Acid-in-diesel emulsions have been extensively used in matrix acidizing and acid fracturing treatments. This acid has several advantages including lower corrosion rate, minimum number of additives, and deep acid penetration. For stimulation purposes, the most important properties of emulsified acid are reactivity, stability, and viscosity. The size distribution of the droplets of the emulsion affects these properties.

The purpose of this paper is to correlate the droplet size of the dispersed phase (acid) to the viscosity and stability of emulsified acids. Measurements of the droplet size were acquired with laser diffraction techniques and analyzed using an advanced image processing system. Viscosities were measured using Brookfield PVS rheometer at varying temperatures and pressures. The stability of the emulsified acid was monitored using an HTHP see-through cell.

Shear viscosity was measured for emulsions with droplet sizes ranging in diameter from 1 to 20 qm. The viscosity covered a shear rate range from 10 to 750 s−1 and temperature range from 25 to 80 °C. All measurements were regenerated for emulsifier concentrations of 1, 5 and 10 gpt. It is noticed that finer emulsions have higher viscosities. Likewise, similar measurements were performed with varying acid volume fractions.

The most stable emulsion was noted at an acid volume fraction of nearly 0.7. Other volume fractions were stable for a few hours before diesel was expelled as a separate layer above the emulsion. Eventually, the remaining emulsion restabilized at acid volume fraction of 0.7.

This paper discusses the effects of acid volume fraction, emulsifier concentration and droplet size distribution on the rheological properties of emulsified acid.

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