ETH Zurich has established the Bedretto Underground Laboratory for Geoenergies (BULG) in the Swiss Central Alps (http://www.bedrettolab.ethz.ch), where hydraulic stimulation techniques and associated induced seismicity will be studied. Purpose of the experiments is to improve the understanding of hydromechanical processes linked to the creation of a deep geothermal reservoir. The BULG is located in relatively homogeneous granite with an overburden of around 1000 m. In context of the initial in-situ stress field and rock mass characterisation campaign, several hydraulic mini-frac tests were conducted. Four 30 m-long vertical and two 40 m-long inclined boreholes were analysed in detail to complement preliminary reported stress magnitudes. Several measurement protocols were utilised within the mini-frac tests conducted at five depth intervals in each borehole.
Mini-frac tests can provide extensive information about the stress magnitudes, stress directions and rock properties like permeability or stiffness. The pressure decay analysis after pump shut-in was used to infer fracture closure pressure, whereas dry packer reopening tests gave an estimation of the fracture reopening pressure and system stiffness. Shut-in times were varied from several minutes to one hour or overnight (12 to 14 h) to display effects on the fracture closure analysis and obtain the local pore pressure. The latter ranges between 2.4 to 5.3 MPa, an indication of tunnel drainage effects. Since fracture closure pressure determination, an approximation to the minimum horizontal stress, is controversial in the literature, several analysis techniques were compared: G-function, square root of time, bilinear pressure-decay and jacking pressure. In most cases, the applied techniques give consistent results in a range of 1 to 2 MPa, but sporadically differences are larger. The fracture compliance method was used to identify the point where the stiffness of the fracture increases, related to the beginning of its closure. Therefore, it is the most accurate indication of minimum principal stress, which ranges between 12.6 to 15.2 MPa. Derived magnitudes for the maximum horizontal stress lie between 17.8 to 23.8 MPa.
Increasing fracture stiffness was correlated to a linear or bilinear flow regime on log-log scale plots of the pressure derivative. Like the fracture closure pressure, the formation breakdown, fracture reopening and instantaneous shut-in pressures show intra- and inter-borehole variations. The proximity to naturally fractured regions, which were located on borehole logs, seems to influence the data quality and pressure values. Slip-tendency calculations indicate that the pressure range reached during the mini-frac tests is sufficient to reactive these fractures. As the induced tensile fracture propagates further away from the borehole with every injection cycle, it becomes more and more likely that it intersects pre-existing fractures. This is seen as multiple closure signature on several of the used diagnostic plots, where it is beneficial to have extended observation times (≥ 1 h) to fully characterise the different closure behaviours.