Small angle neutron scattering studies of diffusion and permeation through pores in clays

An extensive study using small angle neutron scattering (SANS) has been made of the pore structures in naturally compacted London, Oxford and Lower Lias clays and in commercially purified montmorillonite and kaolinite. All the pore volumes were initially filled with H2O. Using SANS contrast variation techniques, the accessibilities of the pores to D2O by diffusion and permeation have been studied and the generic behaviour of each of the materials has been determined. The results are in qualitative agreement with other mass transfer measurements and suggest that access to some of the porosity takes more than a thousand hours. Most noticably in Oxford clay, a disproportionately large fraction of the surface area appears to be contained in porosity which is inaccessible in such times. Characterisation of the accessibility in different parts of the pore structure is intended to provide input data for models of radionuclide transport through clay geological barriers. Similar application is envisaged in connection with enhanced oil recovery. (author)