1.0 Introduction
1.1 Overview
Crude oil is seldom produced alone from reservoirs. It is always produced as a complex mixture of hydrocarbons and formation water. These mixtures undergo extreme agitation under high shear rate and turbulence as they flow from the reservoir pores through perforated casing into the wellbore, to the tubing and finally through the surface production facilities. This occurrence causes the water phase to be dispersed and stabilized as fine droplets in the bulk oil phase and hence forms emulsion. Emulsion is among the many problems encountered in the production, transport, and refining of crude oil and dealing with this complex structural arrangements account for much of the expenses incurred by oil companies in their daily operations. Their control and resolution is among the major challenges encountered both onshore and offshore by production engineers, production chemists and facilities engineers during production, especially in a complex multi-wells system. Emulsion problems are usually more problematic in fields where heavy crude oils are produced.
Emulsions are stabilised by rigid interfacial films that forms a ‘'skin'' on the water droplets and prevents them from coalescing. The stability of these interfacial films, and hence the tightness of the emulsions, depend on a number factors, including the heavy materials present in the crude oil (e.g. asphaltenes, resins, waxes), inorganic solids (e.g. clays, scales and corrosion products), temperature, droplet size and droplet size distribution, pH and brine composition. As the producing field depletes, the nature of petroleum emulsion changes continuously due to changes in some of these factors and production methods. Produced oilfield emulsions can be water-in-oil (W/O), oil-in-water (O/W) or multiple water and oil in water (W/O/W), but most produced oilfield emulsions are of the W/O type. This depends on several conditions, which include but not limited to: fraction of each liquid phase, hydrophilic-Lypophylic balance (HLB) etc. From a purely thermodynamic point of view, a W/O emulsion is an unstable system. This is because there is a natural tendency for a liquid/liquid system to separate and reduce its interfacial area and hence, its interfacial energy. However, most oilfield emulsions are stable over a period of time (i.e. they possess kinetic stability).1,4 Produced oilfield emulsions have been classified on the basis of their degree of kinetic stability. According to this, oilfield emulsions have been classified as loose, medium and tight emulsions. Loose emulsions separate in a few minutes, medium emulsions separate in ten minutes or more, while tight emulsions will separate in a matter of hours or even days.
Emulsions from several production headers in the oilfield are usually commingled at the manifolds and then transported to the central processing facilities for treatment. They are usually very difficult to treat and cause a number of operational problems, such as overloading of surface separation equipments with water, increased cost of pumping wet crude, increased heating cost, tripping of separation equipments, high pressure drop in lowliness increase in cost of demulsifiers, production of off-specification crude oil, thick sludge in stock tank bottom, corrosion in export and subsea pipelines, catalyst poisoning at refineries and sometimes force the shutdown of processing equipments in the Wet Crude Handling Facility (WCHF). The overall effect of this is a significant loss in production and loss of revenue to the operators.
Different treating methods thus exist in the petroleum industry for demulsification of crude oil. They include thermal methods, mechanical methods, electrical methods and chemical treatment. In general, these methods are interrelated. Applying heat to the emulsion reduces the viscosity of the oil and increases the water settling rates. It also results in the destabilisation of the rigid films caused by interfacial viscosity. Application of heat for emulsion breaking should be based on an overall economic analysis of the treatment facility. Furthermore, some of the mechanical equipment available in the breaking of oilfield emulsions include free-water knockout drums, phase separators etc. High voltage electricity is also often used for breaking emulsion. It is generally theorized that water droplets move more rapidly when induced with an electric field, and hence collide with each other, and coalesce.
The distance between the electrodes in some designs is adjustable so that the voltage can be varied to meet the requirement of the emulsion being treated. By far; the most common method of emulsion treatment is adding chemicals. Demulsifier chemicals account for approximately 40% (in value) of the world oilfield production chemical markets. They are deployed at virtually every crude oil processing station worldwide. Chemical additives, recognised as the second ‘aid', are special surface active agents that migrates to the water – oil interface once added to the emulsion.
