Hydraulic and thermal impact modelling at the scale of the geothermal heating doublet in the Paris Basin, France

13 pagesInternational audienceThe Paris Basin is extensively developed for the geothermal district heating (GDH) of approximately 150 000 dwellings. As of late 2010, thirty four GDH systems apply the doublet concept in the Paris suburban area and mine the heat of the Dogger reservoir, a limestone formation of Mid-Jurassic age at depths ranging from 1500 to 2000 m. As the brine is fully reinjected, cold water bodies progressively invade the reservoir around injector wells inducing both thermal and hydraulic interactions at the doublet scale. The premature production well cooling and the sustainable development of the resource highlight two critical parameters, the thermal breakthrough time (tB) and the extent of the cooled fluid bubble(S) respectively. First, a set of benchtest simulations was launched to compare the sensitivities of tB and S parameters to selected reservoir conceptual model typologies. These simulations were applied on a GDH doublet undergoing a suspected thermal breakthrough. Five reservoir modelling teams validated their “in house” simulations by (i) checking an analytical (Gringarten-Sauty, 1979) solution, and (ii) testing three candidate reservoir structures on the doublet considered remotely located (i.e. not interfering with nearby exploitations) for a first step. The outcome resulted in a rewarding insight into the variability of simulation outputs. An additional segment will enable the actors to compare their modelling expertise on the same doublet considered in interaction with the other GDH operations located in its environment. Second, BRGM carried out a survey towards various rehabilitation schemes (a new doublet or a triplet) and their contribution toward sustainability standards. From a hypothetical, twenty five year life, doublet simulation, an initial hydraulic/temperature field was derived. Then, several new well locations were simulated and isotherms, alongside production well cooling kinetics, compared accordingly. A two-stage rehabilitation scheme, i.e. triplet then a new doublet, seems to reconcile the resource longevity and the economic demand. Further work is required to compare the different designs with a method integrating both the impact of the geothermal exploitation on the resource and the lifetime of the exploitation in a single mathematical factor

A simplified fracture network model for studying the efficiency of a single well semi open loop heat exchanger in fractured crystalline rock

International audienceGeothermal energy is a renewable energy source particularly attractive due to associated low greenhouse gasemission rates. Crystalline rocks are in general considered of poor interest for geothermal applications atshallow depths (< 100m), because of the low permeability of the medium. In some cases, fractures may enhancepermeability, but thermal energy storage at these shallow depths is still remaining very challenging because ofthe complexity of fractured media. The purpose of this study is to test the possibility of efficient thermal energystorage in shallow fractured rocks with a single well semi open loop heat exchanger (standing column well). Fordoing so, a simplified numerical model of fractured media is considered with few fractures.Here we present the different steps for building the model and for achieving the sensitivity analysis. First,an analytical and dimensional study on the equations has been achieved to highlight the main parameters thatcontrol the optimization of the system. In a second step, multiphysics software COMSOL was used to achievenumerical simulations in a very simplified model of fractured media. The objective was to test the efficiencyof such a system to store and recover thermal energy depending on i) the few parameters controlling fracturenetwork geometry (size and number of fractures) and ii) the frequency of cycles used to store and recoverthermal energy. The results have then been compared to reference shallow geothermal systems already set up forporous media. Through this study, relationships between structure, heat exchanges and storage may be highlighte

Heat-energy storage through semi-opened circulation into low-permeability hard-rock aquifers

International audienceIn low-permeability environments, the solutions of heat storage are still limited to the capacities of geothermalborehole heat exchangers. The ANR Stock-en-Socle project explores the possibilities of periodic storage ofsensitive heat1 in low-permeability environments that would offer much better performance than that of boreholeheat exchangers, especially in terms of unit capacity. This project examines the storage possibilities of usingsemi-open water circulation in typically a Standing Column Well (SCW), using the strong heterogeneity ofhard-rock aquifers in targeting the least favorable areas for water resources.To solve the main scientific issues, which include evaluating the minimum level of permeability requiredaround a well as well as its evolution through time (increase and decrease) due to water-rock interaction processes,the study is based on an experimental program of fieldwork and modelling for studying the thermal, hydraulic andgeochemical processes involved. This includes tracer and water-circulation tests by injecting hot water in differentwells located in distinct hard-rock settings (i.e. granite and schist) in Brittany, Ploemeur (H+ observatory network)and Naizin. A numerical modelling approach allows studying the effects of permeability structures on the storageand heat-recovery capacities, whereas the modelling of reactive transfers will provide an understanding of howpermeability evolves under the influence of dissolution and precipitation. Based on the obtained results, technicalsolutions will be studied for constructing a well of the SCW type in a low-permeability environment. This workwill be completed by a technical and economic feasibility study leading to an investment and operations model.This study aims to describe the suitability of SCW storage for shallow geothermal energy. In order to reachthese objectives, Stock-en-Socle is constructed around a public/private partnership between two public researchorganizations, Géosciences Rennes and BRGM, and two companies, Antea Group and Soletanche Bachy, expertsin groundwater and geothermal energy.1Sensitive heat: modifies the temperature of water and its surrounding solids without modification of physicalproperties, as opposed to latent heat that causes a phase change, such as vaporizatio