New insights into the reaction of citric acid with calcite

Abstract

The previous studies of citric-calcite have been limited to core flood, 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 study, the rotating disk was used to study citric-calcite reaction at 1,000 psi, temperatures 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 results obtained showed that the citric-calcite reaction is dependent on both the rotational speed and temperature. The reaction of citric-calcite becomes surface-reaction controlled at temperatures of 25, 40, and 50°C at rotational speeds greater than 450 ,500, and 500 rpm, respectively. Although this surface reaction was expected to be enhanced by the chelation ability of citric acid, however it was found that the reaction was mainly governed by “H+ attack” mechanism at pH values < 3, and the “chelation” mechanism is not significant at these pH values. 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. Therefore, the overall reaction rate was mainly governed by the rate of the forward reaction, and hence, it was molded by simple reaction rate equation, rate = k*[Citric]n. The reaction order (n) was found to be less than 1. In addition, the value of the reaction constant (k) was determined at various temperatures. The effect of temperature on the reaction order was found to follow Arrhenius law.