Controlled mud-cap drilling for subsea applications: Well-control challenges in deep waters

This paper describes a new drilling-riser concept and drilling methodology for deepwater operations that will remove some of the well-control challenges and limitations currently experienced when handling kicks and deep gas influxes in deepwater regions, with the following results: Providing improved and more flexible well-control procedures. Reducing the potential of hydrate plug formation during well-control operations. Allowing for drilling longer hole sections than normally considered feasible when using conventional drilling methods, thus reducing the number of casing strings required in the well. Allowing for improved drilling performance in depleted formations The main elements in the system are based on using a small, high-pressure drilling riser [14-in. outer diameter (OD), 12.5-in. inner diameter (ID)] with a split surface/subsea blowout preventer (BOP) and a subsea mud-lift pump connected to the drilling riser and a separate mud-return line. During drilling and well-control operations, the mud level in the riser is maintained considerably below sea level to create a mud/air interface (i.e., a "mud cap") that can be continuously adjusted up or down by the mud-lift pumping system. As a consequence, the bottomhole hydrostatic pressure will be controlled. One of the main purposes of this system is to mitigate the inherent problems with a conventional 21-in.-OD marine drilling riser during well-control scenarios in deepwater operations. The system will compensate for frictional pressures resulting from circulation and adjust the bottomhole pressure (BHP) accordingly.