Circulation Sub is a downhole tool to control flow between the pipe and annulus. Once opened, the downward flow through pipe will be split. Partial of the circulating fluid will flow through the sideway ports to the annulus between wellbore and pipe. This is especially useful in applications such as drilling in slim-hole wells, or wellbore cleanup operations, where different annular flow rates are required in different annular sections to reduce ECD or pipe pressure requirement.
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