An experimental examination of the effect of the salt type on the chemo-osmotic, diffusive, and hydraulic parameters of Wakkanai mudstones

To better understand the groundwater flow and chemistry in argillaceous formations, the dependency of transport parameters such as permeability, diffusion coefficient, and chemical osmotic efficiency on each of the chemical species dominant in groundwater must also be understood. Despite its importance, however, little is known about the dependence of transport parameters on different types of groundwater. Thus, we investigated transport parameters of Wakkanai mudstones using three different types of electrolyte solutions. Here, we report the relationship among the solution salt type, solute concentration, transport parameters, and pore structure characteristics. Wide ranges in osmotic efficiency and intrinsic permeability, 0.0035-0.14 and 1.19 × 10-19 to 4.75 × 10-18 m2, respectively, reflect the difference in the solution composition based on the salt type, difference in solute concentration, and variation in the pore size distributions of Wakkanai mudstones. The effective diffusion coefficients with filtration effects, at 2.98 × 10-11 to 5.92 × 10-11 m2 s-1, vary depending on salt type, but remain invariant among samples, indicating no difference in fluid flow path associated with small values of osmotic efficiency. In contrast, the osmotic efficiencies are related to differences in salt concentrations. Our findings indicate that salt concentration and pore structure characteristics determine not only the semipermeability of the Wakkanai mudstones but also the diffusivity and permeability.