Internal Corrosion Inhibitor Optimization
A sour gas pipeline (0.5% H2S, 2% CO2) is experiencing internal corrosion. The current inhibitor is a water-soluble imidazoline injected at 10 ppm based on total liquid volume. Corrosion coupons show a corrosion rate of 0.2 mm/year, exceeding the target of 0.1 mm/year. The flow regime is stratified with a liquid holdup of 30%. The inhibitor partition coefficient is 0.5 (water/oil). The operator considers increasing inhibitor dosage, changing to an oil-soluble inhibitor, or improving the injection method.
1. What is the most likely reason for the inadequate inhibitor performance, and what solution do you recommend?
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The water-soluble imidazoline has a partition coefficient of 0.5, meaning it preferentially stays in the water phase. However, in stratified flow with 30% liquid holdup, the inhibitor may not reach the entire pipe surface, especially the top of the line where condensation occurs. The corrosion rate is high, indicating insufficient film persistence. Increasing dosage may not solve the distribution issue. A better approach is to use an oil-soluble inhibitor that partitions into the hydrocarbon phase and provides a persistent film, or to improve injection by using a spray nozzle to distribute the inhibitor along the pipe circumference. Additionally, consider a continuous injection with a higher dosage initially to establish a film, followed by maintenance dosage. Also, evaluate the inhibitor's performance under the specific H2S/CO2 conditions; maybe a different chemistry (e.g., phosphate ester) is more effective.
2. How would you monitor the effectiveness of the new inhibitor program?
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Monitoring should include: (1) corrosion coupons at various locations (top and bottom of line) to measure corrosion rates; (2) linear polarization resistance (LPR) probes for real-time monitoring; (3) periodic fluid analysis for inhibitor residual concentration (e.g., using colorimetric or fluorescence methods); (4) monitoring of iron counts in the produced water; (5) inspection using UT or intelligent pigging to detect wall loss. Establish a baseline before changing the program and compare after a few months. Also, ensure that the inhibitor is compatible with other chemicals and does not cause emulsions or foaming.