupling in fractured rock masses. For this reason, a decimeter-scale true-triaxial setup has been developed to conduct injection-driven shear tests under various stress conditions.

To gain a deeper insight into the hydro-mechanical processes involved in hydraulic stimulation, a true triaxial compressive apparatus at the decimeter scale is employed. The experimental setup consists of 30 x 30 x 45 cm cuboidal granite specimens, each containing an oblique saw-cut laboratory fracture with different surface properties. The fracture is crossed by two boreholes equipped with packers to isolate a fracture interval. Fluid injection into the isolated intervals follows the typical HTPF (hydraulic testing of pre-existing fractures) scheme, including stepwise pressure increases and decreases. Stress boundary conditions are introduced by three sets of oil-filled flatjacks, contained within a steel frame which allows a more realistic and accurate replication of the stress conditions experienced by geological formations during hydraulic stimulation experiments. Stresses for individual tests are manipulated from hydrostatic to strike-slip conditions to test for different end member states of slip tendency. Fracture and rock deformations are recorded by 16 linear variable differential transformer (LVDT) sensors mounted externally along the edges of the specimen, volume changes in the flatjacks and a newly developed borehole deformation probe (mini-SIMFIP).