Emulsion Control Using Electrical Stability Potential
Authors: Messenger J.U. 
Published: December 2007

A technique is described whereby the resistance of an emulsion to breaking can be quantitatively determined. Produced oilfield emulsions are usually the water-in-oil type and, accordingly, do not conduct an electrical current. However, theme is a threshold of A-C voltage pressure above which an emulsion will break and current will flow. The more stable an emulsion, the higher the required voltage. A Fann Emulsion Tester, modified so that low voltages (0 to 10 v) can be accurately measured, is suitable. This technique has application in evaluating the effect of a demulsifier on the stability of an emulsion. Emulsions can, in essence, be titrated with demulsifiers by adding a quantity of demulsifier, stirring, and measuring the voltage required to cause current to flow. Any synergistic effect of two or more materials added simultaneously can be followed accurately. A demulsifier that significantly lowers the threshold voltage (from 100 to 400 v to 0 to 10 v for the emulsions in this study) is effective and can cause the emulsion to break. A demulsifier that will bring about this drop in the threshold voltage at low concentration is very desirable. The technique is also well adapted for rapidly screening demulsifiers.

Introduction

Stable emulsions in produced reservoir fluids resulting from certain well stimulation and completion procedures are common problems. The use of suitable demulsifiers can often mitigate these difficulties. At the present time, a rapid and efficient method for selecting satisfactory demulsifiers is not available. It is badly needed. Reliance is now placed primarily on trial-and-error procedures. A new test method has been developed which permits a more rapid and precise selection of demulsifiers. It involves measuring the electrical stability potential* of an emulsion before and after a demulsifier has been added. This paper describes this method and shows where it should have application in field emulsion problems.

NATURE OF OILFIELD EMULSIONS

Two immiscible components must be present for an emulsion to form; we are concerned here with crude oil and water. An emulsifier must be present for an emulsion to be stable. Emulsifiers can be substances which are soluble in oil and/or water and which lower interfacial tension. They can be colloidal solids such as bentonite, carbon, graphite, or asphalt which collect at the interface and are preferentially wet by one of these phases. Unrefined crude oils can contain both types of emulsifiers. A popular theory is that, of the two phases in an emulsion, the dispersed phase will be the one contributing most to the interfacial tension. Usually this phase contains the least amount of emulsifier. The stability of a water-in-oil emulsion is affected by the following:

viscosity; particle or droplet size; interfacial tension between the phases; phase-volume ratios; and the difference in density between the phases.

A stable emulsion is usually characterized by high-viscosity, small droplets, low interfacial tensions, small differences in density between its phases, and slow separation of the phases. It also has low conductivity (high electrical stability potential).Water-in-oil and oil-in-water emulsions are both common; however, oil field emulsions are predominantly water-in-oil emulsions. The emulsions which commonly occur during completion and stimulation operations contain a combination of several of the following: acids, fracturing fluids (oil, water, acid), and formation water and oil. Produced emulsions usually contain formation water and oil. Emulsions form in oil wells because oil and water are mixed together at a high rate of shear in the presence of a naturally occurring or unavoidably produced emulsifier. During the completion and stimulation of productive zones, and while formation fluids are being produced, oil and water are very often commingled. These mixtures are formed into emulsions by agitation which occurs when the fluids are pumped from the surface into the matrix of the formation or produced through the formation to the surfaced restrictions to flow (such as perforations, pumps, and chokes) increase the level of agitation; tight emulsions are more likely to form under these conditions. Often an emulsified droplet is an emulsion itself is Therefore, emulsion-breaking problems can be quite complex. The complexity can be even greater if a third phase (gas) is included. Demulsifiers operate by tending to reverse the form of the emulsion. During this process, droplets of water become bigger, viscosity is lowered, color becomes darker, separation of the phases faster and electrical stability potential approaches zero. Any of these effects could be followed as a means of determining emulsion stability. However, electrical stability potential is the most reproducible and most easily measured parameter for following the stability of a water-in-oil emulsion.

EXPERIMENTAL

APPARATUS

A water-in-oil emulsion is a poor conductor of electrical current.

JPT

P. 1229ˆ