Role of Basal-Charged Clays in Low Salinity Effect in Sandstone Reservoirs: Adhesion Force on Muscovite using Atomic Force Microscope

Abstract

Low salinity water flooding appears to be an important means to improved oil recovery in sandstone reservoirs. Wettability alteration has been identified as the main effect behind low salinity water flooding due to the interaction of oil-brine-rock interfaces, where clay minerals play a significant role. While how edge-charged clays (kaolinite-bearing sandstone) contribute to wettability alteration during low salinity water flooding has been well-studied, the role of basal charged clays (smectite, illite, and chlorite) in wettability alteration remains unclear. We previously confirmed that basal charged clays trigger pH increases (2 to 3) due to ion exchange with added impetus of mineral dissolution by means of geochemical modeling (Chen et al. Fuel 2018, 112a117). In this work, we hypothesized that the pH increase triggers negative zeta potential of both oil-brine and brine-clays, thus increasing double layer expansion, as well as hydrophilicity. To test our hypothesis, we measured adhesion force between functional groups (aCH 3 and aCOOH) and muscovite using atomic force microscope (AFM) at pH of 7 and 11 with NaCl at a concentration of 10※000 mg/L NaCl. To gain a better isotherm thermodynamic understanding, we measured zeta potential of brine-muscovite and oil-brine and computed total disjoining pressure under constant potential condition using flat to flat and sphere to flat thermodynamic models. Zeta potential measurements show that increasing pH shifted the zeta potential of oil-brine and brine-muscovite to more negative values thus increasing the electrical double layer force. Our AFM measurements show that increasing pH from 7 to 11 indeed decreased 80% of the adhesion force for both functional groups. Total disjoining pressure calculation predicts the same trend as the AFM adhesion measurements. However, the sphere to flat thermodynamic model predicts a correct degree of decrease in adhesion force compared to AFM, implying that the sphere to flat model should be applied to interpret AFM results. Together, our results confirm that basal-charged clays can significantly contribute to low salinity effect due to electrical double layer expansion, thus expanding the application envelope of low salinity flooding in sandstone reservoirs bearing basal-charged clays.