Studies on the Improvement of Coalescence Conditions of Oilfield Emulsions
Authors: Meijs F.H. , Mitchell R.W. 
Published: December 2007

The study of the coalescence of oilfield emulsions can be improved by using a "mixing-intensity parameter," defined as the work done on the fluid per unit mass and unit time. This parameter is used in experiments on both oil-in-water emulsions and water-in-oil emulsions, the former in connection with crude-oil dehydration, the latter in connection with the removal of oil from aqueous effluents.

Introduction

Almost every oilfield chemist will have been involved at some stage with the emulsion problems that often result from the simultaneous production of crude oil and water. He may become involved because the crude oil produced does not meet the water or salt-content specifications imposed by pipeline companies or refineries. Another reason for the chemist's involvement may be the need to dispose of dehydration effluents into public waters, which will be permissible only if the effluent's oil content is first reduced below a certain limit imposed by the authorities. It is evident that there is a need for effective methods of separating water from water-in-oil emulsions and oil from oil-in-water emulsions. One means of achieving such separation is to apply a so-called chemical treatment, which involves the following:Destabilization of the emulsion by addition and dispersion of a suitable surfactant;Coalescence of the destabilized droplets by applying suitable mixing conditions; andSeparation of the coalesced droplets from the continuous phase by gravity.

Clearly, the speed at which the dispersed phase can be separated in Step 3 and its efficiency depend on the degree of coalescence obtained in Step 2 and on the effectiveness of the surfactant used in Step 1. Provided that a suitable surfactant is available, the Provided that a suitable surfactant is available, the overall performance of the process can thus be improved by ensuring optimum mixing conditions - i.e., optimum mixing intensity and mixing time. The need for suitable mixing conditions was already realized in 1937 by Roberts, who stated: "The agitation must be of sufficient intensity to cause impacts that lead to coalescence of the very small droplets, but must not be excessively vigorous, since this would cause a shattering of the drops into still smaller droplets. The agitation must be prolonged for a sufficient period of time to permit the chemical to reach the vicinity of the interfaces of all, or at least a large proportion, of the emulsion particles and prepare them for coalescence with other drops. It must also be prolonged until all, or at least a large proportion, of the small droplets have been coalesced to a sufficiently larger size so that they are ready to enter the last stage of the process." That quite realistic statement of the importance of agitation in emulsion treatment, however, had no follow-up in that no adequate means of formalizing mixing intensity were indicated. One attempt in this respect, introduced by Blair, uses Reynolds number as a mixing parameter, but this approach has been applied only in the design of desalting equipment. In chemical-engineering studies involving the breakup of liquid drops under dynamic conditions, the energy dissipated per unit mass of fluid per unit time is used as a mixing parameter.

JPT

P. 563