In 1995, the first horizontal sidetrack successfully drilled using coiled t
ubing (CT) was carried out at Shell's House Mountain 7. oilfield. The objec
tive was to drill underbalanced a 3 7/8-in. diam by 300-m-long horizontal h
ole section from an existing vertical, cased oil well. The existing well wa
s re-entered using a conventional drilling rig; a window milled in the casi
ng, and the build section drilled to an inclination of 90 degrees. 2 3/8-in
. CT was then used to directionally drill the horizontal section underbalan
ced with nitrified water. The zone of interest was a Devonian-age reef-edge
and limestone structure, at a depth of about 2200 m, called the Slave Poin
t.
A case history of directionally drilling the horizontal sidetrack using 2 3
/8-in. OD. CT is provided. Coiled-tubing drilling proved to be a cost-effec
tive alternative to drilling the horizontal sidetrack without using a conve
ntional drilling rig. Shorter trip times, underbalanced drilling, enhanced
rates of penetration (ROP), and the ability to maintain weight an bit were
all achieved. ROPs increased by up to three-fold while drilling underbalanc
ed.