The effect of demulsifiers on the interfacial rheology and emulsion stability of water-in-crude oil emulsions

Abstract

The interfacial rheological properties of the crude oil-water interface have been measured. It is found that the rheological properties are time dependent and the film takes at least 8 h to attain equilibrium. During this period the surface elasticity and viscosity increase markedly with time, with the surface viscosity increasing from 3 × 10 −2 mN s m −1 after 2 h of ageing to 3 × 10 3 mN s m −1 after 8 h of ageing, and the surface elasticity increasing from zero (i.e. the film was simply viscous) to 2 mN m −1. The effect of the addition of the demulsifier Unidem 120 to an 8 h aged film was to decrease the surface viscosity by an order of magnitude. In the demulsification experiments, this corresponded to a high resolution of the emulsion. The addition of the demulsifier BJ18, however, produced markedly different results. If the demulsifier was added to a pre-aged film, there was little change in the interfacial rheology of the film; indeed the interfacial rheological properties seemed to increase. However, if the demulsifier was added to the crude oil before the interface between the crude oil and water had formed, the interfacial properties of that film remained constant with time, i.e. no thick viscoelastic film typical of the crude oil-water interface was formed. This result indicates that this demulsifier is poor at displacing the naturally occurring asphaltene surfactants from the crude oil-water interface, but if they adsorb at that interface first they prevent the formation of the stable, rigid asphaltene films. Despite this, however, BJ18 was effective in demulsifying freshly formed water in crude oil emulsions.