title: Self-Potential and electromagnetic monitoring during ﬂuid injection into magmatic rocks
Creators: name: Haaf, Nadine
Publisher: name: UNSPECIFIED
Year: 2022

description: Still today, induced seismicity is a major challenge during massive hydraulic stimulation in geothermal energy. Several approaches have been made to deal with it: e.g., adapted trafﬁc light systems for real-time monitoring of hydraulic parameters and induced seismicity or testing new injection protocols such as fatigue hydraulic fracturing. In reducing the magnitudes and number of seismic events, the seismic signal is weakened, affecting the seismic monitoring itself again. Therefore, new monitoring techniques are of major interest. Recent studies showed the successful application of electric and electromagnetic monitoring surveys in the context of hydraulic injection experiments. This thesis includes data sets from two different injection experiments at the reservoir- and the mine-scale to investigate possible electrokinetic or seismoelectric effects and to reproduce the observations from other injection experiments, such as Rittershoffen. The experiments at reservoir-scale were electromagnetically monitored using magnetotelluric (MT) data. The results of this study show a temporal relation between decreasing apparent resistivity in the period ranges of 0.15-1 s and 4-8 s and (i) the geomagnetic ﬁeld activity, (ii) the ﬂuid losses up to 60 L/s, as well as (iii) mechanic processes occurring before induced seismicity. Against this background, the full physical meaning of the apparent resistivity changes remains a matter of debate. Note that forward modeling of the effect of the injected water volume could not explain the observed resistivity changes. Hydraulic fracturing (HF) experiments at underground lab scale, HF2 (conventional) and HF3 (fatigue hydraulic fracturing), were self-potential (SP) and electromagnetic radiation (EMR) monitored. Changes in SP have been observed with a temporal delay in both experiments in the near-ﬁeld sensors (about 50 - 70 m distance from the experiment). However, in the far-ﬁeld sensors (about 150 - 200 m from the experiment), such changes are observed only during the HF2 experiment. Furthermore, the background signal is reached about 45 minutes after the last pressure release. Generally, minima and maxima obtained from different electrode offsets between the individual injection steps are in phase. In contrast, after completing the two experiments, HF2 and HF3, the major minima and maxima are characterized by a signiﬁcant phase shift. In addition, during shut-in phases, the results show an inverse correlation between pressure drop and increasing EMR amplitudes during HF2 and HF3. In conclusion, the objectives of this thesis have been achieved by (i) providing a new magnetotelluric monitoring data set of ﬂuid injection in a different rock type, i.e., basalt, (ii) ﬁlling the scale-gap of SP and electromagnetic (EM) data sets by monitoring underground-lab scale injection tests, and (iii) indicating precursor of seismic events under controllable condition. However, some open questions remain, such as if the precursor observations are veriﬁed in other experiments and on underground lab- or lab-scale. And last but not least, is the supposed link between pressure and EM or SP signal linked to ﬂuid ﬂow on micro-fractures or only to pressure.