Iron Sulfide (FeS) scale is known as a troublesome scale during oil and gas production. The prevention of FeS precipitation in the injection water for a sour carbonate reservoir was required with an injection brine of low Fe2+ concentration (10 mg/L) and high Ca2+ concentration (5,000 mg/L). This work was proposed to identify a chemical which is able to effectively chelate Fe2+ in the presence of high calcium to prevent or reduce the formation of FeS.

Anaerobic bottle tests were conducted at 95°C for 24 hours to compare the performance of selected chemicals, i.e., EDDS (Ethylenediamine-N,N’-disuccinic acid), DTPA (Diethylenetriamine Pentaacetic Acid), EDTA (ethylenediaminetetraacetic acid), THPS (Tetrakis(hydroxymethyl) phosphonium sulfate), and citric acid (CA). Several sets of performance test were conducted: a. with other cation ions and without other cation ions to compare the effects of other cations on chelation; b. with the aged and non-aged product, ie. product aging test at 250°F for 7 days, to understand the effects of temperature on chemical stability; c. with 1.0 g/L and 10 g/L pre-existing calcite solids effects on the FeS chelation performance to simulate the carbonate reservoir conditions; d. with various Fe2+ level of 10 ppm, 50 ppm, and 100 ppm.

Citric acid and THPS showed better chelation performance on Fe2+ than other tested chemicals. The effects of the presence of pre-existing calcite solids on citric acid and THPS were also investigated to simulate the interaction of chemicals with the carbonate formation. The chelation performance of citric acid was affected by the addition of calcite due to the reaction of citric acid with calcite solids. THPS is not affected by the presence of calcite solid at either 1.0 g/L or 10 g/L. From the above testing results, THPS is recommended for the field applications.

This study systematically investigated FeS control and prevention chemical selection and presents the laboratory results to identify the best performance. It provided an insight into the influence of several potential application conditions, e.g., high levels of calcium and other cation ions, pre-existing solids from carbonate reservoir, and long-time exposure to application temperature (250°F), on the selected chelator performance. This work also established a guideline for chelator dosages during field applications to successfully manage FeS scale.

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