MOLECULAR-DIFFUSION IN WATER-SATURATED ROCKS - A NEW EXPERIMENTAL-METHOD

An experimental method has been developed to quantify helium molecular diffusion through water-saturated rocks. The procedure is based on th e through diffusion method classically used for diffusion experiments. The first main improvement consists of the ability to control more en vironmental parameters close to the in-situ conditions than previously . The sample is subjected to stable temperature conditions (from 30 to 80 +/- 1 degrees C), total isotropic stress (from 5 to 20 +/- 0.5 MPa ) and pore water pressure (from 1 to 5 +/- 0.2 MPa). The water saturat ed state of the sample is controlled using permeability measurements a nd maintained throughout the experiment. A second important improvemen t is owing to the development of a numerical simulation and a new proc edure for analysing helium which permits the study of the initial tran sient phase of the diffusive transfer. This procedure is based on heli um leak detection using mass-spectrometry. A diffusion experiment perf ormed on a clayey-marl specimen from Callovo-Oxfordian sediments in th e north-east of the Parisian basin constitutes a significant test of t he method. According to the results, an apparent diffusion coefficient as small as 2 X 10(-12) m(2) s(-1) can be determined within a month, using a 1 cm thick sample. (C) 1997 Elsevier Science B.V.