Pore-scale evaluation on the mechanism of water sensitivity for sandstones containing non-swelling clays

The study presented here aims to gain an insight into the mechanism of water sensitivity for sandstones which contained non-swelling clays and were essentially free of swelling clays. The static immersion experiment combining with the NMR T-2 scanning was performed in advance to examine the effect of clay swelling. Results suggested that the clay swelling has not caused effects on the pores. Afterwards, the QEMSCAN was utilized in conjunction with the water shock experiment. The use of QEMSCAN technique enabled the quantitative characterization and visualization on the morphologies of minerals. The image processing method was employed to extract the clays and pores from the mineral phase images (MPIs). Results showed that the water sensitivity in this study was induced by illites. Based on the contact relationships with pores, the illites were classified into three patterns, among which the Pattern II illites that partially exposed to pores could release from the pore walls. The released illites migrated downstream and some of them were trapped by pores and then accumulated. The accumulation led to the formation of Pattern III illite which was spatial combination of multiple Pattern II illites. The new illites blocked the medium pores, especially the mainstream pores, and further caused their decrease. Additionally, the secondary pores formed among the new combined illites, resulting in the increase of small pores. The large pores were not affected by the accumulation but enlarged instead due to the efflux of illites. The blockages of medium pores caused significant decrease of effective flowing space and further the permeability loss.