Page 85 - Hydrocarbon Exploration and Production Second Edition

72 Casing and Cementing

Figure 4.23 Coiled tubing drilling unit.

4.5.8. Coiled tubing drilling
A special version of slim hole drilling has emerged as a viable alternative: coiled tubing
drilling (CTD) (Figure 4.23). Whilst standard drilling operations are carried out
using joints of drill pipe, CTD employs a seamless tubular made of high-grade steel.
1
3
The diameter varies between 1 and 3 in. Rather than being segmented, the drill
4 2
string is reeled onto a large-diameter drum.
The advantages of CTD are several:
nearly no pipe handling
better well control allows at-balance or even underbalanced drilling, resulting in
higher penetration rates and reduced potential for formation damage
less environmental impact
lower cost for site preparation, lower day rates, lower mobilisation and
demobilisation costs
easier completion by using the computerised tomography (CT) as a production
string.
However, CTD is limited to slim holes, and the reliability of some of the drill
string components such as downhole motors is still improving. Presently, the cost of
building a new customised CTD rig limits the wider application of this technology.

4.6. Casing and Cementing

Imagine that a reservoir exists at a depth of 2500 m. We could attempt to drill
one straight hole all the way down to that depth. That attempt would end either