An experimental study on acid-rock reaction kinetics using dolomite in carbonate acidizing

This study presents experimental results on the reaction kinetics of dolomite with fresh and spent acid in carbonate acidizing. Spent acid is defined as a partially reacted acid involving reaction products, such as calcium and magnesium ions, during an acid-rock reaction. A newly designed apparatus was developed to quantify the effect of spent acid on the acid-rock reaction kinetics at reservoir conditions. A total of 4 runs with 36 experiments at same conditions using the apparatus were repeatedly carried out to investigate the reaction kinetics of dolomite rock under the various concentrations of fresh acid and spent acid. The reaction results showed the dolomite disk was more dissolved in the spent acid than the fresh acid. Moreover, it was observed that the kinematic viscosity was higher, and the pH was lower in the spent acid, when compared with the fresh acid. From the results, it was revealed that the dissolution rate and diffusion coefficient for spent acid were higher than those of fresh acid due to the higher kinematic viscosity and the lower pH of spent acid. This means that the chemical reaction becomes more vigorous due to the long contact time and high acidity of the spent acid at the surface of the dolomite disk. These results suggest that the uncommon ion effect by the impurities, such as ferric oxide and aluminum oxide in clay, which is not related to the reaction of HCl and dolomite, can promote the reactions. A conventional approach using fresh acid could result in significant errors in determining the injection parameters, such as injection rate, volume, and pumping schedule in a carbonate acidizing job, because of the underestimated the dissolution rate and diffusion coefficient. Therefore, the effect of spent acid on acid-rock reaction kinetics should be essentially considered to design the injection conditions in carbonate acidizing.