Coiled Tubing Drilling (CTD) continues to play a dominant role in through-tubing sidetracking of wells on the North Slope of Alaska. Recently several CTD candidates were targeted in an over-pressured reservoir. Three wells were successfully CT sidetracked as a pilot project in this over-pressured reservoir. The ultra-slimhole directional drilling system −2.75″ nominal openhole diameter-developed at Prudhoe Bay1 was used in combination with an unconventional approach to maintain overbalanced conditions when sidetracking these wells.
Hydraulic modeling showed that with a kill weight drilling fluid, the calculated Equivalent Circulating Density (ECD) at the sand face could approach the regional fracture gradient. In addition, kill weight mud in the formulation 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 pressure was less than reservoir pressure, over-balanced drilling conditions were dynamically maintained by the high ECD while circulating, and controlled by increasing the surface pressure using a choke when circulating at less than drilling rates.