Abstract

The pH-stabilisation method is a cost effective, reliable and environmental friendly way to control corrosion in gas/condensate pipelines where glycol is used as hydrate preventer. Increasing the pH to 6.5–7.5 facilitates the formation of a protective iron carbonate film on the steel surface and reduces corrosion of the steel.

The carry-over of formation water puts limits on the use of the pH stabilisation technique as carbonate scale may form. Scale formation in glycol carrying pipelines is not easy to predict, as solubility data for CO2 and scale forming compounds in glycol solutions are very scarce. Glycol affects scale formation by changing the solubility and the precipitation kinetics, and the solubility of for instance calcium carbonate is markedly reduced.

The paper reviews the experience from the fields that have been pH stabilised and discuss the various pH-stabilisation methods that have been used. The paper gives data on the application limits with respect to formation water and discuss operational problems, challenges and solution related to the risk for scaling downstream the lean MEG injection point, in heat exchangers and in the MEG regeneration and reclamation systems.

The pH stabilisation technique

Carbon steel pipelines are the only cost effective alternative for long distance transportation of unprocessed gas/ condensate. The water that condenses in such pipelines has a low pH; the corrosion rates are therefore high unless a layer of protective corrosion products or a corrosion inhibitor film covers the steel surface.

At sweet conditions (no H2S) the main corrosion product is FeCO3. Raising pH above 6 increases the CO32- concentration and reduces the amount of dissolved iron needed to precipitate FeCO3 considerably. Thus, pH stabilisation is simply a technique to raise the pH in the aqueous phase in the pipeline to a level where FeCO3 is stable and precipitates easily. The amount of alkalinity needed to raise the pH depends on the CO2 (and H2S) fugacity, but is usually in range 0.01 to 1 mole per kg aquous phase. Figure 1 shows a fully developed film together with films under development1. When a dense film forms, transport of reactants and corrosion products through the film govern the corrosion rate, which becomes much lower than predicted by worst-case corrosion models. Film properties like porosity, thickness and composition are important. Such properties are strongly related to the precipitation process and depend on supersaturation and temperature. As the solubility of FeCO3 decreases and the precipitation rate increases with increasing temperature, protective carbonate films develop faster at higher temperature. Film formation and corrosion rate also depend on CO2 partial pressure, nature and amount of hydrate preventer, flow velocity, pH, the condition of the steel surface and how the pipeline is operated. As an example the presence of mill scale and rust on the steel surface often enhance film formation and reduce the corrosion rate. Periods without flow (shut down) and draining of the pipeline can have the same effect. All parameters must be assessed when the possible application of carbon steel for a new development is evaluated1,2.

The pH-stabilisation technique is now also considered for sour fields, which contain both H2S and CO2. Introductory experiments have shown that small amounts of H2S (a few ppm in the gas phase) facilitate protective film formation. The composition of the protective film is then a mixture of iron carbonate and iron sulfide. At higher H2S partial pressure only iron sulfide forms. It is not obvious that predominantly iron sulfide films will give the same protection as the iron carbonate films, and work is currently going on in the authors' laboratories to determine the application limits for the pH-stabilisation technique in wet gas pipelines containing mixtures of H2S and CO2 with and without hydrate preventers.

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