Decimeter-scale hydraulic testing of pre-existing Fractures (HTPF) under anisotropic stress conditions. Part 2: Test procedure and results

To extract heat from tight deep subsurface formations, hydraulic stimulation is used to create efficient heat exchangers in the context of enhanced geothermal systems (EGS). Successful geothermal reservoir initiation requires detailed characterization of the regional in-situ stress field and local stress field variations of heterogeneous, i.e. fractured and faulted rock masses, which is often achieved by hydraulic fracturing (HT) and hydraulic testing of pre-existing fractures (HTPF).

To further develop and validate these field-scale in-situ stress determination techniques, decimeter laboratory scale HTPF tests into saw-cut and polished fracture planes are performed in a true triaxial testing apparatus (see Cadmus et al. 2023, this conference for details). Tests are carried out under various anisotropic stress boundary conditions to analyze normal opening and shear behavior. Results of two rock samples with different fracture roughness are analyzed and compared in terms of the injection pressure and flow rate evolution, shear displacement and acoustic emissions. The results indicate that both shear displacement and hydraulic jacking initiate as the injection pressure exceeds the fracture normal stress, which is attributed to heterogeneous fluid pressure distribution in the fracture plane. Future experiments on a fracture surface of intermediate roughness with additional observation boreholes aim to further assess the effects of fluid pressure distribution and fracture plane roughness in the context of stress measurement and induced seismicity.