Tournemire Underground Laboratory: Geophysical Monitoring of Claystone Desaturation in a Ventilated Borehole.

Transcription

1 Tournemire Underground Laboratory: Geophysical Monitoring of Claystone Desaturation in a Ventilated Borehole. U.Zimmer, A. Genty, T. Rothfuchs, B. Bonin, J. Cabrera 1 Introduction With this paper, we will report some theoretical and practical results obtained during the desaturation experiments in the Tournemire Underground Research Laboratory from IPSN. The Tournemire Laboratory in southern France near the town of Millau and next to the famous village of Roquefort consists of an old railway tunnel cutting through claystone. For the research experiments, an additional drift perpendicular to the main tunnel was excavated. Natural and artificial barrier systems are the key elements of a final repository site for nuclear waste in deep geological formations. Rocks with an extremely low permeability for water, e.g., rock salt, granite, and clay, seem to be suitable for the storage of radioactive and toxic waste. Unfortunately, the properties of these rocks, e.g., their permeability, change with variations in the stress field caused by mining activity and the storage of waste. Rock desaturation in the vicinity of a newly excavated gallery is a major safety concern, because this desaturation can damage the geological barrier in the near field. The permeability of clay material is closely related to its water content. The determination, monitoring and modeling of the water content in these rocks with nondestructive methods can help to prove the effectiveness of natural and artificial barrier systems underground. Additionally, the obtained results can be used to verify predictive models. IPSN use a finite element code developed to model desaturation effects in clay. If the numerical results of this code are verified by experiments, it will be a powerful tool for the modeling of water contents in clayey rocks. 1

2 For a first qualitative estimate of the in situ water distribution at the surface of a rock, the fast and easy-to-handle thermography method can be applied. With this method, the temperature of the rock is determined by an infrared camera and temperature differences are interpreted in terms of evaporation. For a more quantitative determination of the water distribution in deeper parts of the rock the electric resistivity is measured. In connection with laboratory measurements on rock samples with known water contents, a quantitative correlation with the electric resistivity can be obtained which allows a quantitative interpretation in terms of water content. The geoelectric resistivity method works well in hard rocks like rock salt and granite. The experiment at Tournemire was a first test if this method can also be applied for the quantitative water determination in clay. 2 Finite Element Modeling In the model used by IPSN it is assumed that only gravity and capillary pressure act on the water distribution besides the evaporation. Air and vapor phase are not modeled explicitly. Two-phase flow effects and thermal and mechanical coupling are neglected in this model. The algorithm is implemented in the CASTEM2000 tool which allows an easy model generation. Applying a mixed hybrid element formulation allows the simultaneous computation of hydraulic head and flow velocity. This numerical code was applied to model the desaturation effects in a drying clay core exposed to air at room temperature. Due to the rotational symmetry, only a slice of the core had to be modeled. The predicted moisture distributions after different time steps are shown in figure 1. In this model, the input parameters (permeability, porosity, succion to water content curve...) were determined from sample ans in situ studies. The colours indicate the volumetric water content in the pores. Red colors indicate full saturation whereas blue colors show the desaturation effects due to evaporation. As expected, the desaturation effects started at the surface and continued into the inner parts with increasing time. Since the sample was assumed to be standing on a table, no evaporation took place through the bottom of the core. After 11 hours of drying, the effects were confined to the part near the surface. With increasing time, the desaturation effects reached deeper parts of the sample. But even after more than 5 days, the inner part of the sample remained unchanged. 2

3 Although the calculated results agreed qualitatively well with the expected effects, it is desirable to prove this through explicit measurements of the moisture distribution in the sample. Since in most cases the water content of a rock influences its electric resistivity, it is possible to estimate the water content of a sample by measuring its resistivity. 3 Quantitative Relation: Resistivity-Water Content For the successful application of the geoelectric method it is necessary to prove if electric resistivity and water content in the rock show a close relation. For hard rocks like rock salt or granite this is well known since many years. To prove the relation for the Tournemire clay rock was the objective of a first experiment. To determine a quantitative relation between the electric resistivity and the water content of a rock, measurements on rock samples with known water content are necessary. To measure the overall electric resistivity, the sample is put between two metal plates (figure 2). Through these plates, an electric voltage is set to the sample which induces the flow of an electric current. From these values, the specific resistivity of the sample is computed including a geometric factor which accounts for the sample dimensions. As a result from the measurements on two different samples, quantitative relations between the resistivity and the saturation of the samples were obtained (figure 3). The samples consisted of clay granules mixed with water to produce samples with different, but known water contents. In a logarithmic plot, both samples showed nearly the same relative changes although an offset occured between them. For fully saturated clay, a resistivity of app. 0.5 Ωm can be extrapolated. Compared to other rocks like rock salt and granite, this value is very low. This low value is not surprising and can be explained by the high water content of clay in these experiments. From these measurements it becomes clear that the absolute values of resistivity differ between samples with the same resistivity changes due to desaturation. Although these results were obtained on clay granules and not on natural claystone, it shows that a general relation between water content in clay and resistivity of the rock is existing. 3

4 4 Resistivity Tomography After the measurements on artificial samples showed a close relation, a real sample of Tournemire claystone was exposed to air and its average resistivity and weight were monitored for more than two weeks. The results (figure 4) showed an exponential decrease of the core s weight. Even after two weeks, the drying process was not finished. Parallel to the loss in weight, the overall electric resistivity increased from an initial value of 50 Ωm to more than 1000 Ωm. These values are significantly higher than the values obtained in the other laboratory experiments, owing to the low water content of the Tournemire claystone. Although this experiment did not simulate the in-situ situation of a drying rock formation very well, it provided a first estimation of the expected effects on the electric resistivity. Since the amount of water in the rock is not limited in situ, as it is in the core sample, the in-situ drying effect is probably less than in the isolated clay core. Nevertheless, an increase of the resistivity by a factor of 2 can be expected in situ. The interpretation of the overall resistivity in terms of water content assumes a homogeneous moisture distribution in the sample. However, it is not likely that the drying process will produce a homogeneous moisture distribution at all times in all parts of the core. To get a more detailed picture of the resistivity distribution, a method called resistivity tomography is applied. In this method, the overall resistivity measurements are exchanged by many single measurements along the surface of the core. At two points, usually called A and B, a known voltage is set to the core. This induces a current, AB I, through the sample. At two other points, usually called M and N, the corresponding voltage, U MN, is measured. Including a geometric factor, which accounts for the relative positions of A, B, M, and N, an apparent resistivity is calculated from the measured output voltage and the input current. If these measurements are conducted along two profiles of electrodes, the resistivity distribution between these profiles can be computed from the measured apparent resistivities. To monitor the drying process of the core in more detail, the sample was equipped with four profiles of electrodes which form two intersection planes. 4

5 5 Resistivity Tomograms of a Core Sample With this method, the drying process of a new clay core was monitored during one week. After the inversions of the true resistivity distribution from the measured values, the resistivity tomograms of the intersecting planes were obtained. To give a better image of the drying effects, the resistivity values obtained for a specific time were related to their initial values. In figure 5, green colors indicate small changes in resistivity and orange/red colors indicate high changes in resistivity. After 11 hours of drying, the resistivity had increased very homogeneously along the measured planes. Subsequently the resistivity increased continuously with an emphasize on the upper part of the core. The origin of this effect is not clear yet. Since the experiment was only a first testing of the method on a clay core, the quantitative interpretation of these tomograms should not be overstressed. But the results showed clearly that resistivity tomograms are suitable for the non-destructive determination and monitoring of water and moisture distributions in clay core samples and, of course, in in-situ experiments as well. The method of resistivity tomograms was already successfully applied at numerous sites in granite and salt rock. With the investigations carried out, its suitability for clay has been proved. Since clay shows a different structure compared to hard rocks like granite, the quantitative interpretation requires more investigations. Especially, investigations on samples with limited extensions require a three-dimensional inversion processing. For a direct comparison of the resistivity tomograms with the results of finite element modeling this is an absolute necessity to improve the reliability of the calculated resistivity. Nevertheless, resistivity tomography has the potential for verifying the numerical modeling of desaturation effects in clay. 6 Thermographic Method and Application 5

6 Besides the electric resistivity, evaporation changes the temperature of rocks. Although these temperature differences are rather small, they can be recorded by using a thermal image camera (figure 6). This camera records the radiation emitted by the object, by the atmosphere, and the radiation reflected by the object. From the radiation, an image of the temperature distribution on the object s surface is computed. The insitu application of this method requires only the set-up of the camera and the control unit. The whole system can be moved easily. Additional lights are usually not required. For the interpretation of the recordings, a photography can be compared with the thermal image of the same area (figure 7 & 8). The photography shows the tunnel surface which is structured by calcite bands in a shale to marl matrix and includes some artificial radiation transmitters like wires and a yardstick. From the photography, the tunnel surface seems to be homogeneous. In contrast to this, the thermal image reveals colder areas in the upper part indicating a higher moisture content. Due to its higher moisture content, the area shows a temperature which is only 0.1 C lower than the background. This shows the sensitivity of the thermographic system. Besides this, the artificial radiation transmitters dominate the picture. 7 Geoelectric In-situ Experiment To create desaturation under controlled conditions, a borehole in the clay was ventilated with fresh air. In order to monitor the desaturation effect, the resistivity of the clay around the borehole was measured with electrodes in two other boreholes (figure 9). For logistic reasons, only 4 m of these boreholes, 0.6 m apart from the ventilated borehole, were equipped with electrodes only in a depth of 16 m. With some interruptions, the resistivity in this area was monitored with resistivity tomograms for nearly 9 months. The electrode array was installed in August Since the ventilation started in February 1999, many datasets were recorded describing the initial moisture distribution in the rock. The results obtained between August 1998 and January 1999 show close similarities (figure 10). The average resistivity was around 150 Ωm which agreed well with the values obtained from resistivity tomography on the core sample. The low resistivity anomaly at point D is possibly a result of moisture inflow into the rock during 6

7 the cementation of the electrodes. The origin of the high resistivity anomaly at A is not obvious but it existed throughout all measurements. During August 98 and January 99, the resistivity distribution remained nearly unchanged. The first desaturation experiment was a ventilation test with no extraction of water from the ventilated air. This test lasted until the end of February During this time, the resistivity distribution changed. The average resistivity doubles to app. 300 Ωm. The different anomalies were pronounced by the ventilation. After this first ventilation test, the water was extracted from the air in order to increase its drying capability. In contrast to the expectations, the obtained resistivity tomograms showed no significant change compared to the image from the end of February The extraction of water had no significant effect on the resistivity distribution. The drying effects recorded with these measurements showed an inhomogeneous desaturation throughout the investigated area. But the rate of moisture extraction was probably to small, and the desaturation due to extraction was to short during this beginning experiments to produce observable results. 8 Results For the verification of finite element models and for the monitoring of in-situ desaturation effects in clay, the resistivity tomography was tested. Although the quantitative interpretation requires more investigations it was shown that resistivity tomography is suitable for the determination of moisture distributions in clay material. The in-situ investigation presented in this paper was one of the first experiments ever conducted in claystone. It was designed to prove the general applicability of this method in clay formation which was successfully accomplished. To compare the in-situ results with the finite-element models it is necessary to improve the quantitative reliability of the computed resistivity distributions. A major progress will be the application of 3-dimensional-inversion algorithm which will be available soon. 7

8 Figure 1: Results of finite element modelling on a drying claystone core Input Voltage (U) Output Current (I) Re sistivity = InputVoltage GeometricFactor OutputCurrent Figure 2: Determination of average resistivity in a rock sample 8

9 10 1 Resistivity ρ t [ Ωm ] Saturation S [ - ] Figure 3: Resistivity-Water Content relation of two (red/green) samples of clay granules ρ [ Ωm ] Masse [ g ] ρ [ Ωm ] Mass [ g ] Time [ h ] Figure 4: Average Resistivity and weight of a drying core sample of Tournemire claystone 9

10 Z Z 11 h X Y V35 40 h X V Y Z X Y 74 h 136 V Z X V Y Figure 5: Resistivity distribution in a clay core from Tournemire after 11 h, 40 h, 74 h and 136 h of drying Object Atmosphere Thermal Imager Radiation emitted by the object Radiation emitted by the telescope lens Radiation emitted by the atmosphere Radiation emitted by the thermal imager Radiation emitted by the surroundings and reflected in the object Figure 6: Thermographic method 10

11 Figure 7: The tunnel surface is structured by calcite bands in a shale / marl matrix and artificial radiation transmitter as e. g. wirelines marker 1IR - TM_WS01.IMG 14.0 C 14 darker wet area Tournemire 0798 Figure 8: Darker zones in the thermographic map indicate wet areas in the desaturated tunnel surface 11

12 0.6 m 0.6 m Electrode Borehole 30 mm 16 m Casing Ventilation Borehole Electrode Borehole 30 mm 4.0 m 21 Electrodes 0.2 m spacing Area of Interest Figure 9: Experiment configuration to determine desaturation effects around a ventilated boreole in clay 12

13 X-Koord X-Koord X-Koord Z-Koord Z-Koord Z-Koord Figure 10: Resistivity Tomograms around a ventilated borehole in claystone at Tournemire 13