Role of pH and cations on emulsion formation and stability of crude oils

Abstract

Emulsification plays a crucial role in various areas. However, undesirable emulsions, especially water-in-oil (W/O) emulsions, formed during crude oil production, transportation, processing, and stockpiling, become significant issues in the petroleum industry. Various studies have focused on evaluating the factors affecting emulsion stability and their mechanisms, which hardly explain the relationship with emulsion formation. This study assessed the effects of pH and cations on the transformation of emulsion types and their stability based on the observed emulsion formation process and emulsion particle sizes. It was found that the final W/O emulsions were transformed from water-in-oil-in-water (W/O/W) emulsions for all brine conditions. At neutral pH, more W/O emulsions were formed through the transformation of W/O/W emulsion, but the high pH broke down most of the W/O/W emulsion into free water and oil. Emulsion particles’ zeta potential or electrokinetics plays a significant role in W/O/W emulsion stability at high pH. The transformation of emulsion types in cation solutions was similar to neutral pH; the amount of W/O emulsion depends on cation species. The W/O/W emulsion stability under different pH in the following order: neutral pH < low pH < high pH, whereas final W/O emulsion stability is as follows: high pH < low pH < neutral pH; MgCl2 < CaCl2 < NaCl. The stability of the emulsion is influenced by the properties of the crude oil, such as viscosity and TAN/TBN ratio, which affect the oil film thickness of the W/O/W. Based on these findings, a stability mechanism is proposed for the emulsion under different pH and cations.