| 摘要 |
A downhole circulation apparatus comprises at least one tubular body portion defining a first through-flow passage through which drilling fluid can pass along the drill string. A first sleeve is slidably mounted in the tubular body portion and is biased into a position in which the first port means is closed by a biasing means such as a spring. Second port means is formed through the tubular body portion at a location below the first port means in the drill string. The second port means may comprise a plurality of second nozzles, wherein each of the second nozzles has a diameter greater than the diameter of each of the first nozzles of the first port means. Larger nozzles enable large particle size drilling fluid to be bypassed. A second sleeve is slidably mounted inside the tubular body portion.
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