Rheological Insights: The Role of Divalent Ions on the Stability and Phase Behavior of Oil–Water Emulsions

Abstract This study explores the rheological insights into how divalent ions, specifically calcium (Ca2+) and magnesium (Mg2+), influence the stability and phase behavior of oil-in-water (O/W) emulsions. O/W emulsions are employed for different purposes in the petroleum industry, including drilling muds, viscosity-reducing agents, enhanced oil recovery (EOR), and transportation of crude oil. We investigated the effects of salinity concentration, time, and surfactant dosage on emulsion properties. Eighteen stable emulsions were prepared using CaCl2 and MgCl2 brines at 0.5% (group A) and 5.0% v/v (group B) concentrations of a commercial anionic surfactant, Hercoten, (from a carboxylate family). Our findings demonstrate that brine composition significantly affects emulsion phase inversion, rheological behavior, and viscosity; CaCl2 emulsions reached viscosities of up to 240 cP at 25,000 ppm (day 7), up to one order of magnitude higher than MgCl2 systems. Furthermore, we identified phase inversion in CaCl2 systems and evidence of multiple emulsions (O/W/O) in MgCl2 systems, both at 8000 ppm and 5.0% v/v surfactant. To complement the experimental analysis, we developed a custom Python-based tool, ROJA, for automated image analysis and statistical characterization of emulsion droplets. We suggest that intermolecular forces and increased friction due to electrostatic interactions between Ca2+ and the anionic surfactant led to higher viscosities and smaller, more concentrated droplets (as low as 54.27 μm at 25,000 ppm) at higher salinities. In contrast, Mg2+ emulsions showed lower viscosities due to larger droplets (up to 127.87 μm at 25,000 ppm), resulting in fewer droplet–droplet interactions. The presence and concentration of divalent ions are therefore critical for the performance of O/W emulsions in petroleum industry applications.