https://orcid.org/0009-0008-2039-5341
A novel methodology and experimental apparatus were developed to address the limitations of previous studies and investigate corrosion inhibitor persistency under batch treatment conditions. This approach effectively removed all residues after inhibitor application and prevented O2 ingress during film formation and subsequent steps. A model compound corrosion inhibitor (CI) was utilized to validate the methodology and investigate the effects of solvent on CI persistency. In all experiments, CI was applied in situ on the prepared API 5L X65 steel rotating cylinder electrode inside the empty deoxygenated glass cell using a holder and vial. The setup used a reservoir of CO2-sparged uninhibited brine to continuously dilute the test electrolyte at a constant flow rate. Electrochemical measurements were performed at 20-min intervals to characterize the inhibition behavior over time.
INTRODUCTION
CO2 corrosion of the internal walls of carbon steel pipelines is a significant problem in oil and gas industries. Different methods have been implemented for its mitigation, but the use of corrosion inhibitors (CIs) provides advantages for minimizing pipeline wall loss as costs are lower compared to other techniques and can be adjusted over time. Thus, CIs are widely used as a conventional method to mitigate internal pipeline corrosion using two main methods: continuous inhibitor injection and batch inhibition treatment.1-7 Continuous injection involves constant application of inhibitors, while batch inhibition uses periodic treatments. When continuous inhibitor injection fails to provide adequate protection, batch inhibition is implemented as an alternative approach.
One of the most challenging steps in batch inhibition studies is the methodology that is used in the laboratory to simulate field application. The “ex situ dip and drip” method has been traditionally used throughout the industry for batch inhibition studies in terms of inhibitor film formation. In this methodology, the specimen is first dipped into the neat or diluted inhibitor solution for a defined contact time. Then, the specimen is dripped dry and transferred to the uninhibited brine, and the electrochemical measurements are conducted with or without limited brine renewal.8-12 Since then, some significant improvements have been proposed to address O2 contamination. For example, the inhibitor can be applied to the specimen directly in a deoxygenated glass cell using either the dip and drip method or a spray nozzle to provide a controlled coverage of the specimen. Then, uninhibited brine is introduced to the glass cell without transferring the specimen.13-15 This modified method is called “in situ modified dip and drip” method. Yet, these methods, either ex situ or in situ, still exhibit limitations in terms of brine renewal, which may not ensure the complete removal of CI from the bulk aqueous phase.
The objective of this batch inhibition study was to develop a methodology to simulate the field application of CI with no O2 contamination and to provide continuous and complete removal of inhibitor residuals from the glass cell.
EXPERIMENTAL PROCEDURE
Schematic of the experimental apparatus used in batch inhibition study (left) and the inhibitor vial holder used for inhibitor film formation (right).
Schematic of the experimental apparatus used in batch inhibition study (left) and the inhibitor vial holder used for inhibitor film formation (right).
Experimental Test Matrix for Batch Inhibition Persistency Experiments
For each experiment, detailed steps were as follows:
Sparge 300 gallons uninhibited brine overnight with CO2.
Empty, dry, and sparge glass cell for 30 min.
Install RCE specimen into the glass cell and sparge for another 10 min.
Prepare inhibitor solution (CI dissolved in solvent, either isopropanol or LVT-200) in a separate vial attached to the holder (Figure 1).
Remove stopper and lower the vial into the glass cell and position it around the specimen without touching it.
Set RCE rotation (300 rpm) for a specific contact time (flow with inhibitor contact).
Stop RCE rotation and carefully remove vial from around specimen, avoiding contact between the vial and the specimen.
Remove vial from the glass cell, put stopper back, and sparge for another 5 min.
Introduce presparged uninhibited brine at a constant flow rate and flush out any excess inhibitor from dip and drip procedure with the initial flow of brine before fully wetting the specimen.
After specimen is fully wetted, maintain continuous flow of brine through cell and conduct electrochemical measurements (OCP, EIS, and LPR) at constant time intervals.
RESULTS AND DISCUSSION
(a) Batch inhibition persistency experiment with 15 wt% BDA-C16 in isopropanol (top) and (b) batch inhibition persistency experiment with 7.5 wt% BDA-C16 in LVT-200 (bottom). (30°C, 1,000 rpm, pH 4.0, 1 wt% NaCl).
(a) Batch inhibition persistency experiment with 15 wt% BDA-C16 in isopropanol (top) and (b) batch inhibition persistency experiment with 7.5 wt% BDA-C16 in LVT-200 (bottom). (30°C, 1,000 rpm, pH 4.0, 1 wt% NaCl).
The next research focus was on the effects of solvent on inhibitor persistency. The hypothesis that straight-chain hydrocarbon molecules incorporate with an adsorbed inhibitor layer to enhance the persistency of the inhibitor was tested using LVT-200 as the solvent for BDA-C16. The specimen was dipped into a solution of 7.5 wt% BDA-C16 in LVT-200 (the highest solubility of BDA-C16 in LVT-200). Figure 2(b) shows the results for corrosion rate/residual inhibitor concentration versus time. With an initial mitigated corrosion rate of about 0.4 mm/y, the use of a hydrocarbon increased persistency to approximately 2 h. The hypothesis of incorporation of hydrocarbon into the CI adsorbed layer was shown to be true.
CONCLUSIONS
A new system setup and methodology were developed to investigate batch inhibition persistency without O2 contamination and complete inhibitor removal.
Model compound BDA-C16, while effective in continuous treatment, showed no persistency after the dilution step.
A future manuscript will focus on how this methodology can be applied to other CI formulations and conditions.
ACKNOWLEDGMENTS
The authors would like to thank the following companies for their financial support: Ansys, Baker Hughes, ChampionX LLC, Chevron Energy Technology, ConocoPhillips, ExxonMobil, M-I SWACO (SLB), Multi-Chem (Halliburton), Occidental Oil Company, Pertamina, Saudi Aramco, Shell Global Solutions and TotalEnergies. Technical support from Mr. Alexis Barxias and Mr. Cody Shafer was much appreciated.
