Changes in residual air saturation after thorough drainage processes in an air-water fine sandy medium

In a previous study we investigated the unstable and stable residual air saturations in an air-water two-phase system in a sand medium during a series of consecutive drainage-imbibition cycles with gradually increasing initial air saturations. In a reciprocal study reported here we extended the previous investigation by determining residual air saturations in consecutive imbibition processes starting from four gradually decreasing levels of initial air saturation (and thus increasing water saturation). Three parallel column tests with 9-12 consecutive drainage-imbibition cycles were performed, in which the first three imbibition processes started from the highest initial air saturation that could be obtained with our experimental system. The results show that all the residual air saturations resulting from the imbibition processes were almost constant after thorough drainage processes (even those following imbibition processes starting from low initial air saturations), and thus independent of the initial air saturation. The results also indicate that once the residual air in interconnected pores at the end of an imbibition process was present in the form of connected, pore network-scale air globules, the residual air remained in this state in subsequent imbibition processes, even if they started from low initial air saturations. It may be deduced that the presence of thin water films on the walls surrounding large pores and large volumes of air in their central parts during an imbibition process resulted in residual air being in the form of connected, pore-network scale air globules in interconnected pores. In contrast, thick water films and small volumes of air in the central parts of the pores resulted in residual air in the form of single pore-scale air globules in interconnected pores. Thus, stronger dynamic flow conditions (e.g., higher capillary numbers) may be required to remobilize connected, pore network-scale air globules than single pore-scale air globules in a forced imbibition process.