Pore fluid (e.g., water and methane) phase transition in rocks determines many geology properties of the polar strata, including heat transfer, deformation, and percolation. In this article, low-field nuclear magnetic resonance tests are conducted to investigate the freezing and thawing characteristics of pore fluid in rocks, including saturation, pore fluid distribution, and permeability. During the phase transition, nuclear magnetic resonance tests provide the unfrozen pore fluid saturation variation and unfrozen pore fluid distribution evolution (including unfrozen bound pore fluid and unfrozen movable pore fluid). The experimental results indicate that most movable pore fluid is frozen at the lowest temperature, whereas many bound pore fluids gradually attenuate. Also, the freeze and thaw-induced strains of sandstones are theoretically calculated based on a frost heave model. The calculated results indicate that the phase transition of the pore fluid induces a sharp change in strain, and the difference appears in the two processes. The permeability measured demonstrates that ice growth tends to be between pore cementation and pore-filling. This study quantifies the unfrozen pore fluid saturation, pore fluid distribution, and phase transition- induced strain in sandstones.
In cold regions, the temperature alternates with the seasons (Giorgi et al. 1999). A growing number of geotechnical projects are under construction or planned in cold regions of the world, including Siberia, the polar region, and northwestern China. In addition, polar engineering is carried out under extremely complex geological formations, intrusive water, low temperature, and other environmental conditions, which poses significant challenges to the construction and maintenance of rock and soil infrastructure and the stability of surrounding rocks (Humlum et al. 2003). Thus, understanding freezing and thawing characteristics are critical for rock projects’ safe construction and operation in cold regions.
It is well known that porewater in geomaterials such as soil and rock have no single freezing point and freezes gradually as the ambient temperature drops (Watanabe and Mizoguchi, 2002). When the ambient temperature drops, the pore fluid transforms into a solid phase, reducing saturation and redistribution of the pore fluid. The extra pressure causes the rock to deform in the presence of the solid phase (Winkler 1968). Water exerts erosion and destructive impacts on geotechnical engineering in cold regions, which relates to strain and permeability. Thus, investigating the strain and permeability changes produced by phase transition is essential for the early design and stability maintenance of geotechnical engineering in cold regions. The total strain of rocks is associated with temperature and pore fluid saturation, and the permeability is determined by the flow channels of rocks (Backeberg et al. 2017). Thus, the characterization of unfrozen water saturation and distribution needs to be solved urgently. Nuclear magnetic resonance has been proven beneficial to probe pore fluid content and distribution (Dillinger and Esteban, 2014). Compared with other technologies, including mercury intrusion, sonic method, and differential scanning calorimetry, this technology has the advantages of being fast, non-destructive, and have high-precision.
