Identifiability of diffusion and retention parameters of anionic tracers from the diffusion and retention (DR) experiment

SummaryIn situ diffusion experiments in low-permeability rocks are performed to study radionuclide diffusion and sorption and investigate scale effects. They are carried out in a packed-off interval of a borehole having a circulation system in which tracers are injected and allowed to diffuse into the rock. Tracer concentrations are monitored at the tracer interval during the experiment (dilution data) and measured at the end of the experiment in rock cores (overcoring data). The interpretation of in situ diffusion experiments in clay formations is challenging due to the influence of the filter, the gap between the filter and the borehole wall and the borehole disturbed zone (BDZ). The impact of the filter, the gap and the BDZ on the estimates of the accessible porosity and the effective diffusion, De, of the rock in the presence of random noise has been evaluated for the anionic tracers with synthetic experiments having the geometry and parameters of the in situ diffusion and retention (DR) experiment performed on Opalinus clay. The component of De of the clay parallel to the bedding can be estimated properly from dilution data when the standard deviation of the noise of the data is smaller than 0.05, but the component of De normal to the bedding cannot be estimated from dilution data. On the other hand, both components of De can be estimated properly from noisy overcoring data. Although the parameters of the BDZ and the undisturbed clay cannot be estimated simultaneously from noisy dilution data, they can be estimated from noisy overcoring data when the standard deviation of the noise is smaller than 0.05. Parameter estimation errors are large when the assumption about the BDZ is incorrect. The uncertainties in the De of the filter and in the volume of the circulation system have a noticeable impact on the estimates of the diffusion and porosity of the clay derived from dilution data. On the other hand, acceptable parameter estimates are obtained from overcoring data. The estimation errors of the parameters of the anionic tracers are generally larger than those of the neutral and weakly-sorbing tracers.

► We present the identifiability of anionic tracers.
► The estimates from overcoring data are more accurate than those from dilution data.
► We present the simultaneous estimation of diffusion parallel and normal to the bedding.
► Estimation errors of the parameters of anionic tracers are larger than those of neutral and sorbing tracers.
► Our results will contribute to the optimum design of in situ diffusion experiments.