Diffusivity and Porosity in Rock Matrix Related to the Ionic Strength in the Solution

Abstract

The nature of diffusivity and porosity in rock was studied as a function of various parameters. The phenomena of main interest were dead-end porosity, ion-exclusion and sorption. The rock types studied were rapakivi granite, granite and gneiss, and tracer techniques with ³⁶Cl, ²²Na⁺ and ³H (HTO) were used as a research method. A mathematical solution for outdiffusion from a porous cylinder was developed by applying a corrected form of Fick’s second law for a case where part of the pores are so-called dead-end pores. With this model the theoretical curve could be closely fitted to the measured values. It was found that the rock-capacity factor is an increasing function of the ionic concentration of the solution in the case of Cl indicating ion-exclusion, while the opposite is true in the case of Na⁺ indicating ion-exchange type sorption. The effective diffusion coefficient was also found to vary as a function of the salinity in the case of ³⁶Cl. In the case of ²²Na, the effect was opposite and weaker. The diffusion of tritium through the rock samples was clearly higher than the diffusion of ³⁶Cl. Part of the difference is explained by the smaller effective porosity for ³⁶Cl. The rest can probably be explained by the steric effects on the chloride ion caused by the negatively charged pore surfaces in the narrow pores.