Corrosion inhibitors are additives that must be included in acidizing treatments in order to minimize damage to the tubulars and other downhole equipment. However, these inhibitors may be environmentally damaging and harmful to human health. This paper examines the corrosion-inhibiting properties of several edible stems at downhole conditions with high concentrations of hydrochloric acid (HCl). A group of 10 different edible stems were selected from the shelves of local grocery shops or from online retailers. The objective is to present an alternative variety of corrosion inhibitors that are environmentally friendly and nontoxic.
HCl is commonly used for inorganic scale removal, matrix acidizing, acid fracturing, and many other forms of stimulation treatments. However, HCl may cause severe corrosion of well tubulars and equipment downhole, especially at high temperatures. This can result in safety hazards and disrupt production.
Several techniques, such as emulsification, in-situ generated acids, or switching to weaker acids such as acetic acid, can be used to lower corrosion damage during stimulation treatments. Typically, the cheapest and easiest method to do so is to add corrosion inhibitors to the pumped acids. These corrosion inhibitor molecules have three main mechanisms of protection: they can adsorb to the metal surface to create a hydrophobic barrier that reduces contact between HCl and the metal; form a protective oxide layer; or they form complexes with the corrosive component in solution. Because HCl is required for the stimulation and iron oxide is soluble in it, corrosion inhibitors that form barriers are often favored. This technique has also been successfully applied for many years, causing the addition of corrosion inhibitors to be necessary in any acid treatment, according to the authors.
At temperatures above 200°F, the corrosion rate of HCl increases significantly. As a result, the corrosion inhibitor can no longer sufficiently protect the tubulars from corrosion by itself. Therefore, at these elevated temperatures, corrosion inhibitor intensifiers such as potassium iodide and formic acid are required. Even with the combination of corrosion inhibitors and corrosion inhibitor intensifier, many pumped formulas are unable to provide satisfactory corrosion rates or are banned because of environmental regulations.
As mentioned earlier, commonly used corrosion inhibitors act by adsorbing to the surface of the metal to create a protective layer that reduces contact between the metal surface and the corrosive substance. This inhibits the cathodic/anodic reactions or poisons the production of H2. A variety of organic compounds have been used as corrosion inhibitors, such as acetylenic alcohols, imidazolines, Mannich bases, and quaternary ammonium salts. Generally though, the most common types of corrosion inhibitors used are typically quaternary amines or sulfur-containing compounds. Although commonly used corrosion inhibitors are effective, they are usually harmful to the environment as well as to human health. These mentioned compounds contain sulfur, nitrogen, and oxygen atoms as they help the molecule better adsorb to the surface of the metal. Because many naturally occurring chemicals also contain these groups, they may also possess corrosion-inhibiting properties.
Many authors have examined plant extracts and products as corrosion inhibitors. Authors Khamis and Alandis in a 2021 paper tested the corrosion-inhibiting properties of thyme, coriander, hibiscus, anise, black cumin, and garden cress in 0.1 M sulfuric acid on steel at 77°F. They found thyme showed the best corrosion-inhibition efficiency at 93.8% and the remaining herbs showing various inhibition efficiencies between 37.4 and 85.9%. Ginger extract was also studied as a corrosion inhibitor in 1.0 M HCl on mild steel at room temperature over 22 hours. The authors observed an inhibition efficiency ranging between 86 and 91%, depending on the concentration of extract used.