Interactions Between Chemical Additives and Their Effects on Emulsion Separation
Authors: Yang M. , Stewart A.C. , Davies G.A. 
Published: December 2007

In offshore production, chemical additives are used to combat various operational problems such as corrosion, scaling, foaming and emulsion formation etc. Although individual chemicals may be found to be effective when used alone, there is no guarantee that they will perform as well when present as part of the complex cocktail of additives in the production fluids entering the primary separator. Little is known about interactions between these chemical additives and their effects on gas/oil/water phase separation. This paper will report results from a multi-national sponsored project with the aim of understanding the ways in which chemical additives interact and their effects on emulsion stability with an ultimate objective of developing a strategy for reduction of the chemical inventory offshore. The chemical additives studied include demulsifiers, corrosion inhibitors, scale inhibitors and antifoamants. Both interfacial tension and surface film pressure measurements have been successfully employed to identify interactions between chemical additives and their synergistic effects. Emulsion stability has been measured through conventional bottle tests. It was found that in most cases corrosion inhibitors increased emulsion stability considerably when present both alone and as part of a binary mixture. Interactions between the corrosion inhibitors and some of the demulsifiers were also observed. In contrast the de-foamant and scale inhibitors had little effect on emulsion stability and in most cases did not interact with the other additives.

Introduction

Effective separation of oil and water is vital in ensuring the crude oil quality and low cost of the oil production. This is however severely affected by the presence of quasi-stable mixtures such as water in oil emulsions and foams. To increase the separation efficiency, chemical additives such as demulsifier and anti-foamant are injected into the fluid stream prior its entering into the three phase separators. Demulsifiers are surface-active agents, usually of the non-ionic type, comprising of relatively high molecular weight polymers. They act by weakening the oil-water interface sufficiently to enable droplets of the dispersed phase to coalesce and separate. The chemical composition and dosage rate of the demulsifier is crucial and will vary from field to field. Demulsification of these chemicals can be by one of three mechanisms. They can adsorb at the oil/water interface, displace indigenous surfactant into bulk phases and make interfacial film sufficiently weak and promote coalescence. They can also act as flocculating agents, joining together small droplets, which then coalesce and grow. Alternatively (or simultaneously) demulsifiers can act as wetting agents and wet fine solids present in crude oil and hence reduce their ability to stabilize emulsions.

However, these additives are not the only chemicals injected into the process stream, a number of other processing problems such as corrosion and scale formation are also treated with chemical additives. As a result, by the time the process fluids reach the entrance of the primary separator they will contain a complex cocktail of such additives.

Many of the other chemicals used are also surface active in nature, eg corrosion inhibitors, which utilise saturated long chain amines to protect metal surfaces of pipe and vessels from corrosion. No one is really sure to what extent these chemicals could contribute to the formation of stable emulsions. More importantly, it is not known whether or not the performance of the demulsifiers and defoamants would be affected by their presence. Clearly, if there are any interactions and/or synergistic effects between the demulsifier and any of the other chemicals, a reduction in the efficiency of the demulsifier may result.

Synergistic effects have been found between naturally occurring chemicals in crude oils. For example, purely phenolic fractions which occur abundantly in alkaline extracts were found to be interfacially inactive, however, mixtures of phenols and carboxylic acids exhibiting very high interfacial activity and in some instances the active species appeared to be carboxyphenols. The synergistic effect of phenols and other weekly acidic anionic functional materials on the apparent interfacial activity of carboxylic acid in a California crude was studied by Seifert. Qualitative indications of interactions and synergistic effects between asphaltene fractions and resins were also reported recently by Sjoblom et al.

Little has been found in the literature about synergistic effects and chemical interactions between additives used offshore, though the importance of synergistic effects on destabilising crude oil emulsion was recently mentioned by Urdahl et al. Interactions between demulsifiers (ethoxylated phenol-formaldehyde resins) and asphaltenes was reported to be evident by Taylor, however, there was no quantitative data shown. P. 453