Experimental study on mechanical and permeability properties of sandstone with different granularities

In order to investigate and reveal the influence of granularity on mechanical behavior and permeability characteristics of sandstone, the conventional triaxial tests and triaxial compression and penetration tests are conducted on coarse sandstone, medium sandstone and fine sandstone of a project by using the MTS815 Flex Test GT rock mechanics test system. The researches show that the strength of sandstone and the parameters of shear strength, cohesion c and internal friction angle φ, increase with the decrease of granularity. The strength of sandstone has a decrease of the range between 5.15% and 24.66%. The permeability varies according to the characteristics of deformation: at the elastic stage, the holes inside the sandstone are compacted and the permeability decreases; at the elastic-plastic stage, the permeability increases slowly at first, and then moves up sharply and reaches the maximum value at the post-peak; finally, permeability decreases. The permeability of sandstone with the same granularity decreases with the increase of confining pressure, and the permeabilities of all samples under different confining pressures can be drawn as a linear function. Under the same confining pressure, there is a magnitude difference between the permeabilities of sandstone with different granularities. As for the overall performance of permeability, the permeability of the coarse sandstone is about 10⁵ times that of the medium sandstone, and the permeability of the medium sandstone is about 10 times that of the fine sandstone.