Summary
In-situ-gelled acids have been used extensively in matrix acidizing and acid fracturing for acid diversion and reducing the leakoff rate, respectively. A few studies investigated the rate of dissolution of calcite in polymer-based acids, yet none has addressed in detail the in-situ-gelled acids. Therefore, the aim of this work is to examine the mass transfer and the kinetics of the reaction of 5 wt% HCl in-situ-gelled acids with calcite and determine the effect of Fe crosslinker on the rate of calcite dissolution.
The rate of reaction of 5 wt% HCl in-situ-gelled acid was measured using the rotating-disk apparatus. Rock samples of 1.5in. diameter and 1-in. length were used. The effect of temperature (100-250°F) and disk-rotational speed (100-1,800 rev/min) was investigated using Pink Desert limestone rock samples. Calcium concentration was measured in the collected samples and was used to determine the acid-reaction rate.
Experimental results showed that the rate of calcite dissolution at 150°F was controlled mainly by the rate of mass transfer of the acid to the surface up to a disk rotational speed of 1,000 rev/min and by the rate of the surface reaction above this value. On the basis of the results obtained, the diffusion coefficient of 5 wt% HCl in in-situ-gelled acid at 150°F; the activation energy; and the reaction rate constant at 150, 200, and 250°F were determined for the first time. A power-law kinetic model was used to determine the kinetics parameters. The presence of Fe3+ crosslinker had a significant effect on the rate of dissolution in comparison with reactions with gelled acid (no crosslinker) at the same condition. The reaction rate decreased by a factor of 2.2 and by a factor of 1.4 when the reaction was conducted at 100 and 1,500 rev/min, respectively. A gel layer, formed on the surface, acted as a barrier between the acid and the rock, which reduced the rate of calcite dissolution.