Impact of Methanol on the Scale Deposition – a Case Study from West Africa

Abstract The introduction of methanol to control hydrate risk presents new challenges in terms of increased scale tendency (dehydration of brine) and interactions of the hydrate control chemicals with scale inhibitor performance. The use of methanol for preservation during the full field shutdown in a West African field has shown the increase of the scaling risk of both calcium carbonate (CaCO3) and barium sulphate (BaSO4). This study highlights the challenges faced on field during full field shutdown (FFSD) where after start-up, mineral deposits have been observed at subsea and topside. A full understanding of the evolution of the production water chemistry through the field life cycle has been carried out, including the analysis of scale deposits and supersaturation changes, has been developed. A detailed scaling risk assessment has been performed with and without the presence of methanol, to assess the impact of this hydrate inhibitor on the formation and deposition of calcium carbonate and barium sulphate scales. The scaling risk evaluation has shown the same risks as the previous study carried out in 2019: High scaling risk for calcium carbonate at topside. The use of methanol for preservation during the full field shutdown, carried out in June 2021, has shown the increase of the scaling risk of both calcium carbonate and barium sulphate scales at the risers, the control valve downstream, 1st stage separator, the strainer upstream crude oil transfer pumps and wash tanks. A full monitoring and treatment recommendation is presented in this paper. The impact of methanol on the increase of the scaling risk, using a thermodynamic model has also been discussed. This study outlines the process by which the risk assessment of scale formation and deposition in the presence of methanol was made and how the scale control measures for effective scale management of subsea installation has been carried out. This feedback is intended to be used further on, during the production chemistry studies.