Kinetics of the reaction of citric acid with calcite

Abstract

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.