Abstract

Based on a review of both literature and field data, it is apparent that the role of acetic acid (HAc) in oilfield brines is both extremely complex and somewhat controversial. Although it is commonly believed that the presence of this organic compound enhances both the general and the localized corrosion rate of carbon steel, HAc has recently been reported to also act as a weak inhibitor in specific aqueous environments. These observations prompted a study into whether such behavior is apparent in a CO2 top of line corrosion (TLC) scenario i.e. when HAc dissolves into condensed water which forms on the upper internal wall of carbon steel pipelines during wet-gas or stratified flow. Four different water condensation rates/temperature TLC conditions were selected to investigate the role of HAc on both the kinetics and mechanism of carbon steel dissolution. A miniature three-electrode setup was developed to characterize the real-time TLC response through the implementation of electrochemical measurements. Surface analysis techniques (microscopy and profilometry) were performed to complement the electrochemical results. Collective consideration of the corrosion response and condensate chemistry indicates that similar effects were observed compared to those reported in the literature in bulk aqueous environments, in that the introduction of HAc can result in either an accentuation or diminution in corrosion rate depending upon the operating conditions. In fact, a decrease in corrosion rate was reported at a surface temperature of 20.5°C and water condensation rate of 0.5 ml/m2.s despite a reduction in condensate pH being generated due to HAc presence.

Introduction

Top of line corrosion (TLC) is a specific corrosion mechanism observed in the oil and gas industry. This phenomena occurs under stratified or wet-gas flow regimes when the upper internal pipeline walls are sufficiently cooled (by heat transfer to the surrounding outer environment), promoting local condensation of water vapor. Carbon dioxide (CO2) and organic acids dissolving into the condensed water generate a change in the solution chemistry, ultimately influencing the corrosion kinetics of the contacting carbon steel.

In the oil and gas industry, the majority of hydrocarbon reservoirs produce both acid gases and volatile organic acids in conjunction with formation brine and hydrocarbons. These gases and volatile compounds create highly corrosive condensate chemistries in the context of carbon steel pipelines.

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