Effect Of Hydrogen Sulfide On The Inhibition Of Oil Field Tubing In Hydrochloric Acid
Authors: Hill Donald G. , DeMott Donald N. 
Published: December 2007

Abstract
The increased demands for new oil and gas reserves and the drilling of deeper and hotter wells has resulted in a tremendous challenge in acidizing these wells. Not only higher temperatures but also higher strength acids frequently used to stimulate these wells has demanded the top performance from organic corrosion inhibitors. The use of arsenic is being phased out and new organic based inhibitors and phased out and new organic based inhibitors and inhibitor aids are being developed for high temperature inhibition of hydrochloric acid. These wells frequently contain hydrogen sulfide; therefore an evaluation of the effect of hydrogen sulfide on corrosion inhibition of hydrochloric acid must be included in the inhibitor evaluation program.
In order for the stimualtion engineers to draw out data behind claims and to evaluate the performance of hydrochloric acid inhibitors currently performance of hydrochloric acid inhibitors currently available, a series of corrosion inhibitors and inhibitor aids commercially available were comparatively evaluated by selected test conditions. One inhibitor provided good corrosion inhibition for both 15 per cent and 28 per cent HCl. Also, the performance of this inhibitor was least affected by hydrogen performance of this inhibitor was least affected by hydrogen sulfide. Other inhibitors were less effective in inhibiting higher strength acid solutions and showed a greater detrimental effect on inhibition due to hydrogen sulfide. The inhibitor aids, cuprous iodide and formic acid, allow adequate inhibition of hydrochloric acid at temperatures up to 400 deg. F, however, the addition of hydrogen sulfide limited the maximum temperature where these inhibitors and inhibitor aids can be used.
The inhibitor which offers adequate corrosion inhibition for a wide range of acid concentrations and varied treating conditions, including the detrimental effect of H2S, is more desirable than an inhibitor which only inhibits specific acids for specific applications. Excessive corrosion and possible tubing failure may result if the wrong possible tubing failure may result if the wrong inhibitor recommendation is unintentionally made.

Introduction
The increased drilling of deeper and hotter wells has resulted in a need for more effective high-temperature inhibitors for the higher strength acids frequently used to acidize these wells. Hydrochloric acid is more efficient in stimulating limestone formations and is more economical to use than organic acids such as formic acid and acetic acid. However, hydrochloric acid is more difficult to inhibit than these organic acids. Frequently hydrogen sulfide has been encountered and has magnified the corrosion inhibition problem.
Until recently arsenic inhibitors have been used for the high temperature inhibition of hydrochloric acid. Even though exceptional corrosion inhibition was obtained with arsenic in H2S free environments, the use of arsenic has been eliminated primarily because of toxicity, residual ecological considerations, and its poisoning effect on refinery catalysts.
This elimination has neccessitated the development of a high-temperature, organic-based inhibitor for hydrochloric acid.
New organic based inhibitors and inhibitor aids, or intensifiers, for HCl have recently been developed and introduced for field application. The inhibitive properties of these inhibitors have approximately doubled the inhibition times for hydrochloric acid at intermediate temperatures (200 deg. to 300 deg. F) and extended the temperature limits up to 400 deg. F for short exposure times. Generally, this is about 100 deg. F greater than previously obtained with organic based inhibitors. However, the performance of these inhibitors must be carefully evaluated in order to obtain their maximum performance without exceeding their limitations.
The effect of hydrogen sulfide must be included in the inhibitor testing and evaluation program since the effectiveness of hydrochloric acid inhibitors can be significantly reduced by the addition of hydrogen sulfide to the inhibited acid.