Summary
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.
Introduction
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.
© 2008. Society of Petroleum Engineers
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History
- Original manuscript received:
5 March 2007
- Meeting paper published:
30 May 2007
- Revised manuscript received:
11 September 2007
- Manuscript approved:
10 October 2007
- Version of record:
15 November 2008
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