Abstract

Transportation of wet (water containing) gas and condensate in carbon steel pipelines requires corrosion control. By increasing the pH to a level where FeCO3 precipitates as a protective film on the iron surface the corrosion rate can be reduced to a low level. This method, called pH stabilization, is primarily used for carbon steel pipelines with no co-production of formation water and where glycol is applied as hydrate inhibitor. Increasing the pH promotes precipitation of carbonates of Ca2+, Ba2+ and Sr2+. To prevent excessive scaling, scale inhibitors might be added. The effect these scale inhibitors have on the corrosion rate of steel was the main focus for this study. Two scale inhibitor products were tested. Both products increased corrosion at high pH by preventing growth of protective carbonate films. The scale inhibitors also affected FeCO3 film formed prior to inhibitor addition. In addition they reduced the corrosion rate as such. The resulting corrosion rate was lower than the rate without carbonate film at the same pH, but higher than the rate with protective film. Overall, the corrosion rate was too high.

Introduction

Carbon steel pipelines are often the preferred solution for long distance transportation of unprocessed well stream [1]. Such pipelines are usually subject to CO2 (and H2S) corrosion and need protection. One option for corrosion control is pH stabilization. The technique is used in combination with glycol as hydrate inhibitor and has proven to be effective in gas/condensate fields even when the CO2 pressure is very high (10 bar) [2,3]. The use of pH stabilization is limited due to the risk of carbonate scale formation at high pH levels when formation water is produced. Scale inhibitors are widely used to avoid scaling downhole and the possibility of using them in pipelines is also exploited. Before scale inhibitors are used in pipelines with pH stabilizing action their interaction with the corrosion must be investigated.

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