MWD&LWD and Stethoscope Applications for Subsea Wells

Abstract

Pore pressure modeling and understanding has great importance while drilling a wild cat well in deep offshore. Not only given fracture and leak off margin is so small, but also keeping well in optimum overbalance condition are vital for the safety. After BP’s Moconda incident in the Gulf of Mexico, now understanding of pore pressure regime as well as nature of the high pressure fluid while drilling becoming more and more critical especially in deep water where the riser and well control devices are apart few hundred meters. In addition deep water environment play its critical role being young deposits and buried not deep from the sea bed, and leak off margins are most of the time in “wellbore breathing” range. Phenomenon variously called “Breathing wellbore” or “Ballooning effect” is a result of slow mud returns while drilling ahead followed by mud returns after the pumps have been turned off, such as during a connection or flow check. Usullay any flows during these periods are cause for a concern as they may be due to influx of formation water, liquid hydrocarbon or gas. Any influx from the formation can result in a well control problem, the magnitude of which is dependent on its volume and composition. On the other hand lithologies under deepwater conditions usually show relatively reduced effective stress, due to reduced lithological column. This translates into narrow mud weight windows, driven mainly by shear failure or pore pressure in over pressured conditions, and by minimum horizontal stress gradients. Drilling operations should consider wellbore collapse, kick and losses as the primary hazards. These should be investigated and predicted during well planning, and should also be appropriately monitored during drilling.