Gas Transfer Through Clay Barriers

© 2015 Elsevier Ltd Quantification of gas migration through fully water-saturated geological clay barriers is of relevance in different research fields, like the safe and long-term storage of radioactive waste or carbon dioxide, or natural hydrocarbon exploitation. Reliable forecasts require robust and well-documented experimental data that can be integrated into different petrophysical transport models. In order to provide a broad theoretical background, this chapter first outlines the fundamentals of diffusive gas transport in water-saturated rocks and viscous gas flow controlled by capillary pressure and dilatancy. This is followed by a summary of relevant literature data, providing an overview of applications in different fields and the common ranges of values of individual parameters. Molecular diffusion, driven by chemical potential gradients, is characterized by a low transport efficiency. But it is a ubiquitous process that can act over long periods of time and may then be of relevance on length scales up to hundreds of meters. In comparison, pressure-driven viscous gas flow has a much higher transport capacity. But in originally water-saturated rocks it requires the formation of interconnected gas-filled fluid pathways, either by drainage or dilatation. Below this percolation threshold gas flow remains completely diffusion controlled. The experimental determination of gas transport-related parameters in low-permeable rocks is very challenging. Experiments may take several weeks to months and often a clear discrimination between different transport processes is not possible. Various experimental techniques have been developed and applied, and they usually require specific interpretation methods. In the last section of this chapter, examples are given of experiments performed on different clay-rocks considered as potential host rocks for radioactive waste storage (Boom Clay, Opalinus Clay, Callovian–Oxfordian Clay). Laboratories involved in these studies were those at the Belgian Nuclear Research Centre (SCK-CEN, Belgium), Ecole Centrale de Lille (France), British Geological Survey (UK), and at RWTH-Aachen University (Germany).status: publishe

Interplay of molecular size and pore network geometry on the diffusion of dissolved gases and HTO in Boom Clay

© 2016 Elsevier Ltd Through-diffusion experiments in Boom Clay have been performed with uncharged molecules: tritiated water (HTO) and dissolved gases of different size (He, Ne, H2, Ar, CH4, Xe and C2H6), allowing information to be obtained on the relationship between the diffusion coefficient and the molecular size (characterized by a ‘kinetic diameter’ of the molecules). Experiments have been performed on both clayey and silty Boom Clay samples, to scope for the changes induced by grain size variations on the diffusion process. Experiments on clay cores taken perpendicular as well as parallel to the bedding plane have also been executed, providing additional information on the anistropy of the diffusion process. Empirical relations are proposed to capture the observed decrease of both the diffusion coefficient in water and the effective diffusion coefficient in the Boom Clay porous medium as a function of molecular size. In the same way, the behaviour of the geometric factor G as a function of size is estimated. Although silty samples have a noticeably higher hydraulic conductivity than clayey samples, the difference in diffusion coefficient is less obvious. The anisotropy factor is roughly the same for all investigated components, with an average value of 1.5.publisher: Elsevier articletitle: Interplay of molecular size and pore network geometry on the diffusion of dissolved gases and HTO in Boom Clay journaltitle: Applied Geochemistry articlelink: http://dx.doi.org/10.1016/j.apgeochem.2016.11.022 content_type: article copyright: © 2016 Elsevier Ltd. All rights reserved.status: publishe