Project overview: A geostatistical methodology for modelling groundwater flow and transport in low-permeability media – application on Boom Clay, Ieper Clay and Toarcian argillites

Fluid flow in permeable media has been extensively studied and analyzed in several contexts, especially in the development of groundwater resources and the recovery of petroleum. By comparison, less effort has been spent to groundwater flow and transport of pollutants in low-permeability media. Recently low-permeability layers receive more attention because their low hydraulic conductivities are interesting for several important applications: landfilling of industrial and municipal waste, storage of carbon dioxide, oil and gas reservoir formation and storage of high-level nuclear waste. In all these applications, it is of great importance that the behaviour of groundwater and pollutants is well understood and can be predicted. This is unfortunately not always the case. Too often, phenomena that can not be explained with classical models are observed in low-permeability media: anomalous groundwater pressures, regional permeability values much larger than expected from laboratory tests, concentration measurements that deviate from the predicted values, etc. These problems are often due to (1) inappropriate application of methods originally developed for high-permeability media, (2) the limited data availability about the main transport parameters in low permeability media caused by experimental difficulties, (3) not taking into account the heterogeneity of the different transport parameters and (4) not taking into account of fractures.

In this work, a rigorous methodology was developed for modelling flow and transport in low-permeability media. The development and application of the methodology has resulted in general recommendations for modelling flow and transport in low-permeability for various applications: (1) the relative importance of advection and diffusion should be determined using a Péclet number criterion suitable for low-permeability environments; (2) secondary information such as grain size, lithology and geophysical parameters are very useful to complement and compensate the often limited knowledge of the transport parameters; (3) heterogeneity of hydraulic conductivity should be incorporated; (4) heterogeneity of the diffusion parameters, i.e., the diffusion coefficient and diffusion accessible porosity, should be incorporated and (5) fractures, if present, should be incorporated in flow and transport models of low-permeability media.

Application of the method on potential radionuclide transport in the Boom Clay (Belgium) shows that the effect of K heterogeneity and fractures on flow and transport is limited and that is does not affect the main safety function of the Boom Clay. The heterogeneity of the diffusion parameters has a larger effect on the radionuclide fluxes but neglecting the heterogeneity of the diffusion parameters is a safe conservative assumption since the radionuclide fluxes calculated with heterogeneous models are smaller than the fluxes calculated with a homogeneous model.

Application of the method on the Ieper Clay (Belgium) shows that the heterogeneity of hydraulic conductivity and the diffusion parameters has a large effect on the calculated radionuclide fluxes. Future flow and transport models of the Ieper Clay should thus incorporate the heterogeneity of hydraulic conductivity, the diffusion coefficient and the diffusion accessible porosity.

Application of the method on modeling of chloride transport in the Toarcian argillites (France) shows that incorporating the heterogeneity of the diffusion parameters results in a better correspondence between measured and calculated chloride concentration values.

 

Selected publications