Bullheading of methanol into deepwater wells is a commonly used method to protect wells from hydrates during long shutdowns. A safe, but conservative wellbore protection approach (based on wellhead cooldown alone), was adopted for many early deepwater wells. This bullheading approach generally required large bullheading volumes (often exceeding 50 bbl) for each well. However, for facilities with an increasing number of wells or with platform storage and weight limitations, this conservative approach may create significant operational problems, including crude and water quality concerns (due to the presence of methanol in the stream). Hence, in these cases, alternative operating strategies to minimize methanol usage are needed.This paper describes an optimal bullheading strategy based upon detailed transient assessment of cooldown times of the entire wellbore, as well as upon evaluation of gas-oil-water and gas-liquid segregation boundaries. Bullheading guidelines are developed that include methanol volumes and bullheading schedule, depending on well fluids, wellbore shut-in conditions, and production conditions prior to shut-in. This paper focuses on bullheading strategies for subsea wells; while not explicitly analyzed, the same approach can be applied to direct vertical access (DVA) wells. The paper will also address bullheading differences for subsea wells with uninsulated tubing and wells with vacuum insulated tubing (VIT).Experimental results and models on oil, water, and gas segregation in the wellbore are discussed. Field data are shown for comparison with the simulation results.
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