One of the key issues in the hydrogeologic characterization of sedimentary formations is uncertainty about fluid pressure anomalies that might be generated by chemical osmosis. Groundwater flow under the influence of chemical osmosis is dominated by salinity supply conditions at the boundaries of a semi-permeable formation, as well as by the properties of formation media related to solute and fluid migrations.
To obtain chemico-osmotic, diffusion, and hydraulic parameters from a single rock sample, this study devised a laboratory experimental method consisting of permeability and chemical-osmosis experiments, and developed an experimental system capable of performing both experiments on a natural rock sample in sequence under in situ effective stress conditions. The experimental method devised was successively applied to determine the parameters of siliceous mudstone taken from the Wakkanai formation in the Horonobe research area of the Japan Atomic Energy Agency. In the chemical-osmosis experiment, a pressure of almost 100 kPa (about 10 mH2O) was induced by chemical osmosis under a NaCl concentration difference of almost 0.45 mol/L.
A series of numerical examinations was performed using 1D hydrogeological models, which revealed that the evolution of osmotically-induced fluid pressure, the duration of chemical osmosis, and the direction of net water flux in semi-permeable formation might depend on salinity supply conditions at the boundaries of a semi-permeable formation. A set of type curves for salinity and pressure profiles in the semi-permeable formation were also obtained to facilitate a preliminary examination of the possibility of chemical osmosis in sedimentary formations. The analytical development was performed using a dimensionless number representing the ratio between time constants of pressure propagation and solute diffusion. Therefore, the results are useful for screening formations where chemical osmosis might exist.
