An Evaluation Of Corrosion Monitoring Techniques For Measuring Corrosion Inhibitor Performance In Sour Gas Gathering Systems
Authors: Myers Ronald D. , Cameron Glenn R. , Cameron D.B. 
Published: March 2010

Amoco Canada Petroleum Ltd. operates approximately 50 sour gas wells in the Fox Creek area of Alberta. Historically, anodic polarization probes and corrosion inhibitor residual measurements have been utilized to monitor corrosion inhibitor performance and to verify system inhibition. In this study, a wellhead test loop was installed to compare various corrosion monitoring techniques and to determine the effectiveness and cost performance of several new corrosion inhibitors.

Anodic polarization was useful for measuring relative corrosion rates and relative inhibitor performance. In view of the differences observed between anodic polarization measurements and corrosion coupons, the former technique should be used in conjunction with corrosion coupons, particularly when evaluating new corrosion inhibitors. In the test protocol, electric resistance probes were not useful for short-term inhibitor evaluation, although they warrant further investigation as a long-term monitoring technique in systems where the inhibition criteria have been previously established. The hydrogen probe was not sensitive or responsive to changes in corrosion inhibitors, or inhibitor injection rates.

Although the exposure time of the corrosion coupons was typically six to eight days, the polished rod coupons provided valuable metallurgical response to the various inhibitors tested. Finally, corrosion inhibitor residuals were useful for verification of inhibitor injection rates and correlated well with the optimum concentrations determined by laboratory studies.

Introduction

Corrosion History

Production from the Kaybob South Beaverhill Lake gas field began in 1968, with initial projections for low water production. However, severe water coning occurred immediately and water production rates were much higher than originally anticipated. As the water rates increased, so did the chloride content of the produced water. The combination of these two factors resulted in several internal corrosion problems. The resulting failures were caused by isolated pitting attack in the bottom quadrant of the pipeline in low velocity areas, and in low elevation areas where produced fluids accumulated.

The design of the gas gathering system precluded any form of pigging or detailed line inspection using smart pigs. Consequently, the development of an effective water soluble or highly water dispersible corrosion inhibitor was required to control internal corrosion in the gas gathering system. Initially, control of the chemical injection program was extremely difficult due to two factors a continually increasing water production rate and infrequent well testing, as well as limited access points for obtaining water samples for inhibitor residual testing.

Line failures continued to be a problem until an extensive testing program was initiated, which provided the information required to predict and set maximum flow rates to stabilize water production. A water soluble/dispersible inhibitor was developed during the same period and the combination of these two factors resulted in the successful control of internal corrosion problems in the Kaybob South gas gathering system.

Throughout the life of the field, the following factors have significantly contributed to the development of a successful corrosion inhibition program.Extensive well testing allowed ongoing monitoring and projection of water production rates.The development of a corrosion inhibitor which exhibited significant water solubility dispensability.