Decimeter-scale hydraulic testing of pre-existing fractures (HTPF) under anisotropic stress conditions. Part 1: Experimental setup

Sustainable development of enhanced geothermal systems (EGS) requires a comprehensive understanding of the in-situ stress conditions. Among several techniques developed in the last decades, Hydraulic Testing of Pre-existing Fractures (HTPF) is a common practice to quantify the normal stress acting on hydraulically isolated fractures. To better understand the hydromechanical coupled mechanisms involved during HTPF as well as validation of HTPF analysis methods, a decimeter-scale true triaxial testing (TTT) apparatus was utilized that allows HTPF measurements on fracture planes under anisotropic stress conditions.

This work is primarily focusing on the experimental setup, calibration, and boundary conditions of the TTT apparatus. The apparatus contains three sets of oil-filled flat-jacks, which independently control the stress boundary conditions in the (horizontal) x, y, and (vertical) z directions. Contained within a steel frame, the flat-jacks exert pressures onto loading plates, which transfer and distribute pressure onto the sample’s surfaces. The 30*30*45cm cuboidal granite sample contains a vertical saw cut fracture, oriented 45° in respect to the x and y axis. Boreholes, drilled through the sample to the fracture surface, allow high-pressure fluid injection and monitoring of the fracture fluid pressure. Thirty-two acoustic emission sensors, mounted in the loading plates, allow for active and passive seismic measurement. The deformation of the rock sample is captured by 16 linear variable differential transformer (LVDT) displacement sensors mounted externally to the loading plates. The test procedure and preliminary results of the HTPF tests can be found in Osten et al. (2023; this conference).