Self-potential and electromagnetic radiation monitoring of hydraulic fracturing experiments at the Äspö hard rock laboratory (Sweden)

At the Äspö hard rock underground laboratory in Sweden, six in situ hydraulic fracturing experiments took place at 410 m depth. A multistage hydraulic fracturing approach is tested with a low environmental impact, e.g., induced seismicity. The idea is to mitigate induced seismicity and preserve the permeability enhancement process under safe conditions. The fractures are initiated by two different injection systems (conventional and progressive). An extensive sensor array is installed at level 410 m, including simultaneous measurements of acoustic emissions, electric self-potential, and electromagnetic radiation sensors. The monitoring catalog includes more than 4300 acoustic emission events with estimated magnitudes from the continuous monitoring setup (in-situ sensors between 1-100 kHz). The experiment borehole F1 is drilled in the direction of Shmin, perpendicular to the expected fracture plane. Two electromagnetic radiation sensors are installed and aligned to (i) Shmin and (ii) the expected fracture plane with a sampling rate of 1 Hz and a frequency range between 35-50 kHz. The self-potential sensors are installed at level 410 with a distance of 50-75 m from the borehole F1, including nine measuring probes and one base probe. A second self-potential setup is deployed at level 280 m in the far-field with a distance of 150-200 m from F1. The self-potential data were measured with a sampling rate of 1 Hz. For the first time (to our knowledge), the electric and electromagnetic monitoring results of two hydraulic stimulation at mine-scale are presented. The results are discussed, including the different injection types (one conventional and one progressive experiment) and the acoustic emission events. The self-potential results reveal increases in amplitude during both hydraulic fracturing experiments at both depth levels. A second increase in the self-potential was observed only during the conventional injection and only at level 280 m. This is consistent with the results of the acoustic emission catalog, which show a larger number and larger magnitude of events during conventional injection experiments. The changes in the electromagnetic field are predominantly in the direction of Shmin during both the conventional and the progressive injection experiments.