Kinetics of the Reaction of Citric Acid With Calcite

Abstract

AbstractThe previous studies of citric-calcite have been limited to core flood tests, and bench scale experiments. However, the reaction kinetics of citric acid-calcite has not been measured. This paper gives, for the first time, the kinetics of citric-calcite reaction, which will provide a better way to model the performance of citric acid as a stand-alone stimulation fluid.In this paper, the rotating disk was used to study citric-calcite reaction at pressure of 1,000 psi, temperature values from 25 to 50°C, citric acid concentrations of 1 to 7.5 wt%, and disk rotational speeds of 100 to 1,000 rpm. The reaction of citric acid with calcite is dependent on the initial citric acid concentration, disk rotational speed, and the temperature value. For example, at 50°C, the reaction is reaction rate-limited at high rotational speeds, rpm > 500, using high initial citric acid concentrations (3, 5, 7.5 wt%), while at low acid concentrations (1,2 wt%), the reaction is mass-transfer limited even at high rotational speeds, 1,000 rpm. Calcium citrate formation is also dependent on both the initial citric acid concentration, and the rotational speed. More calcium citrate forms at high initial citric acid concentrations, especially at high rotational speeds.Calcium citrate precipitation occurred at the reacted calcite surface, even at low initial citric acid concentration, 1 wt%. Due to this precipitation, the equilibrium of citric-calcite reaction was disturbed, and the reaction shifted towards the forward reaction (toward the products). Therefore, the overall reaction rate was mainly governed by the rate of the forward reaction, and hence, it was molded 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 equal to 0.833. In addition, the value of the reaction constant (kf) was determined at various temperatures. The effect of temperature on the reaction order was found to follow Arrhenius law, where the activation energy was found to be equal to 63.1 kJ/mol.