In deepwater subsea oil and gas production, methanol is commonly used for preventing gas hydrate formation. Other than some recent laboratory experiments on the effect of methanol on mineral scale precipitation,1&2  there however have been no field reports of methanol-induced or aggravated mineral scale deposition in the oilfield produced fluids. This paper presents a unique and severe calcium sulfate (gypsum) scale deposition problem in the Canyon Express field system, a deepwater subsea tieback in the Gulf of Mexico, and its successful inhibition by using methanol soluble scale control products. In this case, the gypsum scaling was found to be directly caused by continuous injection of large volumes of methanol into the produced fluids for hydrate inhibition. This paper describes and discusses this particular scale problem, its diagnosis, computer prediction of methanol effect on scale formation, qualification of a methanol soluble combination scale/corrosion inhibitor, and the subsequent successful inhibition treatment of the gypsum scale. This paper highlights the drastically detrimental impact of methanol on scale formation and demonstrates the benefits of using methanol soluble scale control products that reduce the need for additional subsea injection lines and eliminate the potential for chemical incompatibility.

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