Abstract

The inter-relationship between scale and corrosion and their control in oil field production systems has been well documented in relation to increased localized "under deposit" corrosion and the potential for detrimental performance interferences between scale and corrosion inhibitors. The impact of various chemical interferences (impact on corrosion inhibitor performance by the presence of scale inhibitor) is routinely assessed in laboratory pre-screening tests. However, although the importance of iron carbonate scales and other passivating scales is well documented, the impact on corrosion control due to the deposition of more conventional inorganic scales and their removal is not generally assessed in detail. In addition, the presence of scaling ions in test brines can significantly impact the recorded corrosion rates in conventional electrochemical tests owing to the build up of deposits on the various electrode surfaces. Omitting the scaling ions can lead to a dramatic change in test conditions (for example much lower test pH's in CO2 saturated systems if bicarbonate ions are omitted from the test brine) therefore reducing the appropriateness of the test.

The work reported describes a range of laboratory tests conducted to assess the impact of inorganic precipitates (scales) and their removal on both inhibited and uninhibited corrosion rates. Initial work summarises the impact of the inclusion or exclusion of scaling ions from test brines on the corrosion rates recorded using conventional electrochemical laboratory techniques. Further tests are then reported which examine the impact on general corrosion rates and corrosion inhibition using partially pre-scaled samples and those in which scale deposits have been chemically removed.

In summary the presence of scale deposits can have a significant impact on the recorded corrosion rates and the performance of conventional corrosion inhibitors which may have important implications for chemical screening.

Introduction

The inter-relationship between inorganic scale and corrosion and their control in oil field production systems has been well documented. This relates not only to the protective or passivating surface films which form as a result of the initial corrosion process, but also to the formation of other, more conventional inorganic scales which are routinely controlled by the use of scale inhibitor chemicals. In addition to the protective nature of such scales, the potential for under deposit corrosion associated with the presence of scale coatings has become an area of increasing concern in the oil and gas production industry. This can lead to increased localized corrosion rates, especially in the presence of trace levels of oxygen such as in water injection systems. Furthermore, a strong inter-dependency is also often recorded when examining methods for the mitigation and control of both scale and corrosion especially when chemical inhibition is used.

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