Summary
Previous studies of citric acid/calcite have been limited to coreflood tests
and bench-scale experiments. However, the kinetics of a citric acid reaction
with calcite has not been measured. This paper gives, for the first time, the
kinetics of citric-acid/calcite reaction, which will provide a better way to
model the performance of citric acid as a standalone stimulation fluid.
In this paper, the rotating disk apparatus was used to study
citric-acid/calcite reaction at a pressure of 1,000 psi, temperatures from 25
to 50°C, citric acid concentrations from 1 to 7.5 wt%, and disk rotational
speeds from 100 to 1,000 rev/min. The reaction rate of citric acid with calcite
was found to be dependent on the initial citric acid concentration, disk
rotational speed, and temperature. For example, at 50°C, the reaction was
reaction-rate-limited at rotational speeds greater than 500 rev/min, using high
initial citric acid concentrations (3, 5, and 7.5 wt%), while at low acid
concentrations (1 and 2 wt%), the reaction was mass-transfer-limited even at
high rotational speeds (1,000 rev/min). During the reaction of citric acid with
calcite, calcium citrate precipitation occurred at different acid
concentrations and rotational speeds. The amount of this precipitation was
found to be a function of both the initial citric acid concentration and disk
rotational speed. More calcium citrate precipitated at high initial citric acid
concentrations, especially at high rotational speeds.
Calcium citrate precipitation occurred at the calcite surface, even at low
initial citric acid concentrations. Because of this precipitation, the
equilibrium of the citric-acid/calcite reaction was disturbed and the reaction
shifted toward the forward reaction (toward the products). Therefore, the
overall reaction rate was governed mainly by the rate of the forward reaction,
and, hence, it was modeled by a new simplified reaction-rate equation; rate =
kf
{Ka1·CB}n/2.
The average value of the reaction order (n) was found to be 0.833. In
addition, the value of the reaction-rate constant (kf) was
determined at various temperatures. The effect of temperature on the
reaction-rate constant was found to follow the Arrhenius law, where the
activation energy was found to be 63.1 kJ/mol.
© 2010. Society of Petroleum Engineers
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History
- Original manuscript received:
21 January 2009
- Meeting paper published:
21 April 2009
- Revised manuscript received:
23 July 2009
- Manuscript approved:
17 October 2009
- Published online:
1 April 2010
- Version of record:
22 September 2010
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