A conventional method of treating produced crude oil emulsions is bydemulsifier injection; these demulsifiers are usually selected for specific crude oils using the well-known bottle test. It is the purpose of this paper to discuss a physicochemical aspect of the bottle test procedure highlighting a pitfall that has been accepted, and indeed used, for many years by both oil industries and service companies. The main problem is centred around the mode of demulsifier injection. Many demulsifiers are insoluble in crude oil and consequently have poor efficacies when injected in a neat form. In contrast, the performance of demulsifiers previously diluted in organic solvents can be either inhibited or enhanced. The reasons for this are highlighted. The interfacial pressure adsorption isotherms obtained for a given demulsifier indifferent solvents adsorbing to a crude oil-water interface, coupled to the demulsifier aggregation behaviour in these solvents, show the effect of solvency power on demulsifier interfacial activity. The consequences of this behaviour are demonstrated in the laboratory bottle test and also on the model dehydrating rig for a demulsifier which only poorly resolved an emulsion of water-in-a North Sea crude oil when added neat. We describe how the bottle test can be carried out to effectively and reproducibly screen demulsifiers.
During the production of crude oil, the coproduction of formation water often leads to the formation of relatively stable water-in-crude oil emulsions. The resolution of these emulsions is normally achieved using chemical surfactants (demulsifiers) in combination with heat treatment, electrical treatment, gravity settling or coalescers. The optimum demulsifier for any specific dehydration process is selected by water/salt dropout using the well-known 'bottle test' procedure (1). This has several advantages and disadvantages. The main advantage is that a demulsifier can be selected quickly, during on-site tests using either produced or simulated emulsions.