Low salinity water flooding in high acidic oil reservoirs: Impact of pH on wettability of carbonate reservoirs

Wettability alteration has been identified as an important mechanism during low salinity water flooding in carbonate reservoirs. Oil composition, in particular, acidic and basic functional groups, plays an important role in regulating wettability. In this paper, we explored the potential of low salinity effect in reservoirs with high acidic components (acid number = 4.0 mg KOH/g and base number = 1.3 mg KOH/g) with a combination of approaches (e.g., contact angle and zeta potential measurements, and surface complexation modelling). We measured the contact angles of oil on calcite surfaces in presence of aqueous ionic solutions at different pH (3 and 8), salinity (0.01 and 1 mol/L), ion type (CaCl2 and Na2SO4) and temperatures (25-100 degrees C). Our results show that both salinity and ion type significantly affect contact angle at pH = 8. However, at low pH (pH 3), the oil-brine-calcite system becomes strongly water-wet with minor effect from salinity, ion type, and temperature. Lowing salinity drives the zeta potential of both oil-brine and brine-calcite to be strongly negative, particularly in the presence of Na2SO4 at pH 8, confirming the contribution of the electrical double layer to low salinity effect. Geochemical modelling reveals that breakage of electrostatic bridges between oil and calcite increases linearly with acid number, implying a higher potential of low salinity effect in high acidic oil-beating carbonate reservoirs. This study provides an insight into how acid number parameter influences the wettability of oil-brine-carbonate system, and underscores the significance of acid components in wettability alteration of carbonate reservoirs, assisting in a better prediction and screening of low salinity FOR in carbonate reservoirs. (C) 2019 Elsevier B.V. All rights reserved.