Zeta-Potential Guided Equivalent Salinity for Screening Microemulsions Under Multi-Ion Brines

Summary The salt concentration parameter within the traditional hydrophilic-lipophilic deviation (HLD) equation applies exclusively to the single inorganic salt sodium chloride (NaCl), rendering it inadequate for screening microemulsion systems under multi-ion brines encountered in practical applications. To address this limitation, we propose a screening methodology for composite-ion microemulsion systems founded on specific ion effects. This approach achieves effective screening of middle-phase microemulsions under composite ion conditions by analyzing ion-induced alterations in microemulsion micellar behavior. Four inorganic salts prevalent in formation water [NaCl, potassium chloride (KCl), calcium chloride (CaCl2), and magnesium chloride (MgCl2)] were selected alongside various anionic surfactants [e.g., ALFOTERRA 123-4S 90, anionic fatty alcohol polyoxyethylene ether sodium sulfate (AES)]. Experimental procedures encompassed observing microemulsion phase behavior transitions via single-ion salinity scanning; measuring micellar zeta potentials at varying ion concentrations through zeta potential experiments; and fitting a linear function model correlating zeta potential with the ionic strength parameter, where the slope relates to surfactant type, and the intercept relates to ion type. Using NaCl as the benchmark, the equivalent ionic strength parameter for composite ions was calculated, enabling determination of the optimal composite-ion salinity, subsequently validated through composite-ion phase behavior experiments. The results demonstrate that the brine solution dielectric constant responds to cations in the order Mg2+ > Ca2+ > Na+ > K+, whereas sulfonate surfactants respond in the order Mg2+ > Ca2+ > K+ > Na+, reflecting the specific ion effect. Zeta potential exhibits a robust linear relationship with the ionic strength parameter at low concentrations. In composite-brine experiments using tetradecane and equivalent alkane carbon number (EACN)-matched crude-containing blended oil as the oil phase, the calculated equivalent optimal salinity consistently produced well-defined Winsor III microemulsions. This confirms the method’s efficacy for screening composite-ion microemulsion systems and suggests its theoretical applicability for analyzing the stability of other charged structures, such as charged micellar or bubble systems.