The formation of Stable Dispersions of Crude oil and Produced Water: The Influence of Oil Type, Wax & Asphaltene Content
Authors: Davies G.A. , Nilsen F.P. , Gramme P.E. 
Published: December 2007

Constituents in oil can have a major influence on the separation of mixtures with produced water. In most cases this results in a reduction in the separation rate and the capacity of separation trains. In this paper the effects of wax, asphaltene content and water cut on the separation of different oils are examined.

Introduction

In the production of natural gas and oil, constituents present in the fluid extracted from petroleum reservoirs are first separated in to three principal streams; gas, oil and water. This separation is usually carried out in large horizontal cylindrical vessels, the difference in the density of the phases determining the separation. The relative quantities of gas and oil are determined by the operating pressure and temperature, thermodynamic equilibrium is generally achieved in the vessel. The rate of phase separation, the kinetics of the process is, in a perfect system, determined by the settling velocity of the dispersed phases and by the coalescence of gas bubbles and liquid droplets.

The structure of the three phase mixture can be quite complex, for example the gas / liquid mixture may contain bubbles of gas in liquid and liquid droplets in the gas. The former would be a foam or froth the latter case would be a dense dispersion. The separation rates of these different forms of mixture are quantitatively different. The problem becomes more acute for mixtures of oil and water. In this case two essentially different types of mixtures can be formed. The first is a dispersion of droplets of one liquid in the other. A dispersion is thermodynamically unstable and will separate under gravity as a result of differences of density between oil and water. The rate of separation is found to be extremely sensitive to the size of the droplets.

The mechanism of separation involves settling and coalescence. The settling velocity is a function of the droplet size, the exact relationship between these two depends on the local velocities in the vessel and on the settling regime, for example Stokesian, Newton or turbulent regime. In most equipment the regime is either Stokesian or Newton. The coalescence of droplets within the dispersion and at the dispersed continuous phase boundary is also a complex function of diameter. This is so since the surface and gravitational forces which control coalescence are both related to the drop diameter or more correctly the local curvature of the interface. It is found that as the drop size decreases, the separation rate also decreases. When droplets become very small, of the order of 30-60 m and below, separation is found to be controlled by settling and the settling rate in these cases is usually so small that the settling time is much greater than the residence time of the fluids in the vessel. Systems of this type, although thermodynamically unstable will not separate in conventional equipment. These are normally classified as secondary dispersions. A knowledge of the characteristics of the dispersion (drop size distribution) is clearly an important factor in the design of separation equipment.

In addition to this a further complication arises since the separation rate of oil droplets from water is not the same as for water drops from oil in the same system even if the droplet size distribution in both cases is the same. Therefore the phase continuity of the mixture is also a parameter which must be known.

The problem in connection with oil water separation is not fully described since emulsions can be formed which are thermodynamically quasi-stable. P. 163