Pore Water Freezing in Sandstone Considering Pore Structure Effects

This paper investigated the impacts of pore structure characteristics on pore water freezing of sandstone at low temperatures. Freeze-thaw cycles (from - 20.0 to 20.0 degrees C) were conducted to change the pore structure of sandstone using two different freeze-thaw treatment methods. Multi-level freezing treatments (- 1.5 degrees C, - 4.0 degrees C, - 6.0 degrees C, - 10.0 degrees C, - 15.0 degrees C and - 20.0 degrees C) and nuclear magnetic resonance (NMR) tests were performed to study the evolution of unfrozen water content (UWC) of sandstone with temperature and analyze the evolution difference caused by pore sizes. The impacts of pore structure characteristics, such as porosity, pore size distribution and fractal dimension, on the UWC at different temperatures were discussed. Moreover, a prediction model based on porosity was proposed for estimating the frozen degree of pore water of sandstone at low temperatures. The results indicate that with decreasing temperature, the pore water in sandstone undergoes supercooling stage (0.0 to - 1.3 degrees C), rapid freezing stage (- 1.3 to - 4.1 degrees C) and slow freezing stage (- 4.1 to - 20.0 degrees C). However, the pore water freezing is affected by pore sizes, with the UWC of micropores decreasing gradually with decreasing temperature, while that of minipores and mesopores first decreasing sharply and then remaining approximately constant. Interestingly, significant correlations are observed between UWC and parameters such as porosity, micropore content, mesopore content and fractal dimension, suggesting their potential in evaluating pore water freezing. Furthermore, the proposed model can effectively predict the frozen degree of pore water in sandstone at low temperatures with an acceptable error.