Search for chemical tracers to identify the thermal breakthrough of Dogger geothermal doublets in the Paris basin (France) - Final report

The study for the search for chemical precursors, associated with the prediction of thermal breakthrough doublets, was carried out in the context of geothermal installations exploiting the Dogger reservoir in the Paris region. The main experimental data used are the chemical composition of the fluids, the properties of the stratified reservoir, and the operating parameters of doublets. The methodological approach involves three aspects: analysis of data from periodic monitoring of operation, modelling of the chemical evolution of the heat transfer fluid, and numerical simulation of transfers within the reservoir. Chemical tracing is generally used to quantify mass transfer processes and to identify the equivalent reservoir structure. Applied to the case and combined with thermal transfers, it enables us to define the concept of chemical precursor in relation to the energy decline to be characterized. The various topics contributing to the analysis of the feasibility of chemical tracing, are presented in five chapters. They cover: - an inventory of the various potential chemical precursors induced by geothermal geothermal fluid (chapter 1); - characterizing the typical response of a doublet following injection of a tracer (chapter 2). The results of the numerical simulation of transfers between wells are intended to quantify the properties of the observable response: general appearance, relative amplitude, evolution dynamics with time; - analysis of implementation conditions and assessment of the feasibility of artificial tracing by brief injection (chapter 3); - study of the phases and reactions constraining the equilibrium of sulfides (chapter 4), in order to verify whether this species can constitute a potentially exploitable precursor of the thermal breakthrough; - the application of the previous technique (measurement of dissolved sulfur concentration) to the case of two doublets in the Paris region, for which we have periodic wellheads production (chapter 5). The comparison of results of the experimental monitoring with those of the transfer simulation is intended to illustrate the value of this method. The conclusion summarizes the main lessons learned from this approach, in order to assess the potential and constraints of chemical tracing when used to clarify the notion of the thermal lifetime of a doublet. The documents presented in the appendices concern the experimental data used, the results of the modeling of the chemical evolution of the fluid, and the evaluation of the cost of the tracer for a brief injection of tracer solutions enriched in heavy stable isotopes (oxygen-18 and deuterium).