Experimental Study on the Permeability Characteristics of Sandstone in Different Chemical Solutions

Groundwater seepage is one of the important factors which affect the physical and mechanical properties of soft rock. In this study, the permeability evolution of sandstone was investigated using water solution seepage tests with different pH values. The results show that the chemical nature of the solutions has a great influence on the evolution of the permeability coefficients of sandstone specimens. Over time, it was determined that the permeability coefficient under the solution with PH = 2 was significantly greater than those under the PH = 7 and PH = 12 solutions used in this study. During the infiltrations, the ions of H+ and OH- which were involved in the water-rock interaction made the pH value of the solution change with different degrees. After the seepage, the ions in the solutions changed significantly. The ion concentrations seeping from sandstone specimen in the acidic solution, such as Ca2+, Mg2+, Fe3+ (Fe2+), Al3+ and K+, were found to be much greater than in the neutral and alkaline solutions. However, the ion concentrations seeping from sandstone specimen in the neutral solution displayed little differences from those in the alkaline solution. The concentration of the Si ion in the alkaline solution was much higher than those of the acid and neutral solutions, and the concentration of the acid solution was slightly higher than that of the neutral solution. The changes in the ion concentration in the osmotic solution reflected that there had been different forms of chemical reactions between the minerals of the sandstone and the solutions. The seepage pressures also had some effects on the permeability coefficients, pH values, and concentrations of ions in the solutions. After the seepage tests, the rock microstructures were observed by the means of a Scanning Electron Microscope (SEM). The results show that some new holes have been formed in the sandstone specimen due to the seepage, especially in the sample with the acid solution. The internal structural changes of the sandstone are found to be the main reasons for the changes in the permeability coefficients.