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Comparative study of convective and diffusive transport phenomena within the Opalinus Clay at Mont Terri

​Catherine Ji Yu has defended her thesis on 9th November 2017 at IRSN, in Fontenay-aux-Roses (France).

Document type > *Mémoire/HDR/Thesis

Keywords >

Research Unit > IRSN/PRP-DGE/SRTG/LETIS

Authors > JI YU Catherine

Publication Date > 09/11/2017

Summary

The safety of radioactive waste disposal facilities in deep geological formation depends on the evaluation of the rock confining properties and the processes governing fluid and solutes fluxes. The thesis is conducted in the framework of Deep Borehole experiment of the Mont Terri consortium, of which the IRSN is a partner. The purpose of this research is to build confidence with regard to understanding relative importance of diffusive and convective phenomena and to identify the impact of a hydraulic and chemical transient behaviour on the transfers of fluid and solutes, and anomalous pressures generation. These anomalies are frequent in sedimentary basins and can be measured in argillaceous formations studied by the IRSN, such as the Toarcian-Domerian clay rock of Tournemire, the Callovo-Oxfordian formation of the Paris Basin and the Opalinus Clay at Mont Terri.

The first stage of this research work consisted in determining the driving forces (pressure, temperature and solute concentration gradients) and phenomenological coefficients, which are key parameters involved in the global diffusive-advective transport equations. These equations include diffusion defined by Fick’s laws (solute flow caused by a concentration gradient), and the advection sensu largo, which corresponds to pure advection coupled with chemical osmosis and thermal osmosis (fluid flow caused respectively by hydraulic, chemical and temperature gradients). Data acquisition was performed with drillcore samples from the BDB-1 deep-inclined borehole and the multi-packer hydraulic system with pressure and temperature sensors installed within the borehole. Representative of the natural formation conditions, temperature and pressure measurements revealed a high geothermal gradient of 8.5 °C.100 m-1 and an excess of hydraulic head of at least 60 m. Several methods were used to obtain the hydraulic parameters: petrophysical model, spectral analysis of pressure series, tests using gas or water in Hassler cell, and in situ hydraulic tests. They revealed hydraulic conductivity values virtually in the same range than the ones acquired so far at the tunnel level, but with higher average values in the Opalinus Clay shaly facies, in the order of 10-12 m.s-1. Specific storage coefficients rather homogeneous in the order of 10-6.m-1 were determined. Leaching experiments and out diffusion tests were performed to acquire the chloride contents along the borehole. The resulting profile confirms the results of the previous studies and shows an asymmetric curved trend with maximum values superior to 12 g.L-1 in the basal part of the clay formation. Several experimental setups were used to acquire the Opalinus Clay diffusive parameters: out and in diffusion in cubic or radial configuration and through diffusion. Effective diffusion coefficients for chloride and bromide were estimated in the order of 4∙10-11 m².s-1 and a low anisotropy ratio of 2.4 was estimated for chloride effective diffusion coefficient in the Opalinus Clay sandy facies. Radial diffusion measurements enabled to highlight the anion exclusion effect with a coefficient in the range of 46 to 60%. Theoretical models based on electrostatic interactions and the modifications of the hydrogen bounds associated with water molecules near the clay surface were used to compute the osmotic transport parameters.

A Bayesian inversion based on a Monte Carlo Markov Chain algorithm was carried out to obtain a paleohydrological scenario explaining the current chlorinity profile. The chloride profile obtained by out diffusion experiments was interpreted by means of a purely diffusive one-dimensional numerical model. The assumption of purely diffusive mass transport phenomena was verified by estimating the Peclet number including osmotic processes in the advection term. The implemented scenario consists in a diffusive exchange between the argillaceous formation and the adjacent aquifers, with deferred activation times of the fresh-water sources linked to the surface erosion of the geological formations. The implemented inversion based on effective diffusion coefficients, initial value of the chloride concentration and two exhumation and thus, hydraulic activation times for the two bounding aquifers allowed to evaluate the best fit parameter sets and their uncertainties not evaluated so far.

The contribution of osmotic transport phenomena was assessed by interpreting the pressure profile measured in the BDB-1 borehole. Theoretical predictive models were applied to compute the osmotic parameters for monovalent and divalent solutions, based on experimental petrophysical parameters, temperature and chlorinity data. Transient simulations considering the temporal evolution of both chlorinity and pressure with the geological scenario determined using the MCMC scheme, and pseudo steady-state simulations, which are simplified first-order calculations considering equilibrium between pressure, temperature and chlorinity profiles were compared. A calibration was applied assuming no membrane properties for the Passwang Formation and more reduced thermo-osmotic coefficients in their uncertainty range, in order to match the pressure data.

The pressure profile can be reproduced by evaluating the coupled advective flow, by taking into account the respective contributions of pure advection, chemo-osmosis and thermo-osmosis (fluid flows caused respectively by pressure, chlorinity and temperature gradients). Pure hydraulic calculation evaluates a Darcy velocity in the order of -10-14 m.s-1, related to a downward flow. Chemical osmosis slightly increases the advective flow in the same direction, but its effect is rather minor compared to the one induced by thermo-osmosis. Indeed, the water movement is inverted when considering this latter process in the transport equation. Then, the mean Darcy velocity is in the order of 10-13 m.s-1 and the flow becomes upward. However, the Peclet number calculation enables to confirm the dominance of diffusion with respect to advective transport processes for fluid and solutes fluxes through the Opalinus Clay of Mont Terri.

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