Phase Transition in Geomaterials Under Unsaturated Conditions

The freezing and thawing processes in sandstones under unsaturated conditions are methodically explored via the low-field nuclear magnetic resonance (NMR) technique. To precisely monitor the water content and distribution during the freezing and thawing processes, a special Carr–Purcell–Meiboom–Gill (CPMG) pulse sequence is proposed, leading pulse-induced temperature perturbations to be minimized. The results exhibit totally dissimilar water distributions in the two processes even under the same saturation: the freezing process retains more water in large pores, while the thawing process contains less water in large pores. This observation is primarily attributed to the ink-bottle effect occurring during the freezing: large pores with narrow entrances do not freeze until the entrance freezes. In addition, phase transition duration depends on the initial water saturation, for instance, the duration of the thawing process increases from 7 to 12 min as the initial water saturation varies from 40% to 80%. Through NMR observation, this phenomenon is associated with the variation of surface-to-volume ratio: high saturation is associated with the presence of water in large pores; the latter owns a small surface-to-volume ratio and accordingly involves a longer phase transition duration. After the assessment of surface-to-volume ratio, the intrinsic kinetic rate coefficient is further estimated to be 4.6 × 10–12 mol cm−2 s−1 ℃−1 for the ice thawing process in the studied rock. This value is five orders of magnitude lower than that found in the literature for the thawing process in bulk condition, i.e., 7.5 × 10–7 mol cm−2 s−1 ℃−1. We suggest that the small intrinsic kinetic rate coefficient addressed in the present work could be related to the constrained effect in rocks.