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 the through diffusion method classically used for diffusion experiments. The first main improvement consists of the ability to control more environmental parameters close to the in-situ conditions than previously. The sample is subjected to stable temperature conditions (from 30 to 80 ± 1°C), total isotropic stress (from 5 to 20 ± 0.5 MPa) and pore water pressure (from 1 to 5 ± 0.2 MPa). The water saturated state of the sample is controlled using permeability measurements and maintained throughout the experiment. A second important improvement is owing to the development of a numerical simulation and a new procedure for analysing helium which permits the study of the initial transient phase of the diffusive transfer. This procedure is based on helium leak detection using mass-spectrometry. A diffusion experiment performed on a clayey-marl specimen from Callovo-Oxfordian sediments in the north-east of the Parisian basin constitutes a significant test of the method. According to the results, an apparent diffusion coefficient as small as 2 × 10−12 m2 s−1 can be determined within a month, using a 1 cm thick sample.