Three-phase displacement events often occur in unsaturated porous media, especially when immiscible con-taminants (e.g., non-aqueous phase liquid, NAPL) inadvertently enter the subsurface. In this study, the influence of initial water saturation on the three-phase displacement efficiency and residual distribution during NAPL invasion into unsaturated porous media was experimentally investigated. The low-field nuclear magnetic reso-nance (LF-NMR) technique was used to dynamically monitor the spatial distribution of water in unsaturated porous media during the invasion of NAPL. The residual distributions (e.g., volume fraction) of NAPL/water/gas were quantitatively analyzed. The results showed that both the initial water saturation and the NAPL injection rate had a significant influence on the three-phase displacement. We found that the displacement efficiency of water/gas exhibited a non-monotonic relationship with the initial water saturation in the unsaturated porous media at certain NAPL invasion rates. A method was proposed to characterize the three-phase displacement regime in terms of the classical two-phase Lenormand's diagram. A significant increase in the volume fraction of residual NAPL after NAPL invasion was observed during the transition from the strong drainage region to the transitional displacement region. However, the initial water saturation was insensitive to the volume fraction of residual NAPL, once the characterized three-phase displacement regime transitioned to the stable displacement region. This study not only highlighted the influence mechanism of initial water saturation and NAPL injection rate on the three-phase displacement efficiency, but also endowed the classical two-phase Lenormand's diagram with the three-phase displacement regime.
