276 Geophysics in Geothermal Exploration magnetic field sensor) developed by the LBNL (Lawrence Berkeley National Laboratory of the University of California), and the detectability of the “cold fronts” had to be validated in situ. The doublet where the measurements were taken is part of Dalkia’s geothermal plant in Evry. In early January 2022, a series of injection points, laid out according to the recommendations from different modellings were prepared in anticipation of the acquisition campaign. On the 1st of March, SDP logging team lowered the probe into the well but was unable to pass through the open-hole section. After some adjustments, the probe successfully detected the surface source signal. Data analysis confirmed that the observed signal was indeed emitted by the surface source, thus validating a key aspect of the technology. Additionally, the models indicated that the secondary field generated by a cold front exceeds the noise level recorded by the downhole probe, demonstrating the detectability of the cold front. Introduction This project is initiated as part of the Géodénergies program. Its goal is to develop a methodology for detecting and monitoring the cold front progress between geothermal doublets of the Dogger formation using Controlled Source Electromagnetic (CSEM) methods. From a theoretical standpoint, the coldwater plume is associated with a variation in electrical resistivity within the reservoir (Revil et al., 1998), which can be detected by geophysical CSEM methods (Wawrzyniak et al., 2016). The project consists of two main parts: 1. At the laboratory scale, a calibration of resistivity variations as a function of temperature and frequency specific to the Dogger formation is conducted to accurately characterize this relationship. 2. Then, at the reservoir scale, a CSEM measurement campaign is carried out on a geothermal doublet using a surface-to-borehole measurement configuration. For this, a magnetic probe developed by LBNL (Lawrence Berkeley National Laboratory at the University of California) is lowered into the production well of the Dogger doublet. This will allow for in situ demonstration of the detectability of the “cold front”. The Evry geothermal doublet operated by Dalkia. In this study case, we will introduce motivations and a summary of the technology used. We then describe the measurement design, including all preparatory work and the campaign’s execution. Finally, we address the data processing and the interpretation of the results, leading to the validation of the cold fronts detectability.
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