PH effect on wettability of –NH+-brine-muscovite system: Implications for low salinity effect in sandstone reservoirs

Abstract

Wettability alteration remains the main mechanism of low salinity water flooding (LSWF) which is controlled by several factors including crude oil composition. Although presence of acidic components in crude oil confirmed to be necessary to enhance wettability during LSWF, yet the effect of basic components remain unclear. In this work we used the concept of surface energy to explicate the dependency of wettability on a high base number (BN) oil as a function of water pH, ion type and salinity. We measured Oil/Brine contact angle and interfacial tension, which then were used to compute Oil/Solid (mica) interfacial tension using Neumann's equation of state. In addition, we measured the zeta potential and simulated the concentration of base components (–NH+ groups) in the oil using a surface complexation model to understand the underlying mechanisms of wettability and interfacial tension alterasion. We found that wettebility is also dependant on the basic components of crude oil where pH plays a major role and salinity and ion type play a secondary role. Increasing pH (from pH=3 to pH=11) decreased the contact angle moving the system to a more hydrophilic or water-wet state (contact angle decreased from 162° to 68° for CaCl2 solution and from 163° to 66° for NaCl solution). We also found that Oil/Solid interfacial tension increased with pH, where changes were more pronounced in the presence of divalent cations compared to monovalent. We hypothesise that changes in the contact angle and Oil/Solid interfacial tension are due to the chemo-physical adsorption of –NH+ groups at the oil surface to the muscovite negative sites. The hypothesis is consistent with the changes in the zeta potential and –NH+ surface concentration computed from the surface complexation model. The results of this study underscore the importance of the basic component on system wettability as a function of pH, providing insights into the low salinity EOR in sandstone reservoirs.