Impact of Calcium Citrate Precipitation on the Reaction of Citric Acid-Calcite

Recently, there has been considerable interest in citric acid for acid fracturing treatments. It has been used as an alternative to hydrochloric acid in acid fracturing treatments because it has slower reaction rate and it can be inhibited at elevated temperatures. These characteristics result in deep acid penetration, longer etched fracture face and hence more effective acid fracturing treatments in deep wells. However, the application of citric acid in stimulation treatments has been met with mixed results. This paper shows for the first time numerous data of citric acid reaction with calcite, which provide a better understanding of the performance of citric acid as a stimulation fluid.

In previous studies (AlKhaldi et al. SPEJ, 2005), the reaction of citric with calcite was examined over a limited range of parameters. To design an effective acid fracturing treatment, it is essential to know the effective diffusivity of the acid system at different field conditions. This paper presents measured reaction rates of citric acid reaction with calcite using the rotating disk apparatus. The theory of the rotating disk apparatus was used to determine the diffusivity coefficient of citric acid from measured reaction rates.

The results obtained indicated that the reaction of citric acid with calcite is mass transfer limited using an initial citric acid concentration range of 1 to 10 wt% at temperature values of 25 to 80„aC, and 1,000 psi. Conditions that lead to the precipitation of the reaction product calcium citrate were identified. In addition, this paper presents calculated diffusion coefficients of citric acid. The diffusion of citric acid decreased when the initial citric acid concentration increased. The effect of temperature and the presence of Na+, K+, Mg2+, Ca2+, Fe3+ on citric diffusion coefficient is also discussed.