Three different cases were selected to study the effect of physicochemical formulation on interfacial rheology properties of surfactant-oil-water (SOW) systems by increasing the complexity of the system from a basic case. This was performed by changing the normalized hydrophilic-lipophilic deviation (HLD N ) to attain the optimum formulation at HLD N = 0. Two types of SOW systems were studied: the first one used an ionic surfactant with a salinity scan, and the second one a mixture of two nonionic surfactants in a formulation scan produced by changing their proportion. Both of them contained cyclohexane as a pure oil phase, without alcohol. Sec-butanol was then added as a co-surfactant with hardly any formulation influence on HLD N . The complexity in interfacial rheology was then increased by changing the oil to a light crude with low asphaltene content. The interfacial rheology is also reported for a realistic system with a high asphaltene content comprised of crude oil diluted in cyclohexane with a conventional surfactant and a commercial demulsifier. The findings confirm that at optimum formulation and whatever the scanning variable (salinity, average ethylene oxide number in the nonionic surfactant mixture, or surfactant/demulsifier concentration), the interfacial tension, and interfacial elastic moduli E, E 0 , and E 00 exhibit a deep minimum. These observations are related to the acceleration of the surfactant exchanges between the interface, oil, and water, near the optimum formulation. Several arguments are put forward to explain how these findings could contribute to the decrease in emulsion stability at HLD N = 0.Keywords Crude oil Á Asphaltenes Á Amphiphiles Á Interfacial rheology Á Formulation Á Emulsion stability Á HLD N

Summary Asphaltene-stabilized water-in-oil (W/O) emulsions can cause severe problems during oil production and transportation. These emulsions are broken by adding a demulsifying agent at a suitable concentration (CD*) to obtain the optimal formulation, with minimal emulsion stability (stability*). Herein, we studied, from a phenomenological point of view, the performance of two demulsifiers on W/O emulsion breaking with high asphaltene content. A very simple polyethoxylated nonylphenol demulsifier (6EO) and a complex commercial demulsifier (COD) were studied. The influence of the chemical nature of the oil phase on the performance of the demulsifiers was evaluated. The emulsion stability* and CD* values of W/O systems of heavy crude oil diluted in cyclohexane (Systems A and B) were compared to W/O emulsions composed by a heavy crude oil diluted in heavy naphtha or in an aromatic synthetic crude oil as the oil phase (Systems C and D). The results show that demulsifier performance improves significantly when the crude oil is diluted in heavy naphtha and in aromatic synthetic crude oil, obtaining unstable W/O emulsions (rupture time of 10−2–10−1 minutes). In the latter cases, the CD* value is significantly lower and with a wide area of low emulsion stability compared to systems formulated with crude oil diluted in cyclohexane. The mechanisms that generate this type of behavior are discussed and strategies to increase performance and robustness analyzed.

The field of action of surfactants encompasses the formulation of dispersed systems in the chemical and petroleum industry and, particularly, in applications such as enhanced oil recovery (EOR) and chemical dewatering (dewatering). In these applications it is necessary to use hydrophilic surfactants that are not sensitive to salinity and water hardness, minimizing their precipitation and improving their performance in terms of interfacial tension decrease. Some of the most widely used surfactants in the industry are linear alkylbenzenesulfonates (LAS-C15-18) due to their low production costs. However, the use of these surfactants in the oil industry has been limited due to problems associated with their precipitation. In this work, we evaluated the action of tetramethyl ammonium chloride (TMACl) as an additive to improve the solubility of the commercial surfactant LAS-C15-18. Solubility tests performed at T = 8, 25 and 40 °C show that the addition of TMACl significantly improves the solubility of the surfactant (Csurf > 1wt%) in moderately-hard-water (Hardness 96.2 ppm Ca, Mg). These results suggest that TMACl acts as a hydrotrope, enhancing the interactions of the LAS-C15-18 surfactant with the aqueous phase. Likewise, the interfacial tension (IFT) curves with respect to theNaCl concentration in the surfactant-heptane-brine system show that the addition of TMACl improves the formulation quality due to the decrease of the interfacial tension in the HLD = 0 condition, from 0.027 mN/m without TMACl to 0.008 mN/m with 1.5 wt% TMACl. In these systems, the addition of TMACl did not result in a change in optimum salinity (S*). Additionally, the IFT-temperature curves of the surfactant/heptane/aqueous phase (NaCl = 1wt%) and surfactant/heptane/moderately-hard-water systems show IFT minima as the TMACl concentration increases in the aqueous phase. Surfactant precipitation was not observed in these systems.