277 10. Feasibility of monitoring cold fronts of geothermal doublets 10.1 Context The risk of thermal breakthrough is a long-term consequence of operating Dogger geothermal doublets with reinjection of cooled brine back into the original aquifer. Currently, measuring the temperature at the wellhead of the production well is the only parameter that enables detection of the arrival of the “cold front”. Thermohydrodynamic (TH) predictive modeling allows for extrapolation of its dynamic behavior (Figure 10.1) but cannot precisely predict its arrival in time and space. Consequently, cases of thermal breakthrough may occur unexpectedly, as was the case in L’Hay-les-Roses (1 °C decrease after 30 years of operation) and Alfortville (5 °C decrease after 30 years of operation). At present, there is no tool available for measuring the aquifer temperature other than direct measurement within a well. The proposed technological development is an important element for monitoring geothermal reservoirs. Figure 10.1 Example of Thermo-Hydrodynamic (TH) modeling used to “predict” the behavior of the cold front over time. Extracted from the activity report submitted by Dalkia for the Evry site. The CSEM method is sensitive to variations in the subsurface’s electrical resistivity. A source injects electric current into the ground using a square wave signal of predetermined frequency and intensity. One or more receivers simultaneously record electromagnetic fields (electric, magnetic, or both) at the surface or in the borehole. These fields combine the primary field (signal emitted by the source) and a secondary field generated by the distribution of subsurface resistivities. After data processing, the calculated transfer functions can then be “inverted” (a mathematical process) to retrieve the subsurface structure in terms of electrical resistivity. Wawrzyniak (2019) proposed using CSEM for borehole detection of the cold front within the CO2 Dissolved project. This work provided guidances and recommendations on the types of sources/receivers and their configurations to use, and it
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