In-situ gelled acids have been used extensively in matrix and acid fracturing for acid diversion, reducing the leak-off rate, and retarding the acid reaction for deep acid penetration. Although literature review shows that previous studies investigated the rate of dissolution of polymer-based acid, none has addressed in details the cross-linked polymer systems. Therefore, the aim of this work was to study the mass transport and the reaction rate kinetics of the reaction of HCl in-situ gelled acid and carbonate rocks.

The reaction rate of 5 wt% in-situ gelled acid was measured using the rotating disk apparatus. Rock samples of 1.5 in. diameter and 1 in. length were utilized. The rate of reaction of in-situ gelled acid was investigated at different temperatures (150, 200, and 250°F) using different rock types (Austin Chalk, Edward limestone, and Pink Dessert limestone) at rotational speeds (100–1800) rpm. Calcium concentration was measured in the collected samples and was used to determine the acid reaction rate.

Experimental results showed that the reaction rate is mass transfer limited up to a rotational speed of 1000 rpm and a surface reaction limited above this value. The diffusion coefficient of 5 wt% HCl in in-situ gelled acid was determined at 150°F and reported for such system for the first time. A power law kinetic model was used to determine the rate kinetics parameters. The reaction activation energy and the rate constant at 150, 200, and 250°F were reported. Effect of Fe+3 (cross-linker) on the reaction rate was studied and was found to decrease the dissolution rate of Pink Dessert limestone by a factor of two in a comparison with gelled acid with the same HCl concentration.

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