Environmental Factors Affecting Corrosion Inhibition of Wet Gas Pipelines With an Oil Soluble Inhibitor
Authors: Achour M. , Stuever W.A. , Buck E. , Kolts J. 
Published: October 2008

Some factors affecting the corrosion inhibition of steel in CO2 containing environments were examined with a single oil soluble inhibitor to determine the limitations for use in a given wet gas pipeline environment. Corrosion inhibition was not strongly influenced by temperature, or CO2 partial pressure. It was strongly influenced by the hydrocarbon phase, inhibitor concentration, and mechanical mixing of oil and water phases. Inhibitor films demonstrated film persistency, lasting at least two weeks.

INTRODUCTION

One factor of importance to the petroleum industry is the increasing confidence in the design and construction of multi-phase wet, gas pipelines1–4, where water, gas condensate and gas are transported. Corrosion protection in these multiphase pipelines is provided by continuous corrosion inhibition. For offshore production, this eliminates the need for weighty offshore dehydration equipment, and often permits non-manned platforms to serve as the primary offshore production facility, increasing safety and decreasing development costs. Such designs permit onshore processing facilities, which can be operated without space or weight restrictions.

Internal corrosion of gas pipelines is an important issue. New pipeline design must account for corrosion control which can be achieved by inhibition, coating, dehydration, or construction from corrosion-resistant alloys. Detailed cost analyses are conducted for alternative methods of corrosion control in all new pipeline projects, and corrosion inhibition of wet gas pipelines is often the most cost effective method of corrosion control.

Internal pipeline environments in newly developed fields are becoming increasingly severe as higher concentrations of CO2 are being considered for transmission in multi-phase wet gas pipelines. The limits of temperature, CO2 partial pressure or fluid flow conditions under which corrosion inhibition can successfully protect these wet pipelines need to be established. Present pipeline design factors5–7 for assessing the limits of corrosion inhibition may be excessively conservative. Thus, corrosion inhibition may be unjustifiably discarded as a technically viable option in preference to more expensive alternatives.

Another issue of importance is life prediction. Multi-phase wet gas pipelines throughout the world have provided useful service. As the service is extended, internal corrosion (inhibitor effectiveness) becomes one of the many factors which might limit the remaining life of the pipeline. The effectiveness is influenced by multiple variables depending on the type of corrosion inhibitor, corrosive environment, method of application, type of pipeline service, and fluid flow conditions. Therefore, evaluation of the effectiveness of corrosion inhibition will require examination of the pipeline topography, fluid flow characteristics, operating practices, and inhibition application methods.

The corrosion processes examined in this investigation included a laboratory study of corrosion in the vapor phase to simulate the top of the pipeline, film persistency of an inhibitor to simulate sphering and interruption of inhibitor injection, inhibitor effectiveness at high partial pressures of CO2, and inhibitor transport mechanisms of an oil-soluble inhibitor.

This work was performed to assist in predicting inhibitor performance under the many different conditions that a single multi-phase pipeline experiences.