Effect of Droplet Size, Emulsifier Concentration, and Acid Volume Fraction on the Rheological Properties and Stability of Emulsified Acids
- Saleh Haif Al-Mutairi (Saudi Aramco) | Alfred Daniel Hill (Texas A&M University) | Hisham A. Nasr-El-Din (Texas A&M University)
- Document ID
- Society of Petroleum Engineers
- SPE Production & Operations
- Publication Date
- November 2008
- Document Type
- Journal Paper
- 484 - 497
- 2008. Society of Petroleum Engineers
- 2.2.3 Fluid Loss Control, 4.1.5 Processing Equipment, 5.8.7 Carbonate Reservoir, 3.2.4 Acidising, 4.2.3 Materials and Corrosion, 2.5.2 Fracturing Materials (Fluids, Proppant), 3.3.1 Production Logging, 4.3.4 Scale, 4.1.2 Separation and Treating, 1.8 Formation Damage
- emulsified acids
- 1 in the last 30 days
- 706 since 2007
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Acid-in-diesel emulsions have been used extensively 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 by use of an advanced image-processing system. The apparent viscosity was measured with a Brookfield PVS rheometer at various temperatures. The stability of the emulsified acid was monitored by use of an HTHP see-through cell.
Steady 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 a temperature range from 25 to 80°C. All measurements were regenerated for emulsifier concentrations of 1, 5, and 10 gpt. It was noted that finer emulsions had 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 stabilized at an acid volume fraction of 0.7.
This paper discusses the effects of the acid volume fraction, emulsifier concentration, and droplet size distribution on the rheological properties and stability of emulsified acids.
Perhaps the first introduction of emulsified acid to the oil industry was by a patent filed by de Groote (1933). According to de Groote, the aim of his invention was to remove formation damage from carbonate rocks with "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 (HCl), nitric acid, and a mixture of the two acids to prepare his emulsion. Crude oil and coal tar distillates, such as naphtha and carbon tetrachloride, were used as dispersing fluids. Sulfonic acid was used as the emulsifying agent, but he did mention the possibility of using asphalt. The procedure he described in the patent for preparing the emulsion is similar to today's practices. However, today's emulsifiers are more efficient. He added approximately 2 to 5% 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/66.7 crude-oil volume ratio.
Interestingly, the objective that inspired the invention of emulsified acid was not to deepen the penetration of the acid, not to decrease the leakoff rate, nor to retard the acid reaction. The inventor objected to the old and previously known chemical treatment because according to him, the raw acid used as the treating agent attacks the metal parts of the well structure and its working parts, which limits the foreseen benefits from the whole treatment. From this point of view, the emulsified acid was invented to address corrosion of well tubulars much more than to be an improved stimulation fluid.
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