In order to develop strategies for the mitigation of induced seismicity in enhanced geothermal systems (EGS), two out of six hydraulic fracturing experiments carried out the Äspö Hard Rock Laboratory (Sweden) in 2017 in a depth of 410 m were investigated in detail. Here we present electric self-potential monitoring during the conventional and the step-wise cyclic injection experiments HF2 and HF3. Electric self-potential data were acquired through two sensor arrays, each including nine measuring probes and one base probe, that were installed at the 410 m and 280 m levels. The experimental borehole F1 is drilled in the direction of Shmin, perpendicular to the expected fracture plane. The self-potential sensors are installed sub-parallel to Shmin at level 410 at a distance of 50-75 m to the borehole F1 and sub-perpendicular to Shmin at level 280 m at a distance of 150-200 m to F1. The self-potential data were measured at a sampling rate of 1 Hz. SP signals were compared with hydraulic, acoustic (AE), and tilt data obtained during HF2 and HF3. Several studies of SP during injection experiments have shown that SP data are commonly interpreted as streaming potential only. Here we present two short modeling studies to additionally estimate the effect of electrochemical and electrothermal potentials on the SP amplitudes.

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