Performance of smart water in clay-rich sandstones: experimental and theoretical analysis

Abstract

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.