This paper describes measured and simulated downhole pressure variations("surge and swab")during drillpipe connections when drilling an ultra-deepwater well offshore Brazil on the Carcara field.Floating rigmotion caused by waves and swell("rig heave")induces surge and swab when the drill string is suspended inslips to make up or break a drill pipe connection and topside heave compensation is temporarily deactivated.This is a known issue in regions with harsh weather such as the North Sea,where pressure oscillations of upto 20 bar have been reported during connections.Recorded downhole drilling data from the Carcara fieldreveals significant pressure oscillations downhole(in the same order of magnitude as in the North Sea)eachtime the drill string was suspended in slips to make a connection in the sub-salt 8 1/2"section of the well.Mudlosses were experienced around the same well depth and they might have been caused by surge and swab.Measured surge and swab pressure variations have been reproduced in an advanced proprietary surgeand swab simulator that considers rig heave,drill pipe elasticity,well friction,non-Newtonian drilling mud,well trajectory and geometry.Moreover,findings in this paper suggest that surge and swab was in factsignificantly higher than recorded by the MWD(Measurement While Drilling)tool.The true magnitude ofsurge and swab is not captured in the recorded MWD data due to low sampling frequency of the downholepressure recording(one measurement every six seconds,a standard downhole pressure sampling rate usedon many operations today).This work shows that surge and swab during drill pipe connections on floaters may challenge the availablepressure window for some wells even in regions with calm weather such as Brazil.Managed PressureDrilling(MPD)is a technique that improves control of the downhole pressure.It is,however,not possibleto compensate fast downhole pressure transients,such as heave-induced surge and swab,using MPD choketopside.This is due to the long distance between the choke and the bit,which translates into a time delayin the same order of magnitude as typical wave and heave periods.A downhole choke combined withcontinuous circulation is one of potential solutions.Surge and swab during drill pipe connections can result in a loss or an influx and should be consideredin the well planning phase when mud weight,section lengths,etc.are selected.
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