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This document synthetizes the results obtained on the determination of the hydrothermal properties of the geothermal aquifer of the Dogger, in the Paris basin (France). This work was carried out by the Paris School of Mines, Laboratory of Mathematical Hydrogeology-ARMINES, in close cooperation with the BRGM (French Geological Survey), the CEA-CENG (Atomic Energy Commission, Grenoble), the IMRG (Institute for Geothermal Research), the SPEG (Company for Geothermal Development), with the help of GEOCHALEUR and the cities of Fontainebleau and Cachan. The work was jointly supported by the AFME (French Agency for Energy), the Commission of the European Communities and the CNRS-PIRSEM (French National Research Organization, Program for Energy and Resources). The main emphasis was put into the determination of the natural velocity of the water in the Dogger. Two methods of measurements were used, one based on the in situ dilution of a radioactive tracer in the well with time, the other on the calculation of the velocity from the permeability and the pressure gradients measured at the bottom of the wells. A detailed thermal logging of the Dogger reservoir was also used for the interpretation of the results. This series of measurements was made initially in Fontainebleau, and then repeated on two additional wells in Cachan. They provide a coherent picture of the velocity in the aquifer. The main conclusions are that it is no longer possible to consider the natural velocity in this aquifer as negligible, as was done so far. The direction of flow is grossly towards the north-west, and its magnitude is estimated as follows: Darcy's velocity Mean pore water velocity Fontainebleau 17 m/ a 130 m/ a Cachan 15 m/ a 100 m/ a It was shown that such a velocity has a significant impact on the life span of a geothermal doublet, especially if the orientation of the wells is chosen in the worst direction with respect to that of the flow. It is therefore suggested that the orientation of any new doublet should be made with a favorable direction with respect to that of the velocity. To this end, preliminary information has been assembled on the general flow direction in the Dogger aquifer (potentiometric maps, salinity maps). This information should however be improved; it is also planned to analyse the situation of the major existing doublets in the Paris region with respect to this new concept. An additional experiment was carried out at Fontainebleau: the measurement of the breakthrough time of a tracer between the two wells of the doublet. To this end, a radioactive tracer (tritium) was injected into the reinjection well and is expected to arrive at the production well after 1989, given the distance between the two wells of the doublet (1 km). This breakthrough time of a tracer is indeed in the order of 4 to 5 times shorter than that of the thermal front. Finally, an experiment to measure the thermal properties of the aquifer has been designed. It requires the injection and then the reproduction of water of a different temperature in one of the wells of the doublet (single well technique). This experiment should be performed prior to 1989 in order to complement the information which will be obtained from the tracer breakthrough time experiment. This report is divided into four chapters and has eight appendices.

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Bibliographic Reference: EUR 10281 FR (1985) MF, 70 P., BFR 150, BLOW-UP COPY BFR 350, EUROFFICE, LUXEMBOURG, POB 1003
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