• The Poitiers site
    The Poitiers site
  • The Poitiers site
    The Poitiers site
  • The Poitiers site
    The Poitiers site
  • Experiments on the Poitiers site
    Experiments on the Poitiers site
  • Borehole cores of the Poitiers site
    Borehole cores of the Poitiers site
  • Hydro-geophysical imagery from the Poitiers site
    Hydro-geophysical imagery from the Poitiers site

 

 

Manager of the Poitiers site : Gilles Porel

The Poitiers Experimental Hydrogeological Site (SEH) was developed by the Hydrasa team (FRE 3114) as part of the Network of National Hydrogeological Sites (SNO H+), under the Poitou-Charentes Region’s “WATER” program (CPER 2002-2006). Located 2 km east of the Science Campus of the University of Poitiers, the SEH occupies an area of 12 hectares on land belonging to the University (Deffend Site: Regional Plant Heritage Botanic Garden). From the geologic viewpoint the SEH occupies the north flank of the “Seuil du Poitou”, a huge Mesozoic carbonate plateau marking the transition between the Aquitaine and Paris sedimentary basins (Fig. 1).

 

 

 Figure 1: Location of the SEH

 

The Jurassic limestones, which overlie a Hercynian crystalline basement, include two stacked aquifers: (i) the Lower and Middle Lias Aquifer (5 to 10 m thick). and (ii) the Dogger Aquifer (100 m thick). These two aquifers are separated by the marly Toarcian aquitard (20 m thick). The studies conducted at the SEH focus mainly on the Dogger Aquifer.

The experimental layout now comprises 35 boreholes, including two vertical and two inclined cored holes. These boreholes were drilled in two separate campaigns: 2002-2003 and 2004. All the holes completely traverse the Dogger Aquifer (depth of boreholes = 125 m). Most of them were drilled on a regular 210 x 210 m grid (Fig. 2).

 

Figure 2: Location of boreholes on the SEH

 

The SEH boreholes are either not cased, or are equipped with perforated casing over the full thickness of the Dogger Aquifer. The piezometric level in the boreholes therefore represents the average hydraulic head over the thickness of the aquifer. Under natural flow conditions the piezometric levels vary from 15 m to 25 m below the ground surface. During drilling, dry argillaceous limestones were continuously observed to a depth of about 30 m, indicating that the Dogger Aquifer is sealed beneath this relatively impermeable formation. Two additional holes were drilled into the crystalline bedrock (Boreholes C2 and IM1, about 160 m deep) so as to be able to record the hydraulic heads in the Lower and Middle Lias Aquifer while hydraulic testing was being conducted in other boreholes. No disturbance of the levels was ever observed in the Lower and Middle Lias Aquifer, demonstrating that the two aquifers are indeed isolated from each other by the Toarcian marls.

Since 2002 the investigations carried out on the SEH have enabled the collection of a large volume of data concerning the Dogger Aquifer. These data, now archived in the H+ database (de Dreuzy et al. 2006), cover:

  • Geologic structure of the reservoir: stratigraphy & lithology of two cored boreholes (Bourbiaux et al. 2007), 3-D seismic imagery of the reservoir (Mari and Porel 2008), gamma-ray well logs, acoustic imagery, and high-resolution photography (Audouin 2007);
  • Petrophysical properties of carbonate rocks: laboratory porosity and permeability measurements on core samples (Chatevaire 2006, Bourbiaux et al. 2007);
  • Flow structure in the boreholes: temperature-conductivity logs, heat-pulse flow-metering (Audouin et al. 2008);
  • Water-table dynamics under “natural” flow conditions (Bernard and Delay 2008) and/or forced conditions: pumping tests (Delay et al. 2004, Bernard 2005, Bernard et al. 2006, Delay et al. 2007, Kaczmaryk and Delay 2007a, 2007b) and hydraulic shock conditions (Audouin 2007, Audouin and Bodin 2007, 2008).

These various investigations have produced a quality of characterization that is unequalled at the national level for this type of environment.

Cross-comparison of the vertical flowmeter data and imagery of the borehole walls indicates that the flows are mainly associated with subhorizontal karstic structures and subvertical fractures. The karstic zones are irregularly distributed over the SEH but appear to be strongly correlated with specific stratigraphic horizons (-50 m, -85 m, and -110 m on the SEH, see Fig. 3). The hydraulic connectivity of the various horizontal karstic zones seems to be provided by vertical fracturing in the limestones (Audouin et al. 2008).

 

Figure 3: Discrete structures with high relative permeability, identified from vertical flowmeter data and high-resolution imagery of borehole walls. Audoin et al. 2008.
 

 

Recently a more detailed characterization of the reservoir’s heterogeneity was undertaken, on the basis of 3-D seismic experiments (Mari and Porel 2008, Fig. 4). The vertical distribution of seismic pseudo-velocities derived from reflection data shows strong correlations with the vertical flowmeter data from the boreholes, the low-velocity zones corresponding to zones of high relative permeability.

 

Figure 4: Vertical sections and maps of seismic pseudo-velocities extracted from the 3-D block at various depths: 50 m (A), 88 m (B), 92 m (C), & 102 m (D). Mari and Porel 2008.
 

 

Two series of pumping-tests were carried out in 2004 and 2005 following the drilling campaigns. The transmissivity figures as estimated by the Cooper-Jacob method are relatively homogeneous (variability factor ~2): T = 2.2 ~ 4.4x10-3 m2/s, while the figures for storage exhibit more variability (3 orders of magnitude): 3.6x10-4 < S < 2.8x10-1. This contrast in heterogeneity between the values of T and S is probably an artifact related to the Cooper-Jacob method (see for example Meir et al. 1998, Sanchez-Vila et al. 1999). Three other methods have been developed and tested: (i) a model based on the laws of fractal scales (Delay et al. 2004, Bernard et al. 2006), (ii) a homogeneous dual-media model (Delay et al. 2007; Kaczmaryk and Delay 2007a,b), and (iii) a dual-media fractal model (Delay et al. 2007, Kaczmaryk and Delay 2007a,b). Each of these models allows the experimental drawdown curves to be adjusted in a satisfactory fashion, but for each series of interpretations the storage values are still much more variable than the transmissivity values. The high relative variability of the storage parameters is explained by the atypical behavior of the drawdown curves, which usually appear to be independent of the distance between the pumping well and the monitoring wells (Fig. 5)..

 

Figure 5: Drawdown curves, 2005.

 

According to Kaczmaryk and Delay (2007b), this atypical behavior could be explained by a very rapid propagation of the pumping cone of depression in preferential flow channels (fractures and/or karstic channels). This interpretation is consistent with the very rapid propagation of pressure disturbances recorded by Audouin and Bodin (2008) in cross-borehole slug tests (Fig. 6). During some of the experiments disturbances were recorded more than 100 m from the well being tested, with velocities up to 10 m/s. The hydraulic conductivities estimated from the cross-borehole slug tests were between 4.4x10-5 and 7.7x10-4 m.s-1. These figures are about ten times higher than those derived from interpreting the pumping tests. In view of the speed of the responses observed, the head disturbance caused by a slug test would seem to propagate only along preferential flow paths, whereas the pumping tests involve the aquifer system as a whole, owing to the limited storage capacities of the fractures and karstic channels. The “selectivity” of the slug tests thus enables specific characterization of the hydraulic properties of the preferential flow paths.

 

Figure 6: Pressure disturbances recorded during cross-borehole slug tests conducted in Well M19.
 

 

Today, the SEH constitutes an operational scientific platform for hosting science projects of national and international scope:

  • INSU/EC2CO/MACH-1 Project: since the end of 2006, eight research teams have been involved in the INSU/EC2CO/MACH-1 Project (Modeling of Aquifers in Heterogeneous Limestones – 1. Flow Dynamics), aimed at investigating the relevance of various conceptual and digital approaches to hydrogeologic modeling, using the H+/SEH database. The principal objective is to determine to what extent the variability and density of the SEH data are relevant and sufficient in terms of constraints, parameterization, and calibration to give the models a certain predictive ability. The modeling exercise consists of predicting the response of the water table (calculation of drawdowns in several piezometers) during two sinusoidal (injection-extraction) pumping tests in two pairs of wells. These pumping tests will be carried out after the event, i.e., once the “digital” predictions have been provided by each team. Such a benchmark for hydrogeologic modeling in a limestone medium will be a ‘first’ on the international scene. Other benchmarks of this type, established in the past, have involved crystalline environments, mainly in connection with international studies on the storage of radioactive wastes in impermeable geologic formations (e.g., the ÄSPÖ TASK 4 and DECOVALEX Projects). These projects have shown the value of making comparisons between models and experimental sites. The MACH-1 Project will make it possible to estimate the necessary and sufficient cost in terms of (i) data (continuous vs. discrete approaches), and (ii) digital calculations (2-D vs. 3-D approaches) in achieving an optimal modeling of the flows at this scale.
  • Hydrasa-Polimi Joint Project: in 2007, two researchers at the Université Politecnico di Milano, Monica Riva and Alberto Guadagnini, spent three months in the Hydrasa laboratory studying the applicability of various stochastic approaches to the SEH data. Pumping-test data were reinterpreted, using the stochastic method of Neuman et al. (2004, 2007), leading to an estimate of the geostatistical parameters describing the spatial variability of the logarithm of transmissivity. The results obtained (joint publication submitted to the Journal of Hydrology) indicate a scale dependence of the variance and length of correlation that is fully consistent with the hierarchic multiscale theory proposed by Neuman and Di Federico (2003) and Neuman et al. (2008).

The themes to be addressed at the SEH in coming years can be classified into two broad areas:

 

Area 1: Characterization of preferential flow channels

  • The spatial density of boreholes on the SEH will be turned to good account in developing new approaches to the characterization and modeling of flows in strongly channeled heterogeneous media.
  • 3-D spatial distribution of the heterogeneities and individual geometry of discrete structures of high relative permeability: a new campaign of high-resolution 3-D imagery is planned for 2009, in collaboration with the French Petroleum Institute. This second campaign will (i) enable validation of the results from the first campaign, and (ii) supplement the spatial coverage of the SEH (currently ~33% covered).
  • Connectivity: relative inter-borehole connectivity will be investigated on the basis of (i) high-resolution 3-D seismic data; (ii) diffusivity figures obtained by interpretation of long-duration pumping tests (see, for example, Knudby and Carrera 2006); (iii) diffusivity figures obtained by interpretation of slug tests; and (iv) short-duration high-frequency (2 Hz) hydrodynamic data, to be acquired during a forthcoming series of pumping tests.
  • Hydraulic properties: the hydraulic properties of the preferential flow channels will be investigated by using (i) data from cross-borehole slug tests; (ii) data from cross-borehole flowmeter tests (experiments to be carried out jointly with the team from Rennes); and (iii) short-duration high-frequency (2 Hz) hydrodynamic data, to be acquired during a coming series of sinusoidal hydraulic testing (experiments with multiple extractions and injections), allowing the preferential flow channels to be stressed at various scales.
  • Limit conditions: the parameterization of conditions at the limits of the MACH1 Project’s models is tricky, because the flows are so strongly channeled, i.e., it is difficult to define the distance beyond which the influence of pumping may, at any given moment, be considered “negligible”. The propagation of pressure disturbances over medium and longer distances outside the SEH will be investigated by installing new “distant” sensors and by carrying out pumping tests with periodic signals.

Area 2: Reactive transport and fluid-rock interactions

  • In situ denitrification: The Hydrasa Laboratory and the Poitiers Water Chemistry and Environment Laboratory are currently collaborating on the development of a pilot in situ biochemical decontamination plant for groundwaters, targeting nitrates (thesis by Marion Chatelier). Laboratory experiments are currently under way to study the kinetics of the biochemical processes involved. Further experiments will be conducted at the field scale, beginning with tracing combined with dual pumping tests so as to characterize the dilution and hydrodispersion flows of the contaminant within the reactor.
  • Origin and dynamics of selenium in carbonate aquifers: the presence of selenium in groundwaters at concentrations greater than the current standards for drinking water (10 µg/l) is a problem encountered in several parts of France. In the Vienne Département, hydrochemical monitoring carried out on several AEP intakes on the Dogger water table (the SEH aquifer) have shown a temporal trend in the concentrations of selenium after the intake has been operating for a certain time. A number of recent lithochemical analyses carried out on SEH drill cores indicate that significant amounts of selenium are trapped in filled paleokarst cavities scattered within the carbonate reservoir. Changes in redox conditions connected to pumping could have led to the release of selenium into the groundwater, and thus to a progressive increase in its concentration in the water extracted. Studies will be undertaken to examine the validity of this theory, based on (i) a detailed physico-chemical characterization (XRD, SEM, XPS) of the speciation of the selenium present in the filled cavities; (ii) a characterization of the mechanisms for liberating the selenium from its solid matrix, via laboratory (batch) experiments combined with thermodynamic simulations (JCHESS, PHREEQC, FITEQL); and (iv) digital modeling of reactive flows at the scale of the reservoir. This modeling will be approached though a “Pipe-Flow” conceptualization of the flow paths, combined with time-domain Lagrangian digital methods (a technique in which the Poitiers team has the expertise for simulating problems of non-reactive transport: see, for example, Bodin et al. 2003, Delay et al. 2003, Bodin et al. 2007, Ubertosi et al. 2007, Delay et al. 2008). The challenge here will be to develop a stochastic algorithm that fits the water-rock interaction processes previously identified in the laboratory.

Main collaborations carried out in the framework of the Observatory H +:

  • Philippe Ackerer, Director of Research, CNRS, Strasbourg (modeling (MACH1 project))
  • B. Bourbiaux, Chef de Projet IFP, Rueil Malmaison (modeling (MACH1 project))
  • P.M. Adler, Director of Research, CNRS, Paris (modeling (projet MACH1))
  • J.-F. Thovert, Director of Research, CNRS, Chasseneuil du Poitou (modeling (MACH1 project))
  • V. Mourzenko, Director of Research, CNRS, Chasseneuil du Poitou (modeling (MACH1 project))
  • C. Grenier, Engineer CEA – LSCE, Gif sur Yvette (modeling (MACH1 project))
  • D. Bruel, Director of Research, Ecole des Mines, Fontainebleau (modeling (projet MACH1))
  • H. Jourde, Lecturer, Université de Montpellier 2 (modeling (MACH1 project))
  • M. Riva, Assistant Professor, Politecnico di Milano (Italie) (stochastic hydrogeology)
  • A. Guadagnini, Professor, Politecnico di Milano (Italie) (stochastic hydrogeology)

Participation in national research programs

  • GDR CNRS 2990 HTHS, Manager F. Delay (Poitiers) 2003- (in progress): Hydrodynamics and Transfers in Underground Hydrosystems
  • INSU/EC2CO Program: MACH1 Project, Manager J. Bodin (Poitiers) 2006- (in progress): Modeling of Heterogeneous Limestone Aquifers – 1. Flow dynamics

Participation in regional research programs

  • State-Regional Planning Contract (CPER), Waters & Soils Program 2001- (in progress)

Personnel associated with SEH (Poitiers)

Lecturer

  • Gilles Porel, Lecturer, Université de Poitiers (30% ETP)
  • Jacques Bodin, Lecturer, Université de Poitiers (30% ETP)
  • Aude Naveau, Lecturer, Université de Poitiers (30% ETP)
  • Mathieu LE COZ, Lecturer, Université de Poitiers (50% ETP)

Technical staff

  • Denis Paquet, Technician CNRS (50% ETP)
  • Benoît Nauleau, Engineer, CNRS (100% ETP)

References :

  • Audouin, O., 2007, Intégration - inversion de données géologiques et mesures hydrodynamiques in-situ pour le conditionnement d’un modèle 3D d’aquifère calcaire : Site Expérimental Hydrogéologique (SEH) de Poitiers, France, Thèse de Doctorat de l’Université de Poitiers.
  • Audouin, O., Bodin, J., 2007, Analysis of slug-tests with high-frequency oscillations, J. Hydrol., 334, 282– 289.
  • Audouin, O., Bodin, J., 2008, Cross-borehole slug test analysis in a fractured limestone aquifer, J. Hydrol., 348, 510-523.
  • Audouin, O., Bodin, J., Porel, G., Bourbiaux, B., 2008, Flowpath structure in a limestone aquifer : multi-borehole logging investigations at the hydrogeological experimental site of Poitiers, France, Hydrogeol. J., 16 (5), 939-950.
  • Bernard, S., 2005, Caractérisation hydrodynamique des réservoirs carbonatés fracturés : application au Site Expérimental Hydrogéologique (SEH) de l’Université de Poitiers, Thèse de Doctorat de l’Université de Poitiers.
  • Bernard, S., Delay, F., Porel, G., 2006, A new method of data inversion for the identification of fractal characteristics and homogenization scale from hydraulic pumping tests in fractured rocks, J. Hydrol., 328 (3-4), 647-658.
  • Bernard, S., Delay, F., 2008, Determination of porosity and storage capacity of a calcareous aquifer (France) by correlation and spectral analyses of time series, Hydrogeol. J., in Press.
  • Bodin, J., Porel, G., Delay, F., 2003, Simulation of solute transport in discrete fracture networks using the Time Domain Random Walk method, Earth Planet. Sci. Letters, 208(3-4), 297-304.
  • Bodin, J., Porel, G., Delay, F., Ubertosi, F., Bernard, S., de Dreuzy, J.-R., 2007, Simulation and analysis of solute transport in 2D fracture/pipe networks : The SOLFRAC program, J. Contam. Hydrol.,89, 1-28.
  • Bourbiaux, B., Callot, J.-P., Doligez, B., Fleury, M., Gaumet, F., Guiton, M., Lenormand, R., Mari, J-L., Pourpak, H., 2007, Multi-Scale Characterization of an Heterogeneous Aquifer Through the Integration of Geological, Geophysical and Flow Data : a Case Study, Oil & Gas Science and Technology, 62 (3), 347-37.
  • Chatevaire, L., 2006, Porosité et perméabilité des roches réservoirs et corrélation avec les observations pétrographiques : application à l’aquifère du Site Expérimental Hydrogéologique (SEH), Rapport Master 2 Géomatériaux-Eaux, Université de Poitiers.
  • de Dreuzy, J.-R., Bodin, J., Le Grand, H., Davy, P., Boulanger, D., Battais, A., Bour, O., Gouze, P., Porel, G., 2006, General Database for Ground Water Site Information, Ground Water, 44 (5), 743-748.
  • Delay, F., Porel, G., Bernard, S., 2004, Analytical 2D model to invert hydraulic pumping tests in fractured rocks with fractal behavior, Geophys. Res. Letters, 31, L16501, 10.1029/2004GL020500.
  • Delay, F., Kaczmaryk, A., Ackerer, P., 2007, Inversion of interference hydraulic pumping tests in both homogeneous and fractal dual media, Adv. Water Resour., 30 (3), 314-334.
  • Delay, F., Kaczmaryk, A., Ackerer, P., 2008, Inversion of a Lagrangian time domain random walk (TDRW) approach to one-dimensional transport by derivation of the analytical sensitivities to parameters, Adv. Water Resour., 31 (3), 484-502.
  • Delay, F., P. Ackerer, and C. Danquigny, Random walk particle tracking for solute transport in porous and fractured formations : A review, Vadose Zone Journal, S.S.S.A., 4, 360-379, DOI 10.2136/vzj2004.0125, 2005
  • Kaczmaryk, A., Delay, F., 2007a, Interference pumping tests in a fractured limestone (Poitiers - France) : Inversion of data by means of dual-medium approaches, J. Hydrol., 337 (1-2), 133-146.
  • Kaczmaryk, A., Delay, F., 2007b, Improving dual-porosity-medium approaches to account for karstic flow in a fractured limestone : Application to the automatic inversion of hydraulic interference tests (Hydrogeological Experimental Site, HES - Poitiers - France), J. Hydrol, 347 (3-4), 391-403.
  • Knudby, C., Carrera, J., 2006, On the use of apparent hydraulic diffusivity as an indicator of connectivity, J. Hydrol., 329, 377-389.
  • Mari, J.-L. and Porel, G., 2008, 3D Seismic Imaging of a Near-Surface Heterogeneous Aquifer : A Case Study, Oil & Gas Science and Technology – Rev. IFP, 63 (2), 179-201.
  • Meier, P.M., Carrera, J., Sanchez-Vila, X., 1998, An evaluation of Jacob’s method for the interpretation of pumping tests in heterogeneous formations. Water Resour. Res. 34 (5), 1011–1025.
  • Neuman, S.P., Di Federico, V., 2003, Multifaceted nature of hydrogeologic scaling and its interpretation, Rev. Geophysics, 41(3), 1014, doi:10.1029/2003RG000130.
  • Neuman, S.P., Guadagnini, A., Riva, M., 2004, Type-curve estimation of statistical heterogeneity, Water Resour. Res., 40, doi:10.1029/2003WR002405.
  • Neuman, S. P., Blattstein, A., Riva, M., Tartakovsky, D. M., Guadagnini, A., Ptak, T., 2007, Type curve interpretation of late-time pumping test data in randomly heterogeneous aquifers, Water Resour. Res., 43, W10421, doi:10.1029/2007WR005871.
  • Neuman, S. P., Riva, M., Guadagnini, A., 2008, On the geostatistical characterization of hierarchical media, Water Resour. Res., 44, W02403, doi:10.1029/2007WR006228.
  • Riva, M., Guadagnini, G., Bodin, J., Delay, F., Characterization of the Hydrogeological Experimental Site of Poitiers by stochastic well testing analysis, submitted to J. Hydrol (2008).
  • Sanchez-Vila, X., Meier, P.M., Carrera, J., 1999, Pumping tests in heterogeneous aquifers : An analytical study of what can be obtained from their interpretation using Jacob’s method. Water Resour. Res. 35 (4), 943–952.
  • Ubertosi, F., Delay, F., Bodin, J., Porel, G., 2007, A new method for generating a pipe network to handle channelled flow in fractured rocks, C.R. Geoscience, 339, 682-691.