Mass transfer process during the reaction of citric acid with calcite was investigated using the rotating disk apparatus. The effects of disk rotational speed, initial citric acid concentration, and temperature on the effective diffusion coefficient of citric acid were examined.
Using various citric acid concentrations (1, 2, 5, 7.5 wt%), the diffusion coefficient of citric acid was calculated at 25, 40, 50 °C. The effective diffusion coefficient of citric acid was found to be a function of the interplay between the calcium citrate precipitation and the presence of the counter calcium ions. At high initial acid concentration, (5, 7.5 wt%), the effects of calcium citrate precipitation and counter calcium ions were significant and the calculated citric acid diffusion coefficients were not comparable with its measured effective diffusion coefficients using the rotating disk. However, the effects of both the calcium citrate precipitation and the counter calcium ions on the citric acid diffusivity were minimal at low initial citric acid concentrations.
The effect of temperature on the diffusion coefficient of citric acid at a constant citric acid concentration was found to follow Arrhenius law, and the activation energy is equal to 37.9 kJ/mol.
Citric acid (C6H8O7) has historically been used in oil field treatments as an iron-control agent(1). It is commonly used to stabilize iron in spent HCl acids and prevent precipitation of ferric hydroxide(2), and/or iron sulfide(3,4). Besides its ironchelating ability, citric acid possesses another important chemical characteristic. Citric acid is a weak acid and this makes it less reactive with reservoir rocks than hydrochloric acid. Because it is weak acid, citric acid is a good alternative to hydrochloric acid in high temperature formations, where concentrated HCl solutions can cause severe corrosion problems and poor etching patterns(5).
Over the last few years, various acid systems, such as gelled acids, viscoelastic surfactants-based acids (VES), emulsified acids, and encapsulated citric acid, have been introduced to address some of the concerns with HCl. Except of citric acid system; these acids have been extensively studied and successfully employed in acid fracturing treatments(6–10).
Citric acid has been used in some field treatments as an alternative to other conventional weak acids such as formic acid (HCOOH) and acetic acid (CH3COOH). It was introduced because it can be used in an encapsulated form, which prevents its reaction with both the production tubulars and the formation up to 180 °F(11,12), and hence this will protect well tubulars, especially those made of corrosion resistant alloys. The use of encapsulated citric acid in the field has met with mixed results. Blauch et al.(11) reported positive field results and that the formation damage because of calcium citrate precipitation was not a major concern at down-hole conditions. However, Burgos et al.(12) reported that there was no improvement in the performance of an acid-fractured well after the application of encapsulated citric acid. They explained the unexpected field results in terms of calcium citrate precipitation.
