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.