Effect of Droplet Size, Emulsifier Concentration and Acid Volume Fraction on the Rheological Properties and Stability of Emulsified Acids

Abstract

Abstract 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 µm. The viscosity covered a shear rate range from 10 to 750 s-1 and temperature range from 25 to 80 oC. 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. Introduction Perhaps the first introduction of emulsified acid to the oil industry was by a patent filed by Melvin de Groote.1 According to de Groote, the aim of his invention was to remove formation damage from carbonate rocks using "an aqueous acid solution emulsified in a suitable vehicle that effectively protects the metallic parts of the well from injury by the acid in the solution while the solution is being introduced into the well." De Groote used hydrochloric acid, nitric acid and mixture of the two acids to prepare his emulsion. Crude oil, coal tar distillates such as naphtha and carbon tetrachloride were used as dispersing fluids. Sulfonic acid was used as the emulsifying agent, but he described the possibility of using asphalt. The procedure he described in the patent for preparing the emulsion is exactly similar to today's practices. However, today's emulsifiers are more efficient than what he described. He added from two to five percent of the emulsifying agent to the continuous phase [crude oil in his case], and then added the acid to the mixture in 33.3 acid to 66.7 crude oil ratio. Interestingly, the objective that inspired the invention of emulsified acid was not to deepen the penetration of the acid, not to decrease the leak-off, nor to retard the acid reaction. The inventor objected to the old and previously known chemical treatment, according to him, in that the raw acid used as the treating agent attacks the metal parts of the well structure and its working parts which limit the foreseen benefits from the whole treatment.. From this point of view, the emulsified acid was invented to be a corrosion inhibitor much more than of it being an improved stimulation fluid.