Page 141 - Practical Well Planning and Drilling Manual

Section 1 revised 11/00/bc 1/17/01 2:56 PM Page 117

1.5.4
Directional Design [ ]

greater than 90° inclination gives you problems in attaining and
maintaining angle. There are also adverse well control implications
since gas bubbles may be impossible to circulate out of the hole.
Finishing just below horizontal would be much better.
3. Computer modeling can be used to predict likely drags and
torques. Sometimes a small change to the wellpath can make a sig-
nificant difference in drags. The final design should be analyzed to
ensure that it is possible to drill and trip with the rig and drillstring
in use. Minimizing hole drags will increase the chances of drilling
and casing the well successfully.
4. It is common (and recommended) to build to horizontal in the
reservoir, set casing and then drill ahead for the horizontal. This
protects the buildup section and allows isolation of the reservoir
from the formations above. If the completion does not call for
cemented liner to be run (i.e., slotted or prepacked liner is
planned) then placing the production casing in the reservoir allows
isolation from higher zones. Further, having steel into the reservoir
should make it easier to run prepacked screens while minimizing
potential damage to the screens.
5. When aiming to hit a narrow depth range that is small in propor-
tion to the accuracy of depth estimates, consider drilling a pilot
hole through the reservoir first, logging it, and then plugging back
to kickoff to horizontal. This will be far cheaper than building up
close to horizontal, going slightly out in relation to target depth,
then having to either drill many feet to get back on track (if high),
or trying to build too fast causing a dogleg and possibly going over
90˚ (if low).
6. Directional measurements come from some distance behind the
bit. If it is necessary to drill along a certain section of the reservoir
and “follow the geology” rather than by hole angle alone, some
means of identifying location in the reservoir is needed at or close-
ly behind the bit. This technique is called “geosteering,” where
LWD tools are used to measure formation parameters that allow
real-time decisions for steering the bit. Resistivity and gamma ray
measurements will show the distance from shale (cap rock) so the
drill bit can be steered along the top part of the reservoir with rea-
sonable confidence. Since new LWD techniques are being devel-
oped all the time, it is important to determine which available tech-
nique suits the characteristics of the target reservoir.

117