Development and application of an innovative method for studying the diffusion of dissolved gases in porous saturated media
Fine-grained sedimentary formations are investigated in numerous scientific fields, e.g., as
a host formation for a deep geological repository for high-level radioactive waste, as cap rocks for oil reservoirs or CO2 geosequestration, or as potential economic resources in case of shale gas extraction. The permeability characteristics of these geological strata are important to assess their
ability to act as a long-term effective barrier. However, apart from some gross parameters (such as hydraulic conductivity etc.), such characteristics are difficult to measure because of the nanometer-sized porosity structures and complexinterconnectivity.
SCK recently developed a versatile technique to measure the diffusion coefficient of dissolved gases in low-porosity materials. The method was validated on Boom Clay samples, and up to now diffusion coefficients for He, Ne, Ar, Xe, CH4, C2H6 and H2 have been measured. The precision
of the obtained diffusion coefficients ishigh (< 10% uncertainty) compared to other existing techniques.
Most recent results show that an exponential relationship exists between the kinetic
diameter of these gases and the measured effectivediffusion (De). An equal relationship exists between the kinetic diameter of these gases and their self-diffusion coefficient in water D0.
The ratio of D0 over Deff is called the formation factor, F. It is believed that this factor
can be linked to structural propertiesrelated to the permeability of the material. One of the objectives of this PhD is to investigate if information on the permeability/structure of the material can be obtained by measuring the diffusion coefficient ofgases.
Another objective of this PhD proposal is to verify whether similar relationships between kinetic
gas diameter and effective diffusion coefficient exist for other low permeability materials. The diffusion coefficient of other gases can then be predicted based on their kinetic diameter. For this objective, we will investigate rocks of interest both for radioactive waste disposal (Callovo-Oxfordian, Opalinus Clay) and for oil industry and shale gas mining.