Applications of Acetic Acid to Well Completion, Stimulation and Reconditioning
Authors: Harris F.N. 
Published: December 2007

Acetic acid has been used successfully many times in the past few months, in various treating mixtures and in a number of different applications. It has been used asa perforating fluid,a retarded acid without viscosity,a treatment for removal of carbonate scale in the presence of chromium-plated pump parts,a stimulation treatment in the presence of aluminum metal at elevated temperatures,a "kill" fluid for wells,a weak aqueous solution for carrying surfactants to remove emulsions and water blocks in the presence of water-sensitive clays,a first-stage treating fluid ahead of hydrochloric acid for a greater drainage pattern anda transitory true gel or emulsion for placement of temporary bridging agents.

Introduction

Only in recent months has acetic acid been widely used as an aid in overcoming many of the problems encountered in well completion, stimulation and reconditioning. Even though this acid has been used in the past for well stimulation, factors such as economics, handling and the lack of technical data have limited its uses over the past few years. Acetic acid does not present many of the operational difficulties often associated with the more conventionally used hydrochloric acid. The corrosive action of acetic-acid mixtures can be greatly minimized even at temperatures in excess of 240F. With proper inhibition, acid-pipe contact time can now be extended for days. The mixtures currently being used have not caused electrolytic corrosion, hydrogen embrittlement or stress cracking of metals.

Chemical Characteristics of Acetic Acid

Unlike hydrochloric acid, acetic acid can be effectively inhibited against almost all types of steel at elevated temperatures for extended periods of time. Table 1 shows laboratory data of corrosion rates on the most common tubular goods. In these tests, the time exposure of steel to acid has been extended to days without damaging or weakening the pipe. The type of corrosion caused by acetic acid differs from that caused by hydrochloric acid, the latter tending to "pit corrode" tubular goods with extended times. This action becomes accelerated with increasing temperatures. By comparison, acetic acid at equivalent test conditions of time, temperature and pressure will have a slight uniform removal of steel from pipe. Without pronounced pit-type corrosion, no serious damage was incurred to laboratory test samples even when the acid was allowed to spend completely. This also has been born out in field applications. A special inhibitor is always added to the acid to allow it to be stored within the casing or tubing for many hours, when operations require it, so that live acid will be present when it is needed. Corrosion tests and stress-cracking evaluations have been made on heat- treated high-alloy steels, as well as on lightweight aluminum alloys and chromium plating. No damaging effect has been noted on these samples, which were tested for four hours under simulated well conditions between 1,000 and 5,000 psi in a temperature medium up to 250F. Acetic acid is inherently slower-reacting than hydrochloric acid. In its action on carbonates, acetic-acid reaction rates are influenced greatly with pressure. While temperature does exert an influence on rate of reaction, the rate is not accelerated so much as in the case of hydrochloric acid. Fig. 1 reveals a distinct lowering of the reaction rates of acetic acid at approximately 50 per cent spent, at static test conditions of 1,000 psi.

JPT

P. 637^