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Influence of Experimental Injection Method on the Inhibition Efficiency of Volatile Corrosion Inhibitors for Top-of-the Line Corrosion Mitigation

Picture for Influence of Experimental Injection Method on the Inhibition Efficiency of Volatile Corrosion Inhibitors for Top-of-the Line Corrosion Mitigation
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In oil and gas industry, during the transportation of wet gas with a stratified flow regime, the temperature difference between the fluid inside the pipelines and the surrounding environment leads to condensation of water on the upper internal surface and causes metal degradation. This phenomenon is known as Top-of-the-Line Corrosion or TLC.


The condensing phases can consist of not only water but also condensable hydrocarbons.

Product Number: 51323-19452-SG
Author: Maryam Eslami, Marc Singer
Publication Date: 2023
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The extent of Top-of-the-Line Corrosion (TLC) can be reduced using volatile corrosion inhibitors (VCIs). The selection of the correct VCIs depends of course on the operating conditions (pCO2, pH2S, temperature, hydrocarbon co-condensation, organic acid concentration) and on the intrinsic properties of the inhibitor itself (water solubility, volatility, active components). Yet, the method used to inject the VCIs inside the producing pipeline could also be of practical importance, especially considering the complex environment of a multi-component condensing system. A concern among the related industries is the lack of a methodology that would allow for conducting experiments mimicking the field conditions and provide a guideline for extrapolating the experimental results to a larger scale. In this study, a methodology was developed to investigate the effect of injection methods on the inhibition efficiency of VCIs (model compounds decanethiol and hexanethiol). For this purpose, a system of two connected glass cells was designed enabling the injection of the VCIs into the system either through the liquid or directly into the gas phase. The experiments were conducted in the presence of a condensable hydrocarbon (n-Heptane) to investigate the partitioning effect. The results show that injection of the inhibitor through the gas or liquid phase has no influence on its inhibition efficiency, as long as the VCI is sufficiently volatile. Moreover, regardless of the injection method, the hydrocarbon phase interferes with the inhibition by decanethiol while it has no influence on hexanethiol. For a larger scale, it can be concluded that any length of pipe can be protected by an inhibitor injected through the liquid or gas phase if the inhibitor is an effective VCI.

The extent of Top-of-the-Line Corrosion (TLC) can be reduced using volatile corrosion inhibitors (VCIs). The selection of the correct VCIs depends of course on the operating conditions (pCO2, pH2S, temperature, hydrocarbon co-condensation, organic acid concentration) and on the intrinsic properties of the inhibitor itself (water solubility, volatility, active components). Yet, the method used to inject the VCIs inside the producing pipeline could also be of practical importance, especially considering the complex environment of a multi-component condensing system. A concern among the related industries is the lack of a methodology that would allow for conducting experiments mimicking the field conditions and provide a guideline for extrapolating the experimental results to a larger scale. In this study, a methodology was developed to investigate the effect of injection methods on the inhibition efficiency of VCIs (model compounds decanethiol and hexanethiol). For this purpose, a system of two connected glass cells was designed enabling the injection of the VCIs into the system either through the liquid or directly into the gas phase. The experiments were conducted in the presence of a condensable hydrocarbon (n-Heptane) to investigate the partitioning effect. The results show that injection of the inhibitor through the gas or liquid phase has no influence on its inhibition efficiency, as long as the VCI is sufficiently volatile. Moreover, regardless of the injection method, the hydrocarbon phase interferes with the inhibition by decanethiol while it has no influence on hexanethiol. For a larger scale, it can be concluded that any length of pipe can be protected by an inhibitor injected through the liquid or gas phase if the inhibitor is an effective VCI.

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TOL corrosion is reported to occur in large diameter wet gas pipeline in stratified flow conditions
due to low fluid velocities1. With increasing distance from the inlet, the wet gas pipeline becomes
cooler as it loses heat to the environment. Such cooling causes water, hydrocarbon, and other
high vapor pressure species to condense on the pipe wall. The upper part of the pipe will
constantly be supplied with freshly condensed water while the less corrosive water saturated
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Top of line corrosion (TLC) is a specific corrosion mechanism observed in the oil and gas industry. This phenomena occurs under stratified or wet-gas flow regimes when the upper internal pipeline walls are sufficiently cooled (by heat transfer to the surrounding outer environment), promoting local condensation of water vapor. Carbon dioxide (CO2) and organic acids dissolving into the condensed water generate a change in the solution chemistry, ultimately influencing the corrosion kinetics of the contacting carbon steel.
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The hydrocarbon exploration in the ocean and deep sea was started as early as early as the 1850s, when the first drilling was carried out in California, USA. Other early oil explorations activities were later recorded in Pakistan (1886), Peru (1869), India (1890) and Dutch East Indies (1893). The development of an offshore industry is directly related to the development of subsea pipelines as well. As the industry expands towards deeper waters, the pipelines are required to have better materials, designs, operation practices and maintenance strategies to withstand the challenging environments. These pipelines are exposed to elevated temperatures, high pressures, and corrosive fluids.
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