ABSTRACT

This paper deals with the practical perspectives of corrosion inhibition in production operations. To this end examples of inhibition of sweet gas wells, wet gas pipelines and flowlines, and oil pipelines are discussed and challenges are identified. It is shown that a number of common problems can be avoided by proper inhibitor selection and qualification tests. Other key factors of equal importance are a proper economic evaluation to support the choice of corrosion inhibition as the first line of defense and a careful design of the system, including verification of the corrosion mechanism and injection hardware design. It is pointed out that inhibition will be successful only if the operators are engaged at an early stage and the procedures are included in the maintenance plan.

INTRODUCTION

Shell has a long and varied experience with the application of corrosion inhibition in its upstream operations. The most extensive experience has been gained with gas wells [1]. Pipeline corrosion inhibition has historically been less popular, because the majority of the lines were operated dry. Lines in wet conditions were typically made of corrosion resistant alloys such as duplex stainless steel. Exceptions are some inter field and intra field lines transporting crude oil, which have been protected by corrosion inhibition for many years. In recent years, there has been a marked shift towards wet operation of gas lines. These lines now tend to rely on corrosion inhibition by default as long as the operating conditions are not too challenging. The limits of the conditions that are deemed suitable for wet operation are gradually stretched [2].

To enable the use of corrosion inhibition as the first line of defense against corrosion, inhibitor selection and testing procedures have been refined during the past years [3]. In addition to this, realization has grown that management of the corrosion inhibition program is essential to ensure long term integrity of the systems in question [4].

In all cases the inhibitors are film forming organic compounds, such as quaternary amines, imidazolines, or phosphate esters. The tests [3] performed by the vendors and by us do not directly address the molecules. They simply test the performance in terms of corrosion inhibition effectiveness and check whether unfavorable side effects occur. Also, the solubility behavior and partitioning in the phases present in the lines are determined to ensure that the inhibitor ends up in the right place, i.e. where the steel is exposed to a liquid water phase.

This paper gives examples of corrosion inhibition, in which the author was involved, either directly or indirectly. Successes are highlighted and pitfalls are noted and discussed. The necessary steps for the design and implementation of a successful corrosion inhibition application are listed.

EXAMPLES OF INHIBITOR APPLICATIONS

Sweet gas well inhibition

Corrosion rates in sweet gas wells are typically too high to be handled with carbon steel completions. Adequate models are available to assess the expected corrosion rates [5 - 8]. In the early days of gas well production this was not fully appreciated. Corrosion inhibition thus first came about to deal with this problem and for many years became the standard practice. Both continuous injection and batch treatments were introduced and are used to this day.

Inhibitor selection for continuous injection is not considered a big challenge for this type of application. Gas wells typically operate in an annular dispersed flow regime. Thus condensate soluble inhibitors are readily transported to the exposed steel surface, where they tend to perform well. The main challenge lies in

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