Correlations for predicting Viscosity of W/O-Emulsions based on North Sea Crude Oils

Abstract

SPE member Abstract This paper describes some simple correlations which can be used to obtain reasonable estimates of relative viscosities and viscosities of water-in-crude oil emulsions. The apparent viscosities of water-in-crude oil emulsions based on eight different North Sea crude oils of different types have been fined to a general empirical equation which provides estimates of relative viscosities at temperatures between 5 and 40 C, water cuts between 10 and 60% and shear rates between 30 and 500 s-1. The estimates are in general within 15-30% of experimental values. A correlation between crude oil viscosity and average molecular weight, can he utilized to obtain estimates of emulsion viscosities above about 20 C, when only the molecular weight of the crude is known. The yielding (or restart) behaviour of some of the emulsions at low temperature (5 C) is described in terms of static yield stress as a function of water volume fraction. Introduction Stable crude oil emulsions may form in systems containing mixtures of crude oil and formation water, either as a result of sudden pressure drop across a choke valve on the wellhead or as a result of turbulent mixing in the wellbore or in a transport pipeline. Such emulsions are mainly of the water-in-oil type as long as the water volume fraction is below the inversion point, i.e, in general less than 60-80%. Good estimates of emulsion viscosity are important for accurate pressure drop calculations and thus estimates of energy requirements for pumping. This paper considers stable emulsions based on stabilized (gas-free) crude oil at atmospheric pressure. This represents a kind of "base case" or starting point for estimation of viscosities in more complicated systems. In a multi-phase pipeline, the presence of one or several free phases (water, oil, gas) and dissolved gas as well as elevated pressure, complicates the estimation of real, effective viscosities. Normally, a general (temperature- and shear rate-independent) exponential function is used to calculate relative emulsion viscosity (i.e. emulsion viscosity divided by the oil viscosity) for given water volume fractions. An estimate of relative viscosity for one volume fraction is required in order to calculate the constants of the exponential equation. This equation is then used in combination with the oil viscosities (calculated or experimental) at various temperatures, to estimate corresponding emulsion viscosities. Such routines for calculation of emulsion viscosity are based on the following assumptions:The calculated oil viscosity is correct (if calculated values are used).Relative viscosity for a given water cut is independent of temperature.Relative viscosity for a given water cut and temperature is independent of shear rate, i.e. Newtonian behaviour.