The effect of microstructural evolution on the permeability of sandstone under freeze-thaw cycles

Understanding the evolution of microstructures in freeze-thaw (FT) cycles is important to reveal the mechanisms of frost damage in porous rocks. This paper investigated the effect of microstructural evolution on the permeability of sandstone under FT cycles as well as the evolution pattern of microstructures. First, scanning electron microscopy (SEM) tests were carried out to investigate the evolution of sandstone microstructures at the same location under FT cycles. The experimental results showed that the FT cycles obviously caused the spalling of grains of the sample. Second, nuclear magnetic resonance (NMR) tests were conducted to investigate the evolution of the pore size distribution (PSD) and the permeability of sandstone samples under FT cycles. The experimental results showed that the FT cycles had obvious effects on pores with relaxation times varying from 1 ms to 100 ms. As the number of FT cycles increased, the percentage of nanoscale throats decreased, while the percentage of throats in the submicron and micron sizes increased. Third, the predictive model of permeability coefficient, dry density, porosity, surface porosity, surface fractal dimension, median pore size and median throat size was developed using a back propagation neural network (BPNN). Fourth, relative importance analysis of the above parameters was carried out, and the contribution of each parameter to the permeability coefficient of the sample was determined. Finally, the mechanism of FT damage was investigated. The present experimental results are helpful in improving the understanding of the leakage mechanisms of rock engineering in cold regions.