Surface Complexation and Equilibrium Modelling for Low Salinity Waterflooding in Sandstone Reservoirs

Abstract

Abstract Changes at oil/brine and sandstone/brine interfaces can improve oil recovery in sandstone reservoirs by low salinity waterflooding (LSWF). There have been several attempts to understand the interfaces through both experiments and modelling, but the results are inconsistent and the existing models did not consider the important mechanisms on prediction. In this study, an integrated model incorporating surface complexation reaction, dissolution/precipitation of minerals, and speciation was developed to simulate LSWF in sandstone reservoirs. A separate surface complexation model was developed for oil-brine and kaolinite-brine interfaces. The effect of pH and divalent ions of calcium and magnesium concentration on surface species of oil and kaolinite is analyzed. These surface complexation models were combined and coupled with phase-equilibrium model to predict LSWF in sandstone reservoir. Deprotonation of Al and Si sites (>Al: SiO−) of kaolinite and both deprotonated carboxylic groups (−COO−) and calcium-adsorbed (−COOCa+) dominates the surfaces at various brines and its diluted composition. The pH rise in the formation water increases the dissociation of both oil and kaolinite and hence high repulsion between the surface. The desorption of oil was calculated from the surface species concentration of both oil and kaolinite. The simulation results on desorption of oil increases with dilution times (up to 20 times) and amount of equilibrated brine of seawater, formation water, 20 mmol/l CaCl2, and 20 mmol/l MgCl2. Consideration of phase-equilibrium significantly impact on the simulation results; the dissolution and precipitation of sandstone minerals increase the desorption of oil by few percentages, 6-10% in seawater flooding. Increased oil recovery in LSWF is strongly depend on the aqueous chemistry of injected solution and surface electrical properties, such as surface site density and equilibrium constants for ionization and ionic adsorption of oil and kaolinite. The simulation results of this study emphasize the importance of surface chemistry of oil and kaolinite as well as phase equilibrium reactions on improved oil recovery in LSWF. The key factors affecting electrostatics of oil and kaolinite and then oil recovery was identified and analyzed. The potential improvement of the model for better prediction is discussed. The proposed model could serve as a useful tool to predict oil recovery during LSWF in sandstone reservoirs.