Impact of temperature and SO42- on electrostatic controls over carbonate wettability

Abstract

The use of modified salinity and modified composition brines (MSMC), referred to as smart water, to alter carbonate surface wettability at different temperatures has gained enormous popularity. However, an effective way to quantify the geochemical, thermodynamic and electrostatic factors responsible for wettability alteration have still not been fully developed. In this work, a surface complexation model (SCM) based on geochemical and thermodynamic interactions at calcite and oil surfaces was used to investigate the effect of temperature, pH, and sulfate on electrostatic interactions. SCM’s for calcite and oil surfaces were validated against zeta potential data at high temperatures and used to interpret improved oil recovery data from spontaneous imbibition experiments. The SCM was correlated to calcite wettability through the calculation of the bond product sum (BPS); a measure of the strength of the electrostatic bond linkages between calcite and oil surfaces. Increasing temperature increased the zeta potential towards more positive values for positive polarities and towards more negative values for negative polarity at both the calcite and oil surfaces. BPS decreased with increasing temperature due to the weakening of [>CO 3 – ][‒NH + ] electrostatic bonds, indicating an increase in water-wetness, consistent with the existing literature. Increasing the sulfate ion concentration in seawater brine reduced the BPS, indicating an increase in water wetness. This was attributed to the reduction in [>CaOH 2 + ][‒COO – ] associated bonds as sulfate concentration increased. Spontaneous imbibition experiments confirmed the BPS analysis, whereby oil recovery increased as sulfate concentration and temperature increased. • A surface complexation model was developed which describes high temperature calcite-brine-oil interfaces. • Increasing temperature decreased oil-calcite electrostatic bonding and increased water wetness in calcite rocks. • Increasing temperature enhanced brine imbibition into limestone, as predicted by the surface complexation model. • Sulfate concentration and brine pH also influenced calcite wettability.