Continuous in-situ monitoring of dissolved gases for the characterization of the Critical Zone with a MIMS

International audienceIn the perspective of a temporal and spatial exploration of the Critical Zone, we developed an in situ monitoringinstrument for continuous dissolved gas analysis (N2, O2, CO2, CH4, He, Ne, Ar, Kr, Xe). With a large resolution(5 orders of magnitude) and a capability of high frequency multi-tracer analysis (1 gas every 1.5 seconds), theMIMS (Membrane Inlet Mass Spectrometer) is an innovative tool allowing the investigation of a large panel ofphysical and biogeochemical processes.First of all, this study presents the results of groundwater tracer tests using dissolved gases in order to evaluatetransport properties of a fractured media in Brittany, France (Ploemeur, ORE H+). The tracer test experimentshowed that the MIMS is perfectly suitable for field work. The instrument provides precise measurements accurateenough to produce breakthrough curves during groundwater tracer tests. The results derived from 4He data givestransport parameters in good agreement with the results obtained with a fluorescent tracer.Combined with a pump and a multi-parameter probe, the MIMS is also capable to perform accurate dissolved gaseswell-logs allowing a real-time estimation of recharge conditions (temperature, excess air), aquifer stratification,redox conditions and groundwater residence time by 4He dating.Therefore, the MIMS is a valuable tool for in situ characterization of biogeochemical reactivity in aquatic systems,the determination of aquifer transport properties, the monitoring of groundwater recharge conditions and thecharacterization of aquifer-river exchanges

Quantification of coupled fluid flow and reactive transport using a dissolved gas tracer test in a fractured media

International audienceIdentification of biogeochemical reactions inaquifers and determining kinetics is important for theprediction of contaminant transport in aquifers andgroundwater management. Therefore, experimentsaccounting for both fluid flow and reactive transportare essential to quantify reactive transport propertiesat field scale.This study presents the results of a groundwatertracer test using the combined injection of dissolvedconservative and reactive gases (He, Xe, Ar, and O2)and NO3- in order to evaluate the transport propertiesof a fractured media in Brittany, France.Dissolved gas concentrations are continuouslymonitored in situ with a CF-MIMS (Continuous Flow- Membrane Inlet Mass Spectrometer) allowing ahigh frequency (1 gas every 1.5 seconds) multi-traceranalysis (N2, O2, CO2, CH4, N2O, H2, He, Ne, Ar, Kr,Xe) over a large resolution (6 orders of magnitude).Along with dissolved gases, groundwaterbiogeochemistry are monitored through the samplingof major anions and cations, trace elements andmicrobial diversity analysis.The results show breakthrough curves allowingthe combined quantification of conservative andreactive transport properties such as dispersivity,fracture aperture as well as reactions kineticparameters. This ongoing work consisting in the fieldcharacterization of biogeochemical reactivity is to beimplemented in other fracture networks in order toinvestigate the link between fluid flow properties andreaction kinetics

Characterization of shallow geothermal efficiency in fractured mediathrough thermal tracer tests and numerical modeling

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 of thelow storativity of the medium. Within this framework, the purpose of this study is to test the possibility of efficientthermal energy storage in shallow fractured rocks. For doing so, several heat tracer tests have been carried on in asingle well between two connected fractures. We completed this experimental work with numerical modeling ofthermal transport in fractures embedded in an impermeable conductive matrix.The thermal tracer tests were achieved in a crystalline rock aquifer at the experimental site of Ploemeur(H+ observatory network). The experimental setup consists in injecting hot water in a fracture isolated by a doublestraddle packer in the borehole while pumping and monitoring the temperature in a fracture crossing the sameborehole at greater elevation. Several tracer tests were achieved at different pumping and injection rates. Thisexperimental set up allowed to estimate temperature breakthrough for different tracer test durations and hydraulicconfigurations from fully convergent to perfect dipole tracer tests. Thanks to those tests and numerical modelingof heat transport in fractures, we demonstrate that temperature recovery is highly dependent on flow rate andstreamlines shape. Thus, thermal storage rate is inversely proportional to flow and is maximized in perfect dipoleconfiguration. These thermal tracer tests and numerical modeling allow to define the most efficient configurationfor optimizing shallow geothermal storage in fractured rock

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

Quantification of conservative and reactive transport using a singlegroundwater tracer test in a fractured media

International audiencedentification of biogeochemical reactions in aquifers and determining kinetics is important for the predictionof contaminant transport in aquifers and groundwater management. Therefore, experiments accounting for bothconservative and reactive transport are essential to understand the biogeochemical reactivity at field scale.This study presents the results of a groundwater tracer test using the combined injection of dissolved conservativeand reactive tracers (He, Xe, Ar, Br-, O2and NO3-) in order to evaluate the transport properties of a fracturedmedia in Brittany, France.Dissolved gas concentrations were continuously monitored in situ with a CF-MIMS (Chatton et al, 2016) allowinga high frequency (1 gas every 2 seconds) multi-tracer analysis (N2, O2, CO2, CH4, N2O, H2, He, Ne, Ar, Kr, Xe)over a large resolution (6 orders of magnitude). Along with dissolved gases, groundwater biogeochemistry wasmonitored through the sampling of major anions and cations, trace elements and microbiological diversity.The results show breakthrough curves allowing the combined quantification of conservative and reactive transportproperties. This ongoing work is an original approach investigating the link between heterogeneity of porousmedia and biogeochemical reactions at field scale.Eliot Chatton, Thierry Labasque, Je ́roˆme de La Bernardie, Nicolas Guihe ́neuf, Olivier Bour and LucAquilina; Field Continuous Measurement of Dissolved Gases with a CF-MIMS: Applications to the Physics andBiogeochemistry of Groundwater Flow; Environmental Science & Technology, in press, 2016

Thermal retardation in fractured media: theory and field measurement from joint heat and solute tracer test experiments

International audienceThe characterization of flow and transport in fractured media is particularly challenging because hydraulic conductivityand transport properties are often strongly dependent on the geometric structure of fractures at differentscales. In addition to advection and dispersion, heat transfer is also affected by thermal retardation and damping,which results from fracture-matrix diffusion. Here, we derive analytical expressions for thermal retardation anddamping for different fracture geometries and we show, from modeling and field experiments, that estimation ofthermal retardation and damping may provide new constraints on fracture geometry. We use the developed expressionsto interpret the results of single well thermal tracer tests performed in a crystalline rock aquifer at theexperimental site of Ploemeur (H+ observatory network). Thermal breakthrough is monitored with Fiber-OpticDistributed Temperature Sensing (FO-DTS), which allows the temperature monitoring with high spatial and temporalresolution. We demonstrate that the observed thermal response indicates that heat transfer is controlled by achannel fracture of large diameter rather than by a parallel plate fracture. These results point to a strong reductionof fracture-matrix exchange by flow channeling. These findings, which bring new insights on the effect of flowchanneling on heat transfer in fractured rocks, show how heat recovery in geothermal systems may be controlledby fracture geometry. This highlights the interest of thermal tracer tests as a complement to solute tracer tests toinfer fracture aperture and geometry

Quantification of conservative and reactive transport using a singlegroundwater tracer test in a fractured media

International audiencedentification of biogeochemical reactions in aquifers and determining kinetics is important for the predictionof contaminant transport in aquifers and groundwater management. Therefore, experiments accounting for bothconservative and reactive transport are essential to understand the biogeochemical reactivity at field scale.This study presents the results of a groundwater tracer test using the combined injection of dissolved conservativeand reactive tracers (He, Xe, Ar, Br-, O2and NO3-) in order to evaluate the transport properties of a fracturedmedia in Brittany, France.Dissolved gas concentrations were continuously monitored in situ with a CF-MIMS (Chatton et al, 2016) allowinga high frequency (1 gas every 2 seconds) multi-tracer analysis (N2, O2, CO2, CH4, N2O, H2, He, Ne, Ar, Kr, Xe)over a large resolution (6 orders of magnitude). Along with dissolved gases, groundwater biogeochemistry wasmonitored through the sampling of major anions and cations, trace elements and microbiological diversity.The results show breakthrough curves allowing the combined quantification of conservative and reactive transportproperties. This ongoing work is an original approach investigating the link between heterogeneity of porousmedia and biogeochemical reactions at field scale.Eliot Chatton, Thierry Labasque, Je ́roˆme de La Bernardie, Nicolas Guihe ́neuf, Olivier Bour and LucAquilina; Field Continuous Measurement of Dissolved Gases with a CF-MIMS: Applications to the Physics andBiogeochemistry of Groundwater Flow; Environmental Science & Technology, in press, 2016

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

Characterization of thermal tracer tests and heat exchanges in fractured media

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 of thelow storativity of the medium. Within this framework, the purpose of this study is to test the possibility of efficientthermal energy storage in shallow fractured rocks with a single well semi open loop heat exchanger (standingcolumn well). For doing so, several heat tracer tests have been achieved along a borehole between two connectedfractures.The heat tracer tests have been achieved at the experimental site of Ploemeur (H+ observatory network).The tracer tests consist in monitoring the temperature in the upper fracture while injecting hot water in the deeperone thanks to a field boiler. For such an experimental setup, the main difficulty to interpret the data comes fromthe requirement for separating the temperature advective signal of the tracer test (temperature recovery) from theheat increase due to injection of hot water through the borehole which induces heat losses all along the injectiontube in the water column. For doing so, in addition to a double straddle packer used for isolating the injectionchamber, the particularity of the experimental set up is the use of fiber optic distributed temperature sensing(FO-DTS); an innovative technology which allows spatial and temporal monitoring of the temperature all alongthe well. Thanks to this tool, we were able to estimate heat increases coming from diffusion along the injectiontube which is found much lower than localized temperature increases resulting from tracer test recovery. Withlocal temperatures probes, separating both effects would not have been feasible. We also show through signalprocessing how diffusive and advective effects may be differentiated. This allowed us to estimate temperaturerecovery for different heat tracer durations and setups. In particular we show that temperature recovery is highlydependent on hydraulic configuration such as perfect dipole or fully convergent heat tracer test

Biofilm development in a hotspot of mixing between shallow and deep groundwater in a fractured aquifer: field evidence from joint flow, chemical and microbiological characterization

International audienceBiofilms play a major role in controlling the fluxes and reactivity of chemical species transported in hydrologicalsystems. Their development can have either positive impacts on groundwater quality(e.g. attenuation of contaminants under natural or stimulated conditions), or possible negative effects on subsurfaceoperations (e.g. bio-clogging of geothermal dipoles or artificial recharge systems). Micro-organisms require bothelectron donors and electron acceptors for cellular growth, proliferation and maintenance of their metabolicfunctions. The mechanisms controlling these reactions derive from the interactions occurring at the micro-scalethat depend onmineral compositions, the biota of subsurface environment, but also fluid mixing, whichdetermines the local concentrations of nutriments, electron donors and electron acceptors.Hence, mixing zones between oxygen and nutriment rich shallow groundwater and mineralized deep groundwaterare often considered as potential hotspots of microbial activity, although relatively few field data document flowdistributions, transport properties, chemical gradients and micro-organisms distributions across these mixinginterfaces.Here we investigate the origin of a localized biofilm development observed in the fractured granite aquiferat the Ploemeur observatory (H+ network hplus.ore.fr).This biofilm composed of ferro-oxidizing bacteria isobserved in an 130m deep artesian well. Borehole video logs show an important colonization of the well by thebiofilm in the shallower part (0 to 60m), while it is inexistent in the deeper part (60 to 130m). As flow is localizedin a few deep and shallow fractures, we presume thatthe spatial distribution of biofilm is controlled by mixing between shallow and deep groundwater. To verify thishypothesis we conducted a field campaign with joint characterization of the flow and chemical composition ofwater flowing from the different fractures, as well as the microbiological composition of the biofilm at differentdepth, using pyrosequencing techniques. We will discussin this presentation the results of this interdisciplinary dataset and their implications for the occurrence of hotspotsof microbiological activity in the subsurface