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, core-flooding 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 kaolinite sandpacks, while clay swelling,
wettability alteration, and a salt-in effect have been reported to
be more significant in montmorillonite sandpacks.
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