ABSTRACT

Electrochemical impedance spectroscopy (EIS) and electrochemical noise analysis (ENA) were used as combined techniques to study and evaluate the film persistency of several commercial batch treatment inhibitors which are used for protecting oil-wells, gas-wells or pipelines from C02 corrosion. It was found that the inhibitor film deterioration was accompanied by typical changes in the EIS spectra, which could be used as indicators Cormonitoring inhibitor film breakdown. ENA was shown to be able to continuously follow and monitor the inhibitor film deterioration processes. The noise resistance was confirmed to be strongly correlated to linear polarisation resistance (Rp) and this correlation was explained based on a concept called statistical linear polarisation. The presence of the hydrocarbon phase and CO2 corrosion product scale were found to be factors which greatly affect batch treatment inhibitor film persistency.

INTRODUCTION

Inhibition is the most cost effective and flexible means of corrosion control in the oil and gas production industry. There are different approaches to inhibitor application such as continuous treatment and batch treatment. Continuous treatment is a technique which simply injects inhibitor into oil or gas well fluid and keeps a certain inhibitor concentration in the fluid for preventing corrosion. However, in deep wells and in three phase gas pipelines batch treatment techniques are often used because of the difficulty of injecting m inhibitor continuously downhole and for the inhibitor to reach the top wall of the gas flowlines. With batch treatment of flowlines a concentrated solution of batch treatment inhibitor is drawn through the tubing between two pigs. In the downhole application the well is shut-in and the inhibitor allowed to drop through the column of gas; or oil and water. Alternatively, in gas wells the inhibitor may be applied by a brush. These processes allow the inhibitor to form a protective coating or film on the steel surface. The process needs to be repeated periodically because the inhibitor film is gradually removed by dissolution in the produced fluids flowing through the tubing.

Here a problem arises as to how often the batch treatment should be repeated. This is a question which is frequently asked everywhere batch treatment inhibitors are used. Inhibitor film persistency evaluation is essential for determining how often the batch treatment must be repeated and for optimizing the batch treatment; procedures. However, the current technology for assessing inhibitor film persistency is a subject of much discussion and, in many cases, disagreement.

In the case of continuous treatment inhibitor, EIS has been shown to be a usefhl technique for studying inhibitor film formation and destruction processes and for evaluating inhibitor film persistencfl?5,G. ENA was also shown to be a very convenient technique for continuously monitoring the corrosion process and inhibitor petiormance7. However, little attention has been paid to the possible application of EIS and ENA in the study of batch treatment inhibitors. EM has been used successfully in various corrosion and protection fields such as organic coatings studies, passive layer analysis and corrosion inhibitor evaluation. ENA, however, is still an empirical and developing technique, although it hw been shown to be a useful tool in corrosion rate measurement and also in localised corrosion studies. For example, the similarity between the noise resistance and polarisation resistance is still controversial and the meaning of this similarity has not been clearly explained.

This work will investigate the possibility of using EIS and ENA to monitor batch treatment inhibitor

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