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Comprehensive Approach to Oxygen Corrosion Analysis of Water Injection Systems

Product Number: 51315-5962-SG
ISBN: 5962 2015 CP
Author: Troy Eggum
Publication Date: 2015
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Oxygen corrosion is a major threat to water injection systems particularly when injecting seawater to maintain reservoir pressure. Utilizing and building on previously published approaches mechanistic and semi-empirical modeling applications were developed for oxygen corrosion in aqueous environments. The goal of this work was to develop a comprehensive multi-platform approach that can be applied to a variety of oilfield water applications (seawater and produced water) and conditions (oxygen concentrations flow velocities temperatures etc.). The Oxygen Reduction Reaction rate (the principal cathodic reaction) was modeled as mixed-control with activation and diffusion components usually but not universally dominated in oil and gas systems by diffusion processes. At high injection flow rates and when residuals of dissolved chlorine normally injected upstream of the deaerator to control microbiological activity is transported down the tubing string (due to shut-in or upset of the oxygen scavenger system) an erosion-corrosion approach is preferable to the above flow-dependent corrosion rate calculations. Having chlorine residuals in the injected water can be detrimental to the tubing string’s integrity due to its contribution to the rate of internal erosion-corrosion. Based on available data (online dissolved oxygen measurements flow rates temperatures and other operating parameters) cumulative damage from these mechanisms can be assessed and remaining life predictions made. The NACE Standard Practice for Corrosion Control and Monitoring in Seawater Injection Systems (SP0499 – 2012) recommends less than 50 ppb dissolved oxygen less than 6 m/s flow velocity and for carbon steel lines predicts a five to seven year service life before repairs are needed. By understanding the interactions of the identified critical operating parameters an analysis of this type will enable the operator to optimize operating conditions inhibitor and biocide treatment protocols and ultimately extend the service life of water injection installations.
Oxygen corrosion is a major threat to water injection systems particularly when injecting seawater to maintain reservoir pressure. Utilizing and building on previously published approaches mechanistic and semi-empirical modeling applications were developed for oxygen corrosion in aqueous environments. The goal of this work was to develop a comprehensive multi-platform approach that can be applied to a variety of oilfield water applications (seawater and produced water) and conditions (oxygen concentrations flow velocities temperatures etc.). The Oxygen Reduction Reaction rate (the principal cathodic reaction) was modeled as mixed-control with activation and diffusion components usually but not universally dominated in oil and gas systems by diffusion processes. At high injection flow rates and when residuals of dissolved chlorine normally injected upstream of the deaerator to control microbiological activity is transported down the tubing string (due to shut-in or upset of the oxygen scavenger system) an erosion-corrosion approach is preferable to the above flow-dependent corrosion rate calculations. Having chlorine residuals in the injected water can be detrimental to the tubing string’s integrity due to its contribution to the rate of internal erosion-corrosion. Based on available data (online dissolved oxygen measurements flow rates temperatures and other operating parameters) cumulative damage from these mechanisms can be assessed and remaining life predictions made. The NACE Standard Practice for Corrosion Control and Monitoring in Seawater Injection Systems (SP0499 – 2012) recommends less than 50 ppb dissolved oxygen less than 6 m/s flow velocity and for carbon steel lines predicts a five to seven year service life before repairs are needed. By understanding the interactions of the identified critical operating parameters an analysis of this type will enable the operator to optimize operating conditions inhibitor and biocide treatment protocols and ultimately extend the service life of water injection installations.
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