Polymer emulsion hydraulic fracturing fluids were widely employed some 10 to 15 years ago, but became less popular after the introduction of cross-linked, water-based gel systems. The latter were thought to have a higher viscosity and be easier to handle, as well as being cheaper. However, during the last few years, polymer emulsions have enjoyed an increasing popularity in Europe because of their flexible viscosity response and their non-damaging properties1 . In addition, their cost per unit volume of proppant placed is now comparable to that of water-based fluids.

Addition of a well-dispersed internal phase will increase the viscosity of any fluid over that of the external phase alone. For polymer emulsion fracturing fluids the continuous phase is gelled water, the viscosity being enhanced by the addition of diesel, kerosene or crude oil at typical volume fractions between 0.5 and 0.7. During the proppant stage, the fluid rheology is further complicated by the addition of a second internal phase (sand) to the emulsion, which will not only increase the viscosity, but will also affect the stability of the emulsion if the total internal phase (sand + oil) exceeds a maximum value. However, the existence of two different internal phases offers the possibility of gearing the volume fractions of two alternate phases to each other in such a way that the slurry viscosity is constant and independent of the sand concentration. This is particularly important in situations where the tubing head pressure is limited and tubing friction becomes critical and cannot be allowed to increase as the sand concentration is increased during the job.

A last, but not the least, parameter affecting the emulsion rheology is the amount of shear applied to the emulsion while generating it. It was noticed that emulsions prepared in the field had a lower viscosity than emulsions prepared in the laboratory. This difference could be attributed to a different oil droplet size distribution in field- and laboratory-prepared samples. The efficiency of the equipment used in the field to generate an emulsion lags behind that of laboratory equipment, especially at high oil/water ratios and high gel loadings. This is an area where further improvements can be made by the service companies.

Polymer emulsion frac. fluids are thus attractive because of the wide span of control available to the operator.

Field data (THP and BHP data) proved that these rheological correlations effectively described the situation in the field, the predictive accuracy of the formulae being such that the fracture shape may be evaluated from THP data.

Some 25 fracturing treatments have been carried out in Europe using polymer emulsion fracturing fluid with a very high success ratio. Productivity improvement factors of nearly 30 were achieved with few operational problems using sand concentrations up to 14 lb/U.S. gal.

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