Application of Emulsion Viscosity Reducers to Lower Produced Fluid Viscosity

Abstract

Abstract One of the major costs in crude oil production is lifting fluids to the surface for processing. During the lifting process the produced fluid composed of brine and crude oil encounters high shears and pressure drops caused by flowing through electrical submersible pumps or other forms of impingement and restrictions. In addition, crude oil generally contains naturally occurring interfacial active materials which act as emulsifiers. When the produced fluids are subjected to such shear they mix and form a water-in-oil emulsion. This emulsion carries a viscosity that can be many times higher than the crude itself. This increase in viscosity is the main reason for lifting difficulties and the associated higher cost of production. Emulsion Viscosity Reducers (EVR) are chemicals designed to interact with the natural emulsifiers at the water-oil interface and facilitate the water-oil separation. The resolution of the emulsion results in lowering the overall viscosity of the fluid which leads to increased flow and production. In this paper we examined the effectiveness of EVR on some typical Brazilian crude emulsions in the laboratory as well as field application data. Introduction In pure form hydrocarbons do not mix with water or brine and the propensity to form an emulsion does not exist. However, due to the presence of naturally occurring interfacial active substances in crude oil and the mixing that occurs in oil production process there is a tendency for crude oil to form an emulsion with water or brine. It is widely known that emulsion formation requires the existence of surfactants and interfacial active components as well as energy in a mechanical form to shear and mix the two separate and immiscible phases of fluid. Crude oil has a complex composition that usually contains interfacial active polar compounds such as asphaltenes, resins, and naphthenic acids.1 These groups are comprised of molecules incorporating oxygen, nitrogen, and sulfur in forms such as carboxylic acids, amides, alcohols, phenols, mercaptans and amines to name a few. During the production of crude oil there exist several points where mechanical energy is imparted to the production fluids and emulsion formation can occur. One such point is the lifting process where the flow of the fluid encounters shear forces and pressure drops generated by the mechanical forces present in an ESP (electric submersible pump) or other lifting method such as gas lift. High shear forces provide sufficient energy to cause intimate mixing of the crude oil and associated brine. As a result, there is a high probability of emulsion formation. Under most oil production conditions it is common to observe an emulsion formed with a water discontinuous (or internal) phase suspended in an oil continuous (or external) phase. These emulsions are formed based on both the interfacial effects of the natural surfactants and on the amount of mechanical energy applied. Resulting emulsions typically have very fine water droplets ranging in size from 100 µm to less than 10 µm as demonstrated in the literature over the years. Emulsification of brine into the crude oil can result in a fluid that has a higher apparent viscosity than the crude itself as a result of the large number of small droplets and the resulting high oil/water interfacial surface area created. The high surface area results in a large number of surface/surface interactions between droplets that begin to effectively transfer shear through the bulk phase of the emulsified fluids, which is recorded as a high viscosity. The viscosity is described as " apparent viscosity?? as the true viscosity of either phase independently is not directly related to the measured viscosity of the fluid. This higher viscosity emulsion can potentially impede the flow of the fluid, wearing down ESPs and result in lower oil production. To overcome this impediment the down-hole addition of EVR (emulsion viscosity reducer) can effectively lower the apparent viscosity of the fluid, thus enable the lifting process to become more efficient.