Properties and Instability Dynamics of Water-in-Crude Oil Emulsions

Due to the influence of active substances such as resin and asphaltene in crude oil and flow shear, a large amount of water-in-crude oil (W/O) emulsions would be formed in the porous media during reservoir development. To achieve highly efficient development, the emulsification and stabilization mechanisms of W/O emulsions are studied from three aspects: the emulsification dynamics of oil and water, the property variation of W/O emulsions, and the factors affecting the stability. First, the adsorption dynamic characteristics of active substances in crude oil at the oil–water interface are described by the emulsion state and droplet size dynamics of W/O emulsions. During the emulsification process, the reduction of the droplet size goes through two stages: rapid and slow. The strength and stability of the W/O emulsion interfacial film should be the highest in the transition stage. When the droplet size is large, there are still a lot of active substances free in crude oil, except at the oil–water interface. When the droplet size is too small, there are less free active substances in the oil phase, and the adsorption density at the oil–water interface decreases. Second, the variation of W/O emulsions was analyzed. With the increase of water content, the number of water droplets in the W/O emulsion increases, and the oil-phase film between water droplets becomes thinner. The viscosity, interfacial tension (IFT), and solid–oil–water three-phase contact angle of the W/O emulsion increase with the increase of water content. As a result, its fluidity in porous media is greatly reduced, which also brings great challenges to the efficient development of the oil reservoir. Finally, the factors affecting the stability of the W/O emulsion are analyzed. The stability of W/O emulsions is mainly determined by the number of water droplets, the viscosity of W/O emulsions, and the interfacial strength. The stability of the W/O emulsion decreased significantly with increasing temperature. The dehydration speed is accelerated in the early stage. With the increase of water content, the dehydration rate of the W/O emulsion first increases and then decreases. With the increase of salinity, the stability of W/O emulsions is enhanced, and the time when W/O emulsions begin to layer is delayed.