Qualification of Scale Inhibitors for Sub Sea Tie Backs with MEG Injection

Abstract The Ormen Lange field are using continuous injection of MEG to prevent hydrate formation. The MEG is regenerated and reused, and corrosion control of the multiphase pipelines are accommodated by a combination of pH stabilised MEG and corrosion inhibitors. In order to avoid scaling if formation water is produced a scale inhibitor will be added to the lean MEG. A comprehensive scale inhibitor qualification program was carried out, and the goal was to qualify one or more scale inhibitors for scale prevention downstream the MEG injection point at subsea wellhead for fully and partial pH stabilisation. 5 suppliers were invited to supply inhibitors to be included in the qualification programme and the suppliers submitted a total of 10 inhibitors. The qualification program included environmental properties, solubility in lean MEG, scale inhibitor screening tests, scale inhibitor dynamic tests, compatibility of chemicals, emulsion and foaming, partitioning, effect on particles formation and settling onshore, consumption/loss of scale inhibitor in the MEG process and stability in MEG regeneration reboiler. The different tests are described and the results are displayed. The results from the qualification program demonstrated that only few inhibitors satisfied all requirements, the chemicals affect the formation and precipitation behaviour of calcium and iron carbonate and that only one combination of scale and corrosion fulfilled all the requirements. If a scale inhibitor has to be used in combination with a corrosion inhibitor compatibility testing is required. Incompatibility between corrosion and scale inhibitor will cause gunking and scaling. A successful combination of corrosion and scale inhibitor will not increase the corrosivity and simultaneously prevent scaling. It has been observed that such combination of chemicals does not prevent formation of particles but prevents the particles to stick to the surface. The scale qualification program demonstrates the complexity in qualifying chemicals and the need for a good dialogue between the involved disciplines.