Jeremy Godinaud

Maria Klepikova

F Larroque

Nicolas Guihéneuf

A Dupuy

Olivier Bour

Clogging detection and productive layers identification along boreholes using Active Distributed Temperature Sensing

Keywords: Distributed Temperature Sensing, Groundwater Flux, Thermal Conductivity, Clogging, ATES plumes extension [Bloemendal

  L'archive ouverte pluridisciplinaire

Introduction

Aquifer Thermal Energy Storage (ATES) is an open loop geothermal system involving at least two reversible boreholes [START_REF] Fleuchaus | Worldwide application of aquifer thermal energy storage -A review[END_REF].

This technology uses the subsurface as transient long-term storage of cold and warm water and it gains more and more popularity with a great potential to reduce green house gas emissions of the building sector [Vanhoudt et al., 2011[START_REF] Gao | A review on system performance studies of aquifer thermal energy storage[END_REF][START_REF] Ghaebi | Economic and environmental evaluation of different operation alternatives to aquifer thermal energy storage in Tehran, Iran[END_REF][START_REF] Schüppler | Techno-economic and environmental analysis of an Aquifer Thermal Energy Storage (ATES) in Germany[END_REF][START_REF] Stemmle | Environmental impacts of aquifer thermal energy storage (ATES)[END_REF].

ATES performance is highly sensitive to hydrogeological conditions. It requires good aquifer permeability to pump and inject water into the aquifer, low natural groundwater flow to avoid stored energy losses due to advection and/or thermal interaction between wells. During system operation, storage efficiency and energy recovery rate depend on the cold and warm thermal lar point and can provide information on fracture connectivity, transmissivity and storativity distribution [START_REF] Molz | Borehole flowmeters: field application and data analysis[END_REF][START_REF] Paillet | Flow modeling and permeability estimation using borehole flow logs in heterogeneous fractured formations[END_REF][START_REF] Roubinet | Development of a new semi-analytical model for cross-borehole flow experiments in fractured media[END_REF]. These tests are simple to realize and can be performed in short time.

Heat or solute tracer tests are used to investigate groundwater velocity, thermal and hydraulic dispersivity, or location of preferential groundwater paths [START_REF] Bodin | Solute transport in a single fracture with negligible matrix permeability: 2. mathematical formalism[END_REF][START_REF] Klepikova | Heat tracer test in an alluvial aquifer: Field experiment and inverse modelling[END_REF][START_REF] Irvine | Heat and Solute Tracers: How Do They Compare in Heterogeneous Aquifers?[END_REF]. But they are more difficult to realize, and a robust interpretation could require a lot of observation points, as well as comprehensive numerical modeling. Thermal Response Tests (TRT) are used to estimate the average thermal conductivity of aquifers at the vicinity of the tested well, but the data collection could require a long time [START_REF] Beier | Reference data sets for vertical borehole ground heat exchanger models and thermal response test analysis[END_REF][START_REF] Dong | A regional study of in-situ thermal conductivity of soil based on artificial neural network model[END_REF]. Finally, cores collection is financially costly, time-consuming, and their analysis provides only local information which could not be representative of the global in-site aquifer dynamics [START_REF] Mostafa | Investigation of buoyancy effects on heat transfer between a vertical borehole heat exchanger and the ground[END_REF].

Preventive actions to tackle clogging development include reduction in the frequency of pump starts and stops, underground removal of iron, avoiding possible oxygen infiltration or reduction of water turbidity and organic content. Nevertheless, a certain clogging extension cannot be avoided during the recharge process, thus leading to the decrease of geothermal system performance [START_REF] Song | A review of recharge and clogging in sandstone aquifer[END_REF]. Surveillance of the clogging evolution is thus a major challenge for ATES systems, and developing adapted countermeasures on site could significantly help to maintain their performance. Recently, a strategy was developed to follow its evolution [START_REF] Gjengedal | Fouling and clogging surveillance in open loop GSHP systems: A systematic procedure for fouling and clogging detection in the whole groundwater circuit[END_REF]. The surveillance procedure aims to conduct a step-test with incremental increases in the groundwater flow rate while measuring the pressure and temperature responses in the system components. Nevertheless, it does not focus on clogging locations in the well, but gives an overall view of each component of the geothermal system (production wells, submersible pump and groundwater heat exchanger).

Distributive Thermal Sensing with Fiber Optic cable (FO-DTS) opens a timely opportunity to improve our understanding of the functioning of ATES systems. This technology enables to collect efficiently long data set and to monitor dynamics process [START_REF] Bense | Distributed Temperature Sensing as a downhole tool in hydrogeology: SUBSURFACE DTS[END_REF]. For geothermal application, it was first used to monitor temperature variations along production boreholes [START_REF] Hurtig | Fibre optic temperature sensing: application for subsurface and ground temperature measurements[END_REF]. DTS methods have seen further improvements with the use of the active mode (ADTS). It consists on deploying a heat source in the well and monitoring the induced temperature changes [START_REF] Bense | Distributed Temperature Sensing as a downhole tool in hydrogeology: SUBSURFACE DTS[END_REF]. The temperature elevation induced by the heat source and the subsequent cooling directly depends on thermal properties of the porous media and on water flux dissipating heat by advection. Among other, it can be used to achieve Enhanced Thermal Response Test (ETRT) to establish thermal conductivity profile for Borehole Heat Exchanger (BHE) application [START_REF] Freifeld | Ground surface temperature reconstructions: Using in situ estimates for thermal conductivity acquired with a fiber-optic distributed thermal perturbation sensor[END_REF][START_REF] Huber | Geothermal Field Tests with Forced Groundwater Flow[END_REF][START_REF] Vélez Márquez | Distributed Thermal Response Tests Using a Heating Cable and Fiber Optic Temperature Sensing[END_REF][START_REF] Zhang | Actively heated fiber optics based thermal response test: A field demonstration[END_REF][START_REF] Hakala | Novel use of the enhanced thermal response test in crystalline bedrock[END_REF]. Furthermore, the method proved its ability to measure groundwater flux distribution in the subsurface [START_REF] Read | Active-distributed temperature sensing to continuously quantify vertical flow in boreholes[END_REF][START_REF] Bakker | An active heat tracer experiment to determine groundwater velocities using fiber optic cables installed with direct push equipment[END_REF][START_REF] Bas | Estimation of the Variation in Specific Discharge Over Large Depth Using Distributed Temperature Sensing (DTS) Measurements of the Heat Pulse Response[END_REF]. Recent laboratory and field experiments demonstrate that analytical methods can be used to inspect the distribution of thermal conductivity and Darcy velocity along a fiber positioning in saturated porous media [START_REF] Simon | Numerical and Experimental Validation of the Applicability of Active-DTS Experiments to Estimate Thermal Conductivity and Groundwater Flux in Porous Media[END_REF][START_REF] Del | Heat Dissipation Test With Fiber-Optic Distributed Temperature Sensing to Estimate Groundwater Flux[END_REF]. These works enable the generalization of ADTS experiments interpretation.

Apart from aquifer thermal properties, the thermal conductivity profile established in a borehole could be also of major interest to inspect eventual low anomalies due to clogging development. Indeed, the value of thermal conductivity for clogging materials such as bentonite, clay or porous iron hydroxides provides a particular low signature useful for identification [START_REF] Tang | A study on the thermal conductivity of compacted bentonites[END_REF][START_REF] Cho | Thermal Conductivity of Compacted Bentonite and Bentonite-Sand Mixture[END_REF][START_REF] Gjengedal | Fouling and clogging surveillance in open loop GSHP systems: A systematic procedure for fouling and clogging detection in the whole groundwater circuit[END_REF][START_REF] Dalla | An updated ground thermal properties database for GSHP applications[END_REF].

To our knowledge, FO-ADTS experiments in boreholes have never been performed with a purpose of detecting both clogging development and productive layers. Most of the experiments achieved in boreholes mainly focus on fractured zone identification [START_REF] La Bernardie | Thermal Attenuation and Lag Time in Fractured Rock: Theory and Field Measurements From Joint Heat and Solute Tracer Tests[END_REF][START_REF] Maldaner | Groundwater Flow Quantification in Fractured Rock Boreholes Using Active Distributed Temperature Sensing Under Natural Gradient Conditions[END_REF][START_REF] Klepikova | Analysis of thermal dilution experiments with distributed temperature sensing for fractured rock characterization[END_REF], Darcy velocity estimation [Selker andSelker, 2018, del Val et al., 2021] or aquifer thermal conductivity [START_REF] Zhang | Actively heated fiber optics based thermal response test: A field demonstration[END_REF][START_REF] Hakala | Novel use of the enhanced thermal response test in crystalline bedrock[END_REF].

In this context, a cross-borehole ADTS experiment was carried out on a site located at the university of Bordeaux in France, and planned to be exploited by a new ATES. The test was realized under constant pumping rate condition and replicated in two different boreholes to verify the reciprocity of the data. The main objectives were (i) to characterize the vertical distribu-tions of thermal conductivity and Darcy velocity in two different boreholes, (ii) to reveal the preferential water pathways during injection (iii) to demonstrate the potential of ADTS to detect clogging area along well screens. Located in the North Aquitaine Basin, the experimental site corresponds to a large multi-layered aquifer system. The actual structure of this basin is the result of Pyrenees retro-foreland basin developed from the Campanian to the Miocene [START_REF] Labat | Influence of an anticline structure on hydrogeological functioning and aquifer interactions in a multilayered aquifer system: the case of Villagrains-Landiras anticline (Gironde, France)[END_REF]. The local geology corresponds to the top of the system formed by a succession of sedimentary deposits during multiple transgression/regression periods, leading to a vertical heterogeneous sequences of limestone, sandstone and clay from Eocene to Plio Quaternary times [START_REF] Larroque | Vertical heterogeneities of hydraulic aquitard parameters: preliminary results from laboratory and in situ monitoring[END_REF]. The proximal shore location of the site during Oligocene period results in highly horizontally and vertically heterogeneous limestone deposits [START_REF] Sztrákos | Révision lithostratigraphique et biostratigraphique de l'Oligocène d'Aquitaine occidentale (France)[END_REF]

Experimental Site

Context

Field data

The available field data, including gamma ray log, flowmeter logs and drill cuttings observations, highlight a heterogeneous limestone aquifer with a vertical and horizontal unequal distribution of incoming water fluxes. The layer crossed between the depth of 40-60 m (Figure 2a), is made of fossiliferous white limestone. A developed porosity is identified from microscopic drill cuttings observation. In this layer, the Pz51 Gamma Ray values fluctuate around an average of 25.0 cps, which indicates a low clay content (Figure 2d).

The GF1 flowmeter reveals that the incoming water is located from the depth of 40 m to 55 m. In this area, a layer of 2 meters thick between the depth of 44 m and 46 m produces about the half of the total discharge (Figure 2e).

Experiment and methodology

Experimental set up

ADTS experiments were conducted in Pz101 and Pz51. A loop of BRUsens fiber-optic cable (SOLIFOS -Fiber Optic Systems AG, Switzerland) was lowered to their annular space. It contains two multi-mode fibers encased in a sealed stainless steel capillary and covered by wire rope and a PVC jacket (Figure 3a). Two electrical cables were connected to the steel armor of both fiber branches of the loop, to allow the injection of electricity from a power controller. These connections were further sealed by epoxy resin. The induced temperature variation was monitored with a Silixa XT DTS unit reporting temperature every 25 cm at a 2 min sampling interval in Pz51 and 1 min sampling interval in Pz101 (Figure 3b).

In order to ensure contact with the porous media, while deployed in the borehole, each branch of the FO cable was slightly stretched and attached outside the polyvinyl chloride (PVC) tube every 3 meters (Figure 3c). This installation was similar to a previous study in 25 m depth boreholes [START_REF] Del | Heat Dissipation Test With Fiber-Optic Distributed Temperature Sensing to Estimate Groundwater Flux[END_REF]. For both piezometers, 50 meters of FO cable of each side of the loop remained outside the borehole for connection to the DTS unit, and for calibration purposes. Once the whole equipment installed, the gravel pack was added down, which embedded the FO cable in the gravel and prevented any cable shaking.

Temperature calibration process and DTS spatial resolution

During the experiment, 10 meters of each branch from sticking out cables were immersed into separate warm and cold calibration baths for temperature calibration purposes. In order to cover the full range of temperatures that could be expected to be measured along the fiber optic cable, the cold bath was filled with ice and the warm bath was equipped with a resistance to maintain water temperature at 31°C (Figure 3b). The temperature was recorded in each bath with a PT100 probe (0.1°C accuracy) and two RBR SoloT probes (0.002°C accuracy). The temperature in both baths was homogenized with small aquarium bubblers. The loop configuration of the fiber The DTS spatial resolution (S.R) during the experience was estimated from the classical 90% step change [START_REF] Tyler | Environmental temperature sensing using Raman spectra DTS fiber-optic methods: DTS FIBER-OPTIC SENS-ING[END_REF] and the derivative method [START_REF] Simon | A Comparison of Different Methods to Estimate the Effective Spatial Resolution of FO-DTS Measurements Achieved during Sandbox Experiments[END_REF]. For Pz101, the S.R was estimated between 0.9 and 1.1 m. As the sampling time (S.T) was twice larger for Pz51 experiment, the S.R was better and range from 0.7 to 0.9 m. The S.R of these experiments were more precise than the 2-3 m S.R reported in the study of Tyler [START_REF] Tyler | Environmental temperature sensing using Raman spectra DTS fiber-optic methods: DTS FIBER-OPTIC SENS-ING[END_REF] and slightly less precise than the 0.5-0.9 m S.R reported in the laboratory study of Simon [START_REF] Simon | A Comparison of Different Methods to Estimate the Effective Spatial Resolution of FO-DTS Measurements Achieved during Sandbox Experiments[END_REF].

Experiments proceedings

The experiment was ran under cross-borehole configuration. It is a common practice for integrated aquifer characterization. It is an efficient method to explain complex hydraulic behavior of aquifers and investigate preferential flow-path in heterogeneous contexts [START_REF] Butler | Hydraulic tests in highly permeable aquifers[END_REF], Le Borgne et al., 2006a,b, Audouin and Bodin, 2008, Brauchler et al., 2010[START_REF] Lods | Groundwater flow characterization of an ophiolitic hardrock aquifer from cross-borehole multi-level hydraulic experiments[END_REF][START_REF] Aliouache | An inverse approach integrating flowmeter and pumping test data for three-dimensional aquifer characterization[END_REF]. This configuration is therefore of great interest to

Experimental data analysis

Before the heating phase, the initial temperature distribution was recorded.

It represented a C-shape with a minimum temperature of 15.4 • C between the length interval of 35-65 m depth (Figure 5a). Such a temperature profile is typical for subsurface temperature regime influenced by an increase in ground surface temperature caused by global warming and/or urbanization [START_REF] Dědeček | Detection and quantification of local anthropogenic and regional climatic transient signals in temperature logs from Czechia and Slovenia[END_REF][START_REF] Visser | The thermal impact of aquifer thermal energy storage (ATES) systems: a case study in the Netherlands, combining monitoring and modeling[END_REF][START_REF] Bayer | Extracting past atmospheric warming and urban heating effects from borehole temperature profiles[END_REF]. Figure 5b represents transient temperature profiles along Pz51 (455 curves) acquired during the heating phase of the experiment.

As pointed out by the heat map, the U-shape of the cable provided two similar data sets at each sampling depth in Pz51. Data from Pz101 (423 curves, not displayed in Figure 5) are analyzed further in the manuscript.

When the heating started (Experiment 1 and 3), there was a quick temperature increase along the cable, followed by a phase of slow change. As 1) induced by the injected water:

∆T inj = T t -T inj.start (1)
where T inj.start is the temperature profile just before the beginning of water injection, and T t the temperature profile measured at the time t after the beginning of the injection. The evolution of ∆T inj profiles revealed three different zones in both piezometers (Figure 6a andc). The two-dimensional heat transport in saturated porous media (Equation 2) involving conduction and advection processes is expressed as [START_REF] Scott Carslaw | Conduction of Heat in Solids[END_REF] :

∂T ∂t = D t ∂ 2 T ∂x 2 + ∂ 2 T ∂y 2 -q ρ w c w ρ b c b ∂T ∂x , ( 2
)
where T is the temperature [K], x and y are the coordinates [m], q is the Darcy velocity [m s -1 ], ρ b c b the volumetric heat capacity of the rock/fluid ma-

trix [J m -3 K -1 ], ρ w c w the volumetric heat capacity of the water [J m -3 K -1 ].
Ignoring thermal dispersion is a common practice for the interpretation of ADTS experiments [START_REF] Bakker | An active heat tracer experiment to determine groundwater velocities using fiber optic cables installed with direct push equipment[END_REF], Simon et al., 2021[START_REF] Del | Heat Dissipation Test With Fiber-Optic Distributed Temperature Sensing to Estimate Groundwater Flux[END_REF]. The consequences of this choice on the estimation of the parameters are discussed in subsection 5.2. Therefore, in Equation 2, only thermal dif-

fusitivy D t [m 2 s -1 ] is involved. It depends on λ the bulk thermal conductivity [W m -1 K -1
] and is expressed as :

D t = λ ρ b c b . (3)
Considering whether or not natural flow in the aquifer occured, Equation 2could be solved for a linear heating source involving the Instantaneous Line Source model (ILS) or Moving Instantaneous Line Source model (MILS) [START_REF] Scott Carslaw | Conduction of Heat in Solids[END_REF]. The ILS solution expressed as follows (Equation 4), considers no-flow within the aquifer and is the solution to Equation 2 without considering the second term:

∆T (r, t) ILS = P 4πλ E i -r 2 4tD t , ( 4
)
where ∆T is the temperature evolution, P [W m -1 ] is the constant heating power per unit length, r 2 = x 2 + y 2 is the distance from the heating source.

The MILS solution (Equation 5) is adapted from the ILS and considers uniform flow across the heat source to take into account heat transfer by advection. It is expressed as :

∆T (r, t) M ILS = P 4πλ exp qr 2D t ρ w c w ρc W (α, β) (5)
with W (α, β) the Hantush well function [START_REF] Hakala | Novel use of the enhanced thermal response test in crystalline bedrock[END_REF]. λ and q are estimated from these two solutions.

The previous equations did not consider thermal effects induced by the heating of the cable itself and the borehole material configuration. Just after heat injection in the FO cable, the temperature increase was mainly

ΔT [K]

Temperature with skin effect Theoritical temperature without skin effect Figure 7: Theoretical temperature evolution in response to a heat pulse in saturated porous media with and without skin effect of the FO cable (adapted from [START_REF] Del | Heat Dissipation Test With Fiber-Optic Distributed Temperature Sensing to Estimate Groundwater Flux[END_REF] position in the gravel pack and mud and its relative distance to the PVC tube. This aspect lead to a difference between the theoretical temperature calculated with ILS and MILS and the ∆T exp measured by FO-DTS. This difference was taken into account in a ∆T F O term (Figure 7). Thus, to model the temperature increase during the experiment, ∆T F O was added to the ILS and MILS solutions. This was solved by trial/error adjustments between ILS model and experimental data.

Parameters quantification

The ILS and MILS solutions were adjusted on the experimental data to estimate the λ and q values associated with the temperature evolution (Figure 8). The method consisted in reducing the Root Mean Square Error between the observed and modeled temperature using the Brent algorithm [START_REF] Brent | Algorithms for Minimization Without Derivatives[END_REF]. Four typical thermal behaviors were identified during the ADTS experiments, ranging from no-flow and low-flow towards medium-flow and high-flow.

For no-, low-and medium-Darcy velocity (< 2.0 × 10 -5 m s -1 ), the experimental points before 1000 seconds were assimilated to the skin effect.

Thus, these points were excluded from λ fitting and estimation procedure (Figure 8 a-c). When only conductive heat transfer occurred, (i.e. no-flow) the temperature increased linearly along a log time scale during the whole experimental time (Figure 8a). In this case, the ILS solution was used to estimate λ. Advection transfer generated a transition phase until temperature stabilization (Figure 8b andc), and both analytical solutions were used to estimate λ and q.

High groundwater flux generated quick temperature stabilization. When the Darcy velocity was too high (> 2.0×10 -5 m s -1 ), the sampling time of the experiments was unsuited to visualize the linear part of the thermal evolution plot (an example is provided in Figure 8d). Consequently, thermal conductivity λ could not be estimated properly and was fixed at 2.8 W m -1 K -1 . This value corresponds to an average value for limestone rocks [START_REF] Dalla | An updated ground thermal properties database for GSHP applications[END_REF]. Further discussion about the impact of λ values on the estimated q value is given in subsection 5.2.

The analytical models had a good fit to the field data, ,with the value of the Root Mean Square Error (RMSE) generally remaining low, i.e. RMSE< 0.1. The maximum value of 0.15 was calculated for the high Darcy velocity interval.

Low q values below 3.0x10 -6 m s -1 should be considered cautiously. It was demonstrated that an experimental time of 24h with an error of ±0.05 • C on the field temperature measurement, lead to errors above 20% on q estimation [START_REF] Simon | Numerical and Experimental Validation of the Applicability of Active-DTS Experiments to Estimate Thermal Conductivity and Groundwater Flux in Porous Media[END_REF]. This error grew exponentially with the decrease of estimated q. Despite the short distance between Pz101 and Pz51 (8.5 m), the estimated q profiles showed major discrepancies in front of the borehole screens.

These q profiles were not equally distributed along the flowing areas of both piezometers, highlighting a reciprocity gap. In Pz51, high Darcy velocities of 5.0 × 10 -5 m s -1 were estimated in the intervals of 43-46 m and 67-68 m (Figure 9a). A lower peak of 1.0 × 10 -5 m s -1 was detected in one branch at the depth of 62 m. From the depth of 46 m to 53 m, the q value decreased from 7.0 × 10 -6 m s -1 to 4.0 × 10 -6 m s -1 . As for Pz101, the ADTS experiment allowed to estimate a high Darcy velocity superior of 2.0 × 10 -5 m s -1 only in the thin layer at the depth of 62 m. A medium flow of an average of 5.0 × 10 -6 m s -1 was estimated between the length interval 54-61 m (Figure 9c).

In front of the screens, the low λ values were always estimated in front of no-or low flux areas. In Pz51, a value close to 1.0 W m -1 K -1 was estimated in the length intervals 35-39 m and 54-67 m. On the contrary, high λ values were calculated where significant q values were calculated. This trend was also detected in Pz101. High λ values of 3.0 W m -1 K -1 were measured in front of the highest Darcy velocity area at the depth of 62 m, whereas the values fluctuated around 1.5 W m -1 K -1 in the rest of the piezometer.

Discussion

Consistency of the ADTS results in relation with available field data

The parameters profiles obtained from ADTS experiments were compared to available independent field data. The q profile in Pz51 emphasized the andh). The λ values of these layers were always inferior to 1.5 W m -1 K (Figure 9b andd). It is typical for clogging materials like clay, bentonite and porous iron hydroxides [START_REF] Dalla | An updated ground thermal properties database for GSHP applications[END_REF][START_REF] Cho | Thermal Conductivity of Compacted Bentonite and Bentonite-Sand Mixture[END_REF][START_REF] Tang | A study on the thermal conductivity of compacted bentonites[END_REF][START_REF] Gjengedal | Fouling and clogging surveillance in open loop GSHP systems: A systematic procedure for fouling and clogging detection in the whole groundwater circuit[END_REF] and does not correspond to the typical values of the thermal conductivity of the limestone, i.e. 3.0 W m -1 K. These observations highlight the potential of ADTS experiments to locate zones of borehole clogging. We believe that the main reason for the clogging is related to the fact that these wells were planned only for monitoring purpose, and, as a consequence, were not sufficiently developed after their completion. The clogged materials did not have the same position along both boreholes .It seems to be the main explanation for reciprocity gaps between parameters distribution in Pz51 and Pz101. The drill cuttings assessment revealed the same geology in both boreholes. But, a fracture or a more productive area might be intercepted in Pz51 and GF1 and not in Pz101 and could be the

Uncertainty and resolution of parameters calculation

In highly productive layers, the groundwater flow generated a fast temperature stabilization. In this context, ADTS experiments with a sampling time of more than 1 minute are unsuited to visualize the linear part of the ∆T exp plot before the temperature stabilization (Figure 8d). Consequently, it is not possible to accurately estimate the thermal conductivity λ. Consequently, values of thermal conductivity consistent with the properties of the porous material [START_REF] Dalla | An updated ground thermal properties database for GSHP applications[END_REF] encountered in the borehole must be chosen as the default value. However, in high productive context, it is possible to demonstrate the low impact of λ on the estimated Darcy velocity (Figure 11). A sampling time of a few seconds would be necessary to visualize the conductive phase in this context. Nevertheless, a short sampling time produce noisier data which can also lead to high uncertainty [START_REF] Bas | Estimation of Temperature and Associated Uncertainty from Fiber-Optic Raman-Spectrum Distributed Temperature Sensing[END_REF].

As mentioned in section 4.2.1, the analytical model used in this study ignored thermal dispersion. By doing this, the thermal diffusivity is overestimated. It could be another source of error that could lead to underestimating q, especially in high-flow areas. It was demonstrated that an estimated q of 5.0 × 10 -6 m s -1 without considering the thermal dispersion will yield a q of 9.0 × 10 -6 m s -1 if the longitudinal dispersivity is set at 0.1 m [START_REF] Bas | Estimation of the Variation in Specific Discharge Over Large Depth Using Distributed Temperature Sensing (DTS) Measurements of the Heat Pulse Response[END_REF]. Nevertheless, in our experiment, the effect of thermal dispersion should be very small, if not negligible, since here, we use the same cable as a heat source and for measuring temperature.

Moreover, the vertical resolution of the DTS is a limit to measure temperature variations induced by layers with a thickness ranging this resolution.

Even if a thin layer is detected by the FO DTS experiment, the measured temperature in front of this layer is a result of the surrounding conditions leading to uncertainty in parameter estimation [START_REF] Bas | Estimation of the Variation in Specific Discharge Over Large Depth Using Distributed Temperature Sensing (DTS) Measurements of the Heat Pulse Response[END_REF][START_REF] Simon | A Comparison of Different Methods to Estimate the Effective Spatial Resolution of FO-DTS Measurements Achieved during Sandbox Experiments[END_REF]. Unlike ADTS, flowmeter tests are not designed to provide information about conductive heat transfers which could be of great interest in geothermal projects. Nevertheless, the single-hole flowmeter is a common survey that can be achieved in a shorter time than ADTS, after a borehole completion.

It has also the advantage of a more simple setup than the ADTS one. It provides a local vertical distribution of the fluxes at the logging well, and is used to estimate permeability variations along the aquifer [START_REF] Molz | The Impeller Meter for measuring aquifer permeability variations: Evaluation and comparison with other tests[END_REF], 1994[START_REF] Paillet | Flow modeling and permeability estimation using borehole flow logs in heterogeneous fractured formations[END_REF]]. The application of the flowmeter test in cross-borehole configuration proved to be also an efficient methodology to have a better characterization of aquifer dynamics at a large scale. It allows highlighting connectivity of preferential groundwater flow paths [Le Borgne et al., 2006a,b, Audouin and[START_REF] Audouin | Cross-borehole slug test analysis in a fractured limestone aquifer[END_REF].

The ADTS experiments give thermal conductivity only in the vicinity of the monitoring well. To characterize aquifer heterogeneity and thermal diffusitivity on large scales, heat tracer tests with multiple monitoring well may be more powerful experimental field test [START_REF] Klepikova | Heat tracer test in an alluvial aquifer: Field experiment and inverse modelling[END_REF]. It provides additional information compared to solute tracer tests, since the thermal diffusitivity is higher than the solute dispersion [START_REF] Irvine | Heat and Solute Tracers: How Do They Compare in Heterogeneous Aquifers?[END_REF][START_REF] La Bernardie | Thermal Attenuation and Lag Time in Fractured Rock: Theory and Field Measurements From Joint Heat and Solute Tracer Tests[END_REF]. Consequently, heat tracer tests are more suited for geothermal sites. However, due to the ability of heat to propagate through solids by diffusion, completion of heat tracer tests may take much longer time compared to solute tracer tests (days vs hours). [START_REF] Irvine | Heat and Solute Tracers: How Do They Compare in Heterogeneous Aquifers?[END_REF][START_REF] La Bernardie | Thermal Attenuation and Lag Time in Fractured Rock: Theory and Field Measurements From Joint Heat and Solute Tracer Tests[END_REF]. It can also be difficult to realize and interpret compared to ADTS experiments.

. Recent studies [START_REF] Gjengedal | Fouling and clogging surveillance in open loop GSHP systems: A systematic procedure for fouling and clogging detection in the whole groundwater circuit[END_REF][START_REF] Gjengedal | Online remote-controlled and cost-effective fouling and clogging surveillance of a groundwater heat pump system: A case study from Lena Terrace in Melhus, Norway[END_REF] This material has a low thermal conductivity of 0.2 W m -1 K -1 . It gives a limited impact on the temperature evolution generated by the warming of the fiber. This impact was included in the skin effect during our interpretation process. Nevertheless, a lot of wells are made of steel or stainless steel. Because of the high thermal conductivity of this material (15-30 W m -1 K -1 ), the energy demand may be significant to create a temperature increase detectable by the DTS device. A numerical model taking into account a well equipped with steel material would be necessary to inspect the feasibility of ADTS experiment in this configuration.

While on the ENSEGID site, the exploited aquifer is found at a shallow depth (35-65 m), in other sites, the geothermal target may be deeper. The longer cable length required for deeper applications will generate an additional economic cost, and the deployment of the technology in the borehole will be more time-consuming. Moreover, longer heated section of the cable will require more power injection to reach similar temperature variations. In this context, it is a major concern to provide suitable devices able to inject safely more electrical power.

Finally, the ENSEGID site is equipped with several wells close to each other, thus offering a good opportunity to perform ADTS experiments in cross-borehole configuration. Monitoring and pumping boreholes are only separated by a distance of 8.5 m and a flowrate inferior to 5.0 m 3 h -1 was enough to induce groundwater flux measurable by the ADTS experiment.

Nevertheless, on geothermal sites just equipped with a warm and a cold well, the high distance (generally more than 100 meters) between the wells to avoid thermal breakthrough, may be a limit to perform cross-borehole ADTS experiments. Depending on the hydrogeological context and the capacity of the installed submersible pump, the groundwater velocity at the monitoring well may not be detectable by the presented ADTS experiment configuration and interpretation scheme. In this case, a single-hole ADTS configuration, like the one proposed by [START_REF] Read | Active-distributed temperature sensing to continuously quantify vertical flow in boreholes[END_REF], may be more suited.

Conclusion

This study demonstrated the potential of ADTS experiments to collect and interpret a large amount of data by placing a FO between the borehole screens and the porous media. Our study allowed to characterize simultaneously the distribution of thermal conductivity and Darcy velocity in a short time (4-6 hours per experiment), and to get useful insights about the water injection pathways in boreholes. Moreover, the data provided information concerning the position of the clogging development along the screens.

In the future, it would be certainly of interest to test ADTS experiments on a new operational geothermal site. The aim would be to determine initial parameters profiles before the beginning of ATES operation in the production/injection wells, and then, monitor their evolution during a long-term

Figure 1 :

 1 Figure 1: (a) Location of experimental area, (b) ENSEGID site map with boreholes location and focus on the experimental area

Figure 2 :

 2 Figure 2: Field data : (a) Geological section of the site, (b) and (c) technical logs of piezometers with clogged area detect during video inspection, (d) Pz51 gammma ray, (e) GF1 and Pz101 flowmeter logs, and (f,g,h) video inspection screenshots

  Another flowmeter realized in Pz101 one year after the end of borehole works completion (and before the beginning of ATES operation) indicates a more homogeneous flux distribution along this borehole (Figure2e). Between the depth of 60 m and 67 m, the drill cuttings observations emphasize gray limestones, a less developed porosity and a significant clay content. From 67 m, to 71 m, blue marl is encountered. A mean transmissivity value of 7.0 × 10 -4 m 2 s -1 is estimated from a two days pumping test in GF1.Borehole camera inspections show the presence of clogging material in piezometers Pz101 and Pz51 (Figure2f, g and h). From the depth of 35 m to 39 m, the Pz51 screens are totally clogged by a mix of yellow brown bentonite clay/mud and white bio-fouling bacteria (Figure2f). The screens are clean in the interval length 40-54 m. The Pz51 bottom is filled with clay and bentonite and encountered at the depth of 54.5 m instead of the 71 m initially drilled. We believe that the main reason for the fast clogging is related to the fact that these wells are planned only for monitoring purpose . As a consequence, the piezometers were not sufficiently developed. As for Pz101, the screens are totally clogged by bentonite and mud from the depth of 35 m to 53 m (Figure2g). Below the depth of 53 m to the Pz101 bottom at 60 m, the screens are partially clogged (Figure2h). The clogged intervals of the piezometers are represented on the technical logs (Figure2b and c).

Figure 3 :

 3 Figure 3: Device installation scheme : (a) transverse section of the FO cable, (b) scheme of experimental configuration with acquisition well and FO-DTS set up, (c) traverse borehole section representing fiber configuration in annular space

  in the boreholes allowed to create a double-ended connection providing accurate temperature measurements under field conditions [van de Giesen et al., 2012]. The Python toolbox [des Tombe et al., 2020] was used to check the quality of the DTS measurement and indicated a good calibration of the temperature measured by the DTS device.

Figure 4 :

 4 Figure4). The water level was monitored with vented Level Troll probes (In Situ) in both piezometers. The temperature evolution along the FO cable induced by the heat injection was monitored by the DTS device. At the end of the first stage, without stopping the heating of the fiber, the pumped water was injected in the monitoring well to monitor thermal signals induced by the injection (Experiment 2 in Figure4). The injected water came from the aquifer under natural conditions, consequently it was colder than the water warmed by the fiber in the monitoring piezometer. Experiments 3 and 4 (Figure4) correspond to ADTS experiment replicated in Pz51.

Figure 5 :

 5 Figure 5: Temperature distribution along the FO cable in Pz51: (a) natural in-situ profile before the heating and (b) temperature variation with both time and depth along the two side of the FO loop. The temperatures measured at each side of the U cable are similar

Figure 6 :

 6 Figure 6: Experimental injection results : (a) and (c) relative temperature anomalyprofiles ∆T inj at different times since the beginning of the injection;(b) and (d) technical log with identified clogged area from video inspection and flux pathway deduced from injection experiment represented by blue arrows

  associated, first, to the conductive skin effect in the FO cable components and secondly to the borehole material arrangement[START_REF] Del | Heat Dissipation Test With Fiber-Optic Distributed Temperature Sensing to Estimate Groundwater Flux[END_REF], Simon et al., 2021]. It generated a rapid increase of the temperature during the first time after heat injection. The effect of the cable was the same along the borehole. Nevertheless, the effect of the borehole material on the temperature increase could be different along the FO, depending on the cable

Figure 8 :

 8 Figure 8: Determination of mean thermal conductivity λ and Darcy velocity q (piezometer Pz51) when, (a) only conduction occurs, (b) low flux is detected, (c) medium flux and (d) high flux occur with an early temperature stabilization due to higher groundwater flow.

Figure 9 :

 9 Figure 9: Parameter profiles estimated from FO-ADTS experiments in front of screened areas : (a and c) groundwater flux profiles, shaded areas correspond to estimated values with high uncertainty, (b and d) thermal conductivity profiles

  same high productive layer as the one detected in the GF1 flowmeter log at the depth of 44-46 m. The Darcy velocity reached a value of 5.0 × 10 -5 m s -1 at this depth. Nevertheless, the flowmeter log achieved one year after borehole completion in Pz101 suggested a homogeneous flux distribution probably influenced by the clogging development along this borehole. But, the ADTS detected a medium Darcy velocity of 5.0 × 10 -6 m s -1 where the screens are just partially clogged below the depth of 53 m.

Figure 10 :

 10 Figure 10: Interpretation scheme of boreholes clogging and flux location according to video inspection, parameters distribution and injection flow paths

  reason of a high flux in Pz51 and GF1, involving the dilution of the bentonite in Pz51 and not in Pz101.The video camera showed bentonite and clay at the bottom of both piezometers. The position of the plugs, at the depth of 54.5 m in Pz51 and 60 m in Pz101, corresponded exactly to the limits highlighted by Experiment 2 and Experiment 4 (Figure6). It confirmed that during injection, the water did not flow in the lower part of the piezometers. Nevertheless, during Experience 1 and Experience 3, some fluxes were detected below the well bottoms, at the depths of 62 m in Pz101 and 62 m and 68 m in Pz51 (Figure10). It may point out that at these depths, the boreholes were clogged but the deposits were not completely compacted/encrusted into the annular space. High thermal conductivity values (> 3.0 W m -1 K -1 ) may confirm a likely dilution of the mud in these intervals.

Figure 11 :

 11 Figure 11: Analytical solutions generated from the framework to reproduce experimental temperature evolution at 45.2m in Pz51 with a large scale of λ from 1.5 to 4.5 W m -1 K -1

Finally

  , parameters estimation, especially λ estimation, is influenced by the test configuration. The resolved parameters are unlikely to provide the true aquifer properties since the FO cable is located in the borehole annular space, close to the PVC tube, surrounded by the gravel and clogging material. Despite these uncertainties, one should rather focus on relative parameter variation to identify clogging locations and productive layers along the borehole. 5.3. Advantages and limits of the proposed ADTS experiments 5.3.1. Comparison with common field surveys These experiments demonstrated that ADTS could provide in short time information regarding two crucial concerns for ATES operation: (i) the method provides important insights regarding the aquifer heterogeneity in thermal and hydrodynamic properties and (ii) it enables the detection of clogging locations in the borehole. ADTS experiment presents the advantage to estimate a distribution of parameters while pumping tests and TRT give an average value of hydrodynamics and thermal properties. Moreover, these surveys require much longer time of data collection (i.e., at least 48h versus 4-6 h required for the completion of ADTS tests). However, TRT and pumping tests are very simple to deploy, and their well-known and relatively quick interpretation can give a first overview of the global aquifer dynamics.

  based on short step-test combined with temperature and hydraulic performance monitoring, first demonstrated a new technique enabling the detection of clogging and its further evolution characterization in geothermal systems. While this method allows the detection of clogging in the entire system, it does not precise the clogging location in the boreholes. The new characteristic of our ADTS interpretation is to image clogging distribution in the monitoring well. Therefore, this new strategy and the one proposed by Gjengedal et al. [2020] could complement each other. Once deployed in the borehole, and all cable connections realized, ADTS technology could be used as a permanent in-situ device. It presents a certain advantage as it can collect a large amount of data without opening the borehole and removing the submersible pump. The aim would be to monitor clogging evolution and aquifer properties changes over time and to plan and adapt maintenance works before major deterioration of the wells. This aspect would be time and financial saving compared to diagnostic well works, or video camera inspections to visualize clogging development. It is also an advantage to have a permanent fiber in the borehole. It allows to warm the cable and collect the data with the DTS unit without deploying the fiber in the borehole before each experiment like the methodology proposed by Maldaner et al. [2019].5.3.2. Potential limits of cross-borehole ADTS experiments in a geothermal operational contextOur experiments were achieved in wells made of PVC tubes and screens.

Acknowledgements

This research was partly funded by the ANR project EQUIPEX CRI-TEX (grant number ANR-11-EQPX-0011). Jeremy Godinaud acknowleges

Vertical distribution of λ and q were computed for each sampling depth along the two branches of the U cable (Figure 9). In each piezometer, the 2 branches gave similar parameter estimations, suggesting the consistency of the data acquisition and its processing. The uncertain q values below 3.0 × 10 -6 m s -1 were represented in a shaded area on the corresponding profiles (Figure 9a andc).