Emergence of non-aqueous phase liquids redistribution driven by freeze-thaw cycles in porous media based on low-field NMR

Increasing human activities in cold regions have led to serious environmental problems due to non-aqueous phase liquids (NAPLs) contamination. However, the NAPLs redistribution driven by freeze–thaw (FT) cycles is not well understood. In this study, the redistribution of NAPLs in porous media subjected to FT cycles in a temperature range from 20 °C to −15 °C was investigated using the low-field Nuclear Magnetic Resonance (NMR) technique. The redistribution of NAPLs in the prepared samples with NAPLs saturation ranging from 13.56% to 72.05% was confirmed using the transverse spin-spin relaxation time (T2) distribution and the magnetic resonance imaging (MRI). Experimental results revealed that substantial remobilization and distribution of NAPLs occurred between the macropores (T2 greater than 300 ms) and the mesopores (60 ms < T2 < 300 ms) while the NAPLs in the micropores (T2 < 60 ms) remained almost unchanged during 30 FT cycles. The main reason is attributed to the fact that the redistribution rate in micropores is much lower than in macropores and mesopores. Furthermore, the NAPLs content and the number of FT cycles were positively correlated for macropores while negatively correlated for mesopores. For the samples with low NAPLs saturation (e.g., SN=13.56% and 13.56%), the redistribution rate of NAPLs versus FT cycles exhibited an overall linear relationship for macropores and mesopores while a non-liner relationship for micropores. For all pore categories, the increment in SN can cause a remarkable deviation extent. The balance between capillary pressure and freezing-induced pressure was evaluated by the newly introduced dimensionless pressure ratio ΔP, which could help determine the ultimate mobilization and redistribution of NAPLs during FT cycles. Experimental tests suggested that the ΔP was much larger in macropores than in mesopores and micropores. This can explain the dramatic difference in the redistribution rate among different pore categories. This fundamental study is helpful in understanding the mobilization and redistribution mechanisms of NAPLs in cold regions.