Acetic Acid Demonstrates Greater Carbonate Dissolution Than Typically Expected
Authors: Metcalf A.S. , Parker C.P. , Boles J.L. 
Published: March 2010

Abstract
Stimulation of carbonate formations by acid dissolution has been an efficient method of improving hydrocarbon production. In high temperature applications, corrosion issues can limit usage, especially in chrome completions. Acetic acid has been used with some success. Experimental data is presented that illustrates the advantage of in situ dilution of acetic acid for removal of carbonate rock under high temperature conditions.

Introduction
Completions, well clean-outs, and stimulation operations at high bottomhole temperatures have for years been conducted using organicacids because of their low corrosivity(1, 2). In addition, more significant control of wormhole formation has been observed in matrix acid stimulation of carbonates(2, 3). Hydrochloric acid is cheaper, reacts faster and completely, while organic acids with small dissociation constants react slower and not to completion(4, 5).
In the treatment of oil reservoirs with acids, potential asphaltene sludging is possible. Rietjens(6) found that acetic acid tested with three different oils, and using several strengths did not induce sludge in most cases; even in the presence of 1,000 ppm Fe(III). Another study demonstrated acetic acid stimulation fluid with 3,000 ppm Fe(III) and only produced a trace of precipitate(3).
The reaction of acetic acid and other "weak" acids on calcite have been studied by many(8–12). Investigation into reaction of acetic acid on calcite using rotating disks determined that over the pH range 2.3 to 2.9, dissolution was mass transfer limited(11). However, the dissolution rates were lower than expected based on reactan diffusion. It was decided that diffusion of the reactants to the surface and products away from the surface interacted to reduce the dissolution rate. It was also found that above pH 3.7, the surface reaction rate has a major effect on the dissolution. Chatelain et al.(12) determined that influence by the products transport away from the surface was the primary reason for the difference in dissolution by acetic acid. Fredd et al.(11) determined the transition region to be at a pH less than 3.7. Several researchers have put together models for dealing with kinetic expressions governing weak acid reactivity and the associated equilibrium constants(11, 12).
Work presented here involves the extension of previous work by Chatelain et al.(12) in which equilibrium measurements of acetic acid converted by reaction with calcium carbonate at various temperatures and initial acid concentrations were determined. Examination under non-equilibrium conditions of the amount of calcium carbonate dissolved by acetic acid as it is diluted in situ with fresh water shows how an increase of reactivity is possible. Corrosion values are documented for various acetic acid strengths on carbon and chrome steels.

Background
Organic acids have small dissociation constants (see Table 1), and are therefore termed "weak" acids. Research has been performed to investigate the reactivity of acetic acid and to utilize thermodynamic properties to define models for the reactivity(3, 7–12). These models, and parameters produced from them, allow for better design of acid stimulation treatments of carbonate formations.