Role of diffusion osmosis on the mechanical and petrophysical alterations of shale when interacting with aqueous solutions

ABSTRACT Several experimental procedures were employed to investigate the impact of salt type and concentration on shale’s effective porosity and compressive strength. The use of a special gravimetric technique made it possible to quantify water and ion transport into shale when interacting with NaCl, KCl, and CaCl2 solutions. Results showed an increase in shale’s effective porosity when dilute solutions (less than 5% w/w) interacted with shale. On the other hand, a decrease in shale’s effective porosity was observed when the concentration of salt increased to 10%, 15% and 20% w/w. It is argued that the increase in porosity at low salt concentrations was due to cementation degradation caused by ionic invasion without loss of grain cohesion and mechanical stability. Decrease in porosity when higher concentrations of salts were used was apparently due to loss of grain cohesion and pore structure integrity caused by the combined effects of ionic diffusion and diffusion osmosis. These observations were confirmed by shale’s compressive strength measurements. The results showed a negligible change in compressive strength of shale when it interacted with 5% NaCl, KCl, and CaCl2 solutions. In contrast, a considerable reduction in shale’s compressive strength was recorded for higher salt concentrations (10%, 15% and 20% w/w).