Smart water (SW) has been recognized as an effective yet environmentally friendly technique for enhanced oil
recovery in both carbonate and sandstone reservoirs. However, owing to complexities of oil properties, rock compositions, and
ion characteristics, the performance of smart water is not well-understood. This paper attempts to derive insights on how smart
water performs in clay-rich sandstones. A comprehensive mechanistic study is carried out on synthetic sandpacks that contain
different clay types (kaolinite and montmorillonite) and clay concentrations (3 and 8 wt %), under injection of three SWs
(0.3 wt % NaCl, 0.05 wt % NaCl, and 0.3 wt % CaCl2). Extensive experiments and modeling are utilized to investigate
wettability alteration at microscopic and macroscopic scales, including swelling index test, zeta potential measurement, coreflooding
test, contact angle measurement, particle analysis of effluent, differential pressure analysis across the sandpacks, and
disjoining pressure isotherm analysis. The theoretical results of disjoining pressure isotherm analysis show that wettability
alteration is more accurately indicated by the maximum peak of the disjoining pressure curve than by the area below the positive
section of that curve. This is confirmed by contact angle measurements and recovery factors (RFs). In addition, monovalent
cations are found to have stronger impact on changing wettability toward a water-wet state than are divalent cations. We also
find that there might exist a minimum salinity below which the expansion of the double layer reaches its maximum. Decreasing
the salinity below this minimum value is found not to affect the sample’s wettability. Coreflooding tests show that total RF in
the montmorillonite sandpacks is higher than in those made up of kaolinite. In general, a direct relationship is found between
clay concentration and RFs. Furthermore, it is found that fines migration and wettability alteration are the dominant mechanism
in kaol