Reaction of Gelled Acids With Calcite

Abstract

Summary Hydrochloric (HCl) acid is used to stimulate carbonate formations in both matrix and fracturing treatments. However, the reaction rate of the acid with calcite is fast. In addition, the viscosity of regular HCl solutions 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 acidizing and reduce acid leakoff rate during acid fracturing. Gelled acids are extensively used in matrix and acid-fracturing treatments performed in carbonate formations. However, a few studies examined the impact of these polymers on the reaction of HCl acids with calcite. This paper used a rotating disk instrument to measure the dissolution rate of calcite by use of gelled acids. 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 with 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. Evidence of reverse and toroidal flows was noted for the first time by examining the etching patterns of the reacted disks. The etching pattern on the surface of the disk depended, among other factors, on the disk rotational speed and polymer concentration. There was a significant increase in the apparent viscosity of gelled acids and a major decrease in the dissolution rate as the polymer concentration was increased from 0.5 to 1.5 wt%. The reaction of gelled acids with calcite was controlled by a surface reaction at 25°C, and by mass transfer at 65°C. Temperature increased the dissolution rate of calcite at all conditions examined. It did also reduce the viscosity of the gelled acid, which affected the way the acid reacted with calcite.