Laboratory determination and analysis of effective diffusion coefficients for low-permeability rock and clay

Low-permeability rock and clay are focuses in hydrogeology and environmental geology research objects. Physical movement of dissolved species through these media is mainly governed by diffusion processes. In this study, intact clay and silty clay are used as examples, and Cl- as a tracer. The migration model and radial diffusion process of Cl- are established depending on the radial diffusion experimental apparatus. Data fitting and average absolute residual analysis of the theoretical concentrations and the measured values are performed by using the Comsol software, based on the initial and boundary conditions of the experimental model. The effective diffusion coefficients are considered in the simulation of transport coupled with the experimentally calculated values. The results show that the effective diffusion coefficients of G1-Core (with the depth of 101.2~101.5 m) and SY1-Core (with the depth of 147.1~147.3 m and 170.48~170.68 m) samples are 6.0×10-10 m2/s, 4.5×10-10 m2/s and 3.9×10-10 m2/s with the lowest average absolute residual, respectively. The linear relationship of the effective diffusion coefficient and porosity also demonstrates the feasibility of the test method. Furthermore, the relationship between the effective diffusion coefficient of Cl- and the diffusion-accessible porosity can be described by the Archie’s law with the exponent n=1.9 of the fine-grained soil. It is applied to predict the effective porosity of samples.