Study on the Permeability Characteristics of Sandstone under Hydrostatic Pressure and Model Improvement

In order to explore the influence of stress state on the permeability of western weakly cemented sandstone, and to clarify the seepage and mechanical properties of rock strata during mining, the rock seepage test and nuclear magnetic resonance test under hydrostatic pressure were carried out, respectively. On this basis, the rock permeability was inversed, and the change law of the pore structure in the rock during the seepage process was analyzed, and then the nuclear magnetic permeability model was modified. The results show that: (1) The confining pressure has a significant effect on the permeability of rock, which makes the pore structure in rock change; The increase of the osmotic pressure difference will cause the expansion of the pores in the rock, increase the seepage channels and increase the permeability. (2) There is a limit value for the closure of micropores and fine pores caused by the increase of confining pressure, and the confining pressure required for micropores and fine pores to reach the closure limit is obviously lower than that of large pores; The influence of micropores and fine pores on rock permeability decreases with the increase of confining pressure; Macropore closure contributes the most to the decline of rock permeability. (3) The increase of confining pressure will lead to the transformation of large pores into micropores and micropores, and the T₂ cut-off value will increase; The change of pore structure caused by the increase of confining pressure will lead to the transformation of free pores into bound pores in the rock, resulting in the change of permeability. (4) After considering the influence of confining pressure on the change of pore structure, the NMR permeability model is modified. The modified permeability model can better reflect the permeability of rocks, which provides an effective basis for the permeability inversion of this kind of rock in the NMR test.