Summary
Unlike other acid systems, such as gelled and viscoelastic surfactant-based
(VES) acids, where the mobility of hydrogen ion controls the overall rate of
the reaction, emulsified acid/calcite reaction involves the transport of acid
droplets in the diesel phase to the rock surface, breaking of acid droplets,
and then the actual reaction on the surface. A limited number of papers have
been published on the reaction kinetics of emulsified acid. However, none of
the published work considered the effect of acid droplet size on the reaction
of emulsified acid. The objective of this work is to examine the effect of the
acid droplet size on the reaction rate of emulsified acid with calcite.
The acid was 15 wt% HCl emulsified in diesel with 70 to 30 acid-to-diesel
volume ratio. The emulsifier concentration was varied from 1 to 10 gpt. All
emulsions were characterized by measuring the droplet size distribution,
viscosity, and thermal stability. Diffusivities were measured using the
rotating disk device. The experiments were carried out at 25, 50, and 85°C,
under 1,000 psi pressure, and disk rotational speeds from 100 to 1,000 rev/min.
Samples of the reacting acid were collected and analyzed to measure calcium
concentration in the reactor.
The effect of the acid droplet size on the overall reaction rate was
significant. The diffusion rate of acid droplets to the surface of the disk was
found to decrease with increasing emulsifier concentration because of higher
viscosities and smaller droplet sizes. The effective diffusion coefficient of
emulsified acid was found to increase linearly with the average droplet size.
Emulsions with low emulsifier concentrations (1 gpt) had average droplet sizes
of nearly 13 µm. These emulsions were found to have high effective diffusion
coefficients (5.093×10−9 cm2/s) and low retardation. On
the other hand, emulsions with high emulsifier concentrations (10 gpt) had
smaller average droplet sizes (nearly 6 µm) and found to have low effective
diffusion coefficients (4.905×10−11 cm2/s) and high
retardations.
The new sets of data can be used to determine the optimum emulsified acid
formulation to yield deeper acid penetration in the formation. It is suggested
that droplet size can be adjusted to produce the desired diffusion coefficients
for acid fracturing treatments.
© 2009. Society of Petroleum Engineers
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History
- Original manuscript received:
20 November 2007
- Meeting paper published:
13 February 2008
- Revised manuscript received:
17 February 2009
- Manuscript approved:
2 March 2009
- Published online:
16 July 2009
- Version of record:
22 December 2009
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