Residual air saturation changes during consecutive drainage–imbibition cycles in an air–water fine sandy medium

To clarify changes of residual air saturation under special (dynamic, nonequilibrium) conditions, the saturation–capillary pressure (S–p) relationship including the residual air saturation under consecutive drainage–imbibition cycles, generated by scheduled water level fluctuations, in an air–water two-phase sandy column was investigated. Water saturation and capillary pressure were measured online and recorded using TDR (Time Domain Reflectometry) probes, T5 tensiometers and a datalogger. The results show that under dynamic flow conditions the residual air saturation changed suddenly from an unstable to a stable state with changes in initial air saturation during the series of imbibition cycles, and stable nonzero residual air saturation only occurred when the initial air saturation of the imbibition process exceeded 0.49, ca. double the value of the main imbibition process. Once stable residual air saturation was obtained, all the subsequent residual air saturations remained stable and constant in the following imbibition cycles. Before this threshold point, the residual air saturation gradually fell given sufficient imbibition time. The results also indicate that the unstable and stable residual air saturations were mainly due to discrete pore-scale air globules trapped in the interiors of pores and pore-network scale globules located in interconnected pores, respectively.