Iron sulfide scale deposition in sour gas wells is a corrosion induced flow assurance issue. Corrosion inhibitor batch treatment is one of the techniques used to mitigate corrosion and iron sulfide deposition on downhole tubing. The objective of this study is to introduce the newly developed Downhole Corrosion and Scale Monitoring tool (DCSM) to evaluate the performance of batch treatment of corrosion inhibitor under real downhole conditions.

An advanced DCSM tool has been designed and developed to monitor and evaluate the performance of the corrosion inhibitor batch treatment in a sour gas well under actual downhole environment and multiphase flow regime. The tool was retrieved after 3-5 months of exposure to the reservoir condition for post-laboratory analysis. The retrieved coupons were thoroughly characterized to assess the performance of corrosion batch treatment and to understand the mechanisms of corrosion and scaling. X-ray diffraction (XRD) was used to analyze the mineraological composition of the surface scale deposition. Scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDS) analyses were performed to characterize the morphology and elemental analysis of the surface deposition. A surface profilometer was used to quantify the size/depth of the general or localized corrosion, as well as the surface deposition.

To evaluate the performance of the corrosion inhibitor batch treatment, two sets of trial tests were conducted in a selected high-temperature sour gas well. One set of the coupons was retrieved after three-month exposure without corrosion inhibitor batch treatment; the other one was retrieved after five-month exposure with corrosion inhibitor batch treatment. The results showed that a very thin layered iron sulfide scale was formed on the surface of test coupons in both cases. General and localized pitting corrosion were found which indicates that corrosion happened first followed by scale layer deposition on metal coupon surface. Less harsh general and localized pitting corrosion were observed in the presence of corrosion inhibitor batch treatment. The corrosion inhibitor treatment did reduce the general corrosion and localized pitting corrosion under scale deposit at a certain level.

The newly developed DCSM tool is an advanced design which allows direct corrosion and scaling monitoring for metal coupons under downhole conditions. Combined with post-laboratory analysis, it provides corrosion and scale mechanism, evaluation on the performance of mitigation, and proper corrosion and scale management programs to minimize corrosion and scale. The DSCM tool can also be deployed and applied in sweet gas wells, oil wells, and water supplier wells to monitor the corrosion and scale under real downhole conditions.

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