(28c) Low-Temperature Demulsification of High Waxy and High Water Cut Pickering Crude Oil Emulsion

Low-temperature demulsification of crude oil emulsion containing high concentrations of wax and water is rendered difficult due to Pickering stabilization by wax particles.

Secondary oil recovery methods are employed to extract oil as an oil well ages. This leads to an increase in the water content of the crude oil recovered from the well. Pickering emulsions are formed when the immiscible mixture of crude oil-water is transported from the well to the platform through the low-temperature sea bed, where the wax gets cooled well below the Wax Appearance Temperature. While precipitating out of the bulk crude, the wax associates with resins and asphaltenes, which are the polar fractions of the crude oil. The wax-resin-asphaltene composite exhibits amphiphilic behavior, enabling the wax composite particles to grow at the interface around the water droplets. This leads to the formation of wax-stabilized Pickering water-in-crude oil emulsions, which are difficult to destabilize. The extraction of crude oil from this stable emulsion is achievable only at high temperatures (greater than 353K). As a consequence of the increase in the water content of the emulsion, the heat capacity of the emulsion also increases. The limitation of available heating capacity in the heater-treater system, along with space constraints at the offshore platforms, necessitates that demulsification be carried out at lower temperatures, with high efficiency, in order to handle high water-cut crudes and realize high throughputs of de-emulsified crude oil. This demands chemical demulsifiers that can assist with the swift demulsification of these wax-stabilized Pickering emulsions, even at low temperatures. Low-temperature demulsification implies demulsification at temperatures close to the pour point of the emulsion and up to 10℃ below it. High shear mixing may be required before settling, for temperatures much below the pour point.

From preliminary bottle test experiments, a specific combination of surfactants was found that could give 100% water separation within 5 minutes, even at low temperatures. The mechanism of destabilization, in the presence of these surfactants, is understood by studying the three-phase contact angle and interfacial rheology. The influence of surfactants on the three-phase contact angle between wax particles and the oil-water interface is understood using a customized setup comprising a camera and an infrared light source. Infrared light enables visibility through crude oil. The three-phase contact angle is determined by post-processing of the images using ImageJ software. Interfacial rheological analysis of the crude oil - brine interface is conducted using Du- Noüy Ring Apparatus (ARES-G2, TA Instruments).

The surfactants that act as effective demulsifiers wet the wax composite particles at the interface and displace them from the oil-water interface, thereby decreasing the three-phase contact angle to zero. Due to the displacement of the wax composite particles from the oil-water interface, the interfacial elasticity and yield strain reduces significantly. From the above understanding, we propose a guideline for the selection of surfactants for low-temperature demulsification of wax-stabilized Pickering crude oil emulsions, wherein the surfactant mixture must include a medium-HLB wetting agent in combination with a high-HLB phase inverting surfactant. The medium-HLB surfactants that act as good wetting agents are two-tailed, irrespective of whether ionic or non-ionic in nature. Due to their medium-HLB, they neither favor the oil nor the water phases and preferentially adhere to the interface. After the interface is devoid of the wax composite, a second high HLB surfactant assists in accelerated phase inversion of the water-in-oil emulsion to bi-continuous phase, which favors gravity settling of water. Multiple surfactant combinations, which were filtered based on the above - mentioned criteria, have been tested in the oil field and have exhibited 100% separation efficiency within 5 minutes.