Abstract

The effects ofdemulsifier concentration, temperature and shear on the separation ofwaterfrom a water in oil emulsion have been studied experimentally. The oil was a heavy oil from which free water had previously been removed. Shear was applied at controlled rates for various periods of time and the gravitational separation process was accelerated using a heated centrifuge.

The experiments showed that a demulsifier was essential for removal of the emulsified water. The separation rates suggested that droplet sizes in the presence of the chemical were ofthe order ofone micron. Shear wasfound to promote separation by dispersing the demulsifier and by promoting droplet coalescence. The experiments indicated that treating would only be slightly more difficult if a crude oil were transported as an oil in water emulsion containing an emulsifying surfactant.

Introduction

Oilfield treating vessels are designed to provide time and space for phase separation to occur. Although centrifugal forces are sometimes applied in cyclone separators. gravity settling is the most common mechanism. The settling velocity V for a spherical water drop of diameter d, density Pw and viscosity /lw in a highly viscous oil of density p is given by a version of Stokes' law: Equation (1) (Available in full paper)

Thus if separation is to be rapid, high temperatures are desirable to reduce /lw. Any shear processes which occur during flow also play an important role by bringing droplets into contact so that coalescence and diameter increases can occur. For very small droplets, coalescence or aggregation may not take place unless demulsifier surfactants are added.

Because the fluid flow patterns within the treater vessel are complex and unknown, a priori prediction of treater performance from first principles is not possible at present. Nevertheless because the physical processes can be identified, a first step in a rational analysis of treater performance is to identify the roles played by the individual mechanisms. The present investigation was intended to consider the factors involved in selecting an appropriate surfactant for a heavy crude oil.

EXPERIMENTAL PROCEDURE

Fluid characterization: Most of the experiments were, conducted with Pan Canadian ‘Frog Lake’ crude from which free water had been removed by heating the oil at 85 °C for 48 hours. Density measurements were performed for the crude (14.8% water) and the brine which was obtained as free water. The densities were determined by weighing aliquots of hot sample in a 10 mL cylinder whose volume was calibrated in the range 60–90 °C. The measured densities and the computed density of the dry oil are shown as functions of temperature in Figure 1. It is significant that the density difference is insensitive to temperature in this range.

The viscosity of the crude oil was measured with a concentric cylinder viscometer. The oil was Newtonian and in the range of the tests, was correlated with temperature (K) by the empirical equation: Equation (2) (Available in full paper)

Chemical addition and shear: Figure 2 gives a schematic outline of the apparatus which- was used for this step.

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