Ahmed Mohamed Gomaa

Reaction of In-Situ-Gelled Acids With Calcite: Reaction-Rate Study

This paper (SPE 133501) was accepted for presentation at the SPE Production and Operations Conference and Exhibition, Tunis, Tunisia, 8–10 June 2010, and revised for publication. Original manuscript received for review 9 August 2010. Revised manuscript received for review 3 March 2011. Paper peer approved 9 March 2011. 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 Fe 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.