| Title | Continuous and time-lapse geothermal monitoring at Rittershoffen EGS project, northern Alsace, using magnetotellurics |
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| Authors | Abdelfettah, Y; Sailhac, P; Schill, E; Larnier, H; Matthey, P-D |
| Year | 2016 |
| Conference | European Geothermal Congress |
| Keywords | Magnetotelluric, geothermal reservoir characterization, monitoring, ECOGI, phase difference tensor |
| Abstract | Two fieldwork measurements were achieved in 2013 and 2014 in frame of short geothermal well test productions achieved at Rittershoffen geothermal project to perform continuous magnetotelluric monitoring. In 2013, time series were recorded during short period (~4 weeks) in two sites; 1) at Rittershoffen (RITT) seismic station, about 1.6 km eastward to the GRT-1 geothermal well, and 2) at OPS4, at the seismic observation station of the Soultz-Sous-Forêt (~5 km to the West of the GRT-1 borehole). This field experiment is considered as a test-period and as an introduction to a second field campaign achieved during 2014 and another one started in 2016. In 2014, we recorded the time series quasi-continuously at RITT site from June to mid-December. Moreover, 5 other sites were acquired before and after well-test production to achieve timelapse monitoring. These sites were located around GRT-1/2 geothermal wells. In total, six MT sites were acquired in 2014; one continuously and 5 temporary. MT Remote reference site was installed at Welschbruch Geophysical Station, about 70 km South from the studied area. The data were recorded by blocks of 24 hours using a sampling frequency of 512 Hz. To handle natural transient effects in the electromagnetic sources such as variable intensity, polarities and frequencies, data processing was carried out using the well-known, robust Chave’s code to estimate Z tensor. The phase tensor is computed from modulus and phase of each component of MTimpedance Z. This phase tensor has been shown to be independent of distortion. For monitoring applications where the objective is to quantify the transient effects, e.g. prior and after stimulation or pumping/fluid injection, the relative phase difference tensor defined from the phase tensor at two different dates is introduced. The obtained results shows unexplained oscillations: there are several maxima of the signal (anomaly) that do not clearly correspond to known geothermal activity except two of them during wellstimulation periods, i) during acid injection and ii) during brine injection. Current work is done to test different hypothesis about these signals that could be generated, not just by geothermal activity at depth but additionally to the man-made geothermal activity, or from the compression and extension of the geothermal reservoir induced by earth tides causing electromagnetic signal. |