Low Riser Return and Mud-Lift System

摘要

A novel drilling system is presented that allows for longer hole sections to be drilled in deep waters,rnthus reducing the number of casing strings required in the well. The main element in the system is arnsubsea mud pump connected to a high pressure drilling riser, taking returns from a lower level in thernriser. The mud level in the riser is dropped down to a level considerably below sea level, where thernmud/air interface can be continuously adjusted up or down by the mud-lift pumping system. As arnconsequence, the bottom hole hydrostatic pressure will also be controlled. The system offersrnincreased flexibility, i.e., the need to adjust mud-weights is reduced and ECD effects can in manyrnways be neutralized. The system is based on using a smaller size (14” - 12,5” ID) drilling riser, whichrnreduces the volume of drilling fluid to be handled and stored on the rig, reduces the riser tensioningrnrequirements and ultimately the deck load requirements. This implies that lower specification drillingrnrigs may be used in deep waters. Further, the system can be used for drilling of severely depletedrnreservoirs and for in-balance and under-balanced drilling operations. This drilling riser system offersrnseveral advantages when selecting completion strategies. Used on floating production and drillingrnunits, preliminary investigations point to considerable weight and space reductions, hence lowerrnCAPEX can be anticipated if such a system is chosen. The potential well cost reduction will be in thernorder of 30 – 50 % in the GOM. For normally pressured deepwater areas, the Low Riser Return andrnMud-lift System (LRRS) will offer added benefits in that more attractive completion and reservoirrndrilling strategies can be implemented.