Hydrochloric acid is used to stimulate carbonate formations (both matrix and fracturing treatments). However, the reaction rate of the acid with carbonate minerals, especially calcite, is very fast. In addition, the viscosity of HCl solution is relatively low. Acid-soluble polymers are usually added to the acid to increase its viscosity, which is needed to enhance acid diversion during matrix acidizng and reduce acid leak-off rate during acid fracturing. Gelled acid is extensively used in matrix and acid fracturing treatments performed in carbonate formations. However, a few studies have evaluated the impact of these polymers on the reaction rate of the acid.

This paper uses a rotating disk instrument to measure the reaction rate of gelled acids with calcite rocks. Measurements were conducted over a temperature range of 25 to 65°C, a pressure of 1,000 psi, and rotational speeds of 100 to 1,000 rpm. Acid formulations that are typically used in the field were examined. Polymer concentration was varied from 0.5 to 2 wt%. The apparent viscosity of the gelled acid was measured using a Brookfield viscometer. Measurements were done for the same solutions tested with the rotating disk instrument. The temperature was varied from 25 to 100°C, while the pressure was maintained at 300 psi. The shear rate was varied from 57 to 1,700 s−1.

The results obtained indicated that the apparent viscosity increased notably as the concentration of polymer increased from 0.5 to 1.5 wt%. On the other hand, there is significant decrease in the dissolution rates as the concentration of polymer was increased from 0.5 to 1.5 wt%, where there was no measure difference as the concentration increased to 2 wt%. Reverse and toroidal flows were noted within the rotational spends examined. The etching pattern on the surface of the disk depends, among other factors, on the disk rotational speed and polymer concentration. The dissolution rate was found to be a function of temperature, rotational spend and polymer concentration.

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