Effect of electrical double layer and ion exchange on low salinity EOR in a pH controlled system

Abstract

Wettability of an oil/brine/sandstone system is an important petro-physical parameter, governing subsurface multi-phase flow and residual oil saturations. While how edge-charged clays (e.g., kaolinite) contribute to low salinity effect has been extensively investigated, few work have been done on basal-charged clays (e.g., illite, smectite, and chlorite), and few have look beyond the application of the low salinity effect in reservoirs bearing basal-charged clays. We thus integrated atomic force microscopy (AFM) (at a given pH = 7), disjoining pressure calculations, together with surface complexation modelling to understand the importance of basal-charged clays during low salinity water flooding in sandstone reservoirs. We found that low salinity effect can take place without pH increase, confirming that electrical double layer is at least one of the mechanisms to yield low salinity effect, which can be interpreted using disjoining pressure isotherm, whereas ion exchange between oil and basal charged clays leads to an opposite effect at a given pH. However, ion exchange likely favors low salinity effect without controlling pH due to the chemical reaction: >Na + H+ = >H + Na+, which in return decreasing the electrostatic bridges between NH+/clays, COOCa+/clays. Knowing the contribution of EDL and ion exchange on oil/brine/basal-charged clays interaction, the potential of low salinity effect in sandstone reservoirs can be quantified.