Dynamically overbalanced coiled-tubing drilling on the north slope of Alaska

Coiled-tubing drilling (CTD) continues to play a dominant role in through-t ubing sidetracking of wells on the North Slope of Alaska. Recently several CTD candidates were targeted in an overpressured reservoir. Three wells wer e successfully CT sidetracked as a pilot project in this overpressured rese rvoir. The ultraslimhole directional-drilling system with a 2.75-in. nomina l openhole diameter developed at Prudhoe Bay(1) was used in combination wit h an unconventional approach to maintain overbalanced conditions when sidet racking these wells. Hydraulic modeling showed that with a kill-weight drilling fluid, the calcu lated equivalent circulating density (ECD) at the sand face could approach the regional fracture gradient. In addition, kill-weight mud in the formula tion required for CTD operations would be a significant expense. To address these concerns, the wells were CT sidetracked using a less expensive under balanced drilling-fluid system. Although the drilling-fluid hydrostatic pre ssure was less than reservoir pressure, overbalanced drilling conditions we re maintained dynamically by the high ECD while circulating, and controlled by increasing the surface pressure using a choke when circulating at less than drilling rates.