It has been shown in the literature that a secondary low salinity waterflood can unprove the oil recovery by 5-20%. A possible mechanism is that the low salinity causes desorption of organic material, which may increase water-wetness and lead to more favorable relative permeability behavior. A less well-known mechanism is enhanced solvent (e.g., carbonated water) recovery as low salinity enhances the aqueous solubility of neutral components, which after injection will be transferred from the aqueous phase to oleic phase thus decreasing the oil concentration in the oleic phase and diluting the residual oil. By way of example we consider a low salinity carbonated waterflood into a reservoir containing oil equilibrated with high salinity carbonated water. For a given pH, the CO2 equilibrium concentration in low salinity injection water is higher than in the high salinity initial water. PHREEQC, a geochemical aqueous equilibrium programme, can be extended to obtain the accurate partition coefficient of neutral species that are soluble both in the oleic and the aqueous phase For this we use the Krichevsky-Ilinskaya extension of Henry's law for solubility of gases in liquids. Gibbs phase rule shows that the phase behavior only depends on the pH and the chloride concentration In PHREEQC, we use Pitzer's activity coefficients to extend the validity up to 6M. The output of PHREEQC can only be successfully incorporated in multiphase flow simulation programmes, e.g. COMSOL(TM), after applying a smoothing procedure for which we choose symbolic regression (EUREQA(TM)). An optimal formulation avoids spurious broadening of the concentration profiles in contact "discontinuities''. We obtain the saturation, composition and the total Darcy velocity profiles The significant new insight is that by changing the salinity at constant pH the oil recovery by carbonated water flooding can be enhanced This insight can be applied to optimize enhanced oil recovery with a low salinity waterflood.
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