Determination and Fate of Formic Acid in High Temperature Acid Stimulation Fluids

Abstract

Abstract Formic acid is present in some corrosion inhibitors and is used as an intensifier in the acid treatment of oil and gas wells. This organic acid is important for the protection of low-carbon steels from acid corrosion at high temperatures and pressure. In this paper, the fate of formic acid used during acid treatments at temperatures up to 250°F in deep gas wells is investigated in detail. To the best of the authors' knowledge, this is the first study of this type. The following conclusions were made from this work:Most of the commercial organic additives for corrosion inhibitors contain formic acid while one inhibitor intensifier contains both formic and acetic acids.Formic acid concentration in flow back samples from high temperature gas wells was higher than 85% of the injected value in most of the treatments.In concentrated HCl at 250°F, the amount of formic acid present in corrosion inhibitors and intensifiers decreased with time. After 8 hours corrosion inhibitors and intensifiers contained approximately 60% of the initial formic acid concentration. Acetic acid concentration did not significantly change with time in these tests. This work shows the fate of formic acid during acidizing of high temperature wells. Formic acid concentration in acid treatment flowback samples was near the injected value, showing that it remains available to act as a corrosion inhibitor intensifier. However, since it degrades with time, long shut-in times should be avoided. High formic acid concentrations in flow back samples may promote corrosion since the corrosion inhibitor concentration in these samples is significantly reduced. Introduction Formic acid is the strongest unsubstituted carboxylic acid and is used mainly by the textile and leather industries.1 The use of formic acid in many oil and gas field applications has substantially increased over the last three decades. Namely, in well production enhancement,2 in protection of high temperature well tubular goods,3–9 and in the chemical cleaning of industrial equipment such as scale removal from boilers and heat exchangers.10,11 At Saudi Aramco, the use of formic and acetic acids is of major importance in well stimulation. Nasr-El-Din et al.,12 for instance, have successfully applied an acid formula containing acetic acid to stimulate sour water injectors in a sandstone reservoir in central Saudi Arabia. Live acid for well treatments is generally mixed with many commercial additives. These additives protect well tubulars against the aggressive attack of acid at high temperatures, allow higher pumping rates, reduce fluid loss from fractures or prevent iron precipitation.2 Corrosion inhibitors and corrosion inhibitor intensifiers are the additives of primary interest in this work. Corrosion inhibitors can be classified in several ways based on their role in the electrochemical process. Inhibitors may retard anodic metal dissolution, cathodic hydrogen evolution or both anodic and cathodic processes.13 Most organic inhibitors protect steel by forming a film and retard both anodic and cathodic reactions.13 Corrosion inhibitors can also be classified as primary or secondary, depending on the reaction of the organic molecule. A primary inhibitor will not be chemically changed, while a secondary inhibitor will react and produce a substance that is the real inhibition molecule.13,14 For example, the secondary inhibitor dimethylformamide will hydrolyze to formic acid and dimethylamine at bottomhole conditions (Equation 1). Equation 1 At high temperatures in concentrated HCl, corrosion inhibitors commonly require the addition of inhibitor intensifiers in order to achieve adequate corrosion inhibition.4,5 Formic acid is commonly used as an inhibitor intensifier for low carbon steel.3–5 Formic acid may be present in an internally intensified corrosion inhibitor or may be added as a separate component.4