Fluid-solid coupling analysis of aquifer sandstone based on Nano-CT scanning reconstruction

The stress-pore-seepage coupling mechanism in sandstone aquifers affected by mining disturbances is crucial for safeguarding roof aquifers and predicting contaminant transport during coordinated coal and uranium mining operations. This study employs nano-CT scanning, nuclear magnetic resonance (NMR) data calibration, and three-dimensional model reconstruction to perform fluid-solid coupling numerical simulations at the microscale within sandstone. Based on pore size distribution, the NMR T2 spectrum can be broadly categorized into four segments: pores smaller than 1 nm, pores between 1 nm and 1 μm, pores between 1 μm and 6 μm, and pores larger than 6 μm. The NMR T2 spectrum exhibits two peaks at pore sizes of 1 nm–1 μm and above 6 μm. NMR results during seepage indicate that, under an external load of 2 MPa and an increase in water pressure from 0 MPa to 0.5 MPa, nanopore (1 nm to 1 μm) porosity decreases from 5.06% to 4.65%, while micropore (>1 μm) porosity increases from 0.14% to 0.6%. As external stress continues to rise, both nanopore and micropore porosity decrease linearly: nanopores decline from 4.65% to 4.24% (a reduction of approximately 8.82%), and micropores decrease from 0.6% to 0.46% (a reduction of approximately 23.3%). The proportion of total porosity remains largely unchanged, with nanopores accounting for about 90% and micropores comprising approximately 10%. A sandstone pore reconstruction method was developed utilizing NMR and nano-CT data, with a minimum representative element size of 100 pixels. Microscopic simulations reveal that increasing external stress leads to complex stress concentration and attenuation near pores. During seepage, flow lines converge toward pore regions and preferentially traverse zones with lower effective stress. Furthermore, reverse flow and boundary layer effects occur internally during seepage, intensifying under increasing stress.