Page 133 - Practical Well Planning and Drilling Manual

Section 1 revised 11/00/bc 1/17/01 2:56 PM Page 109

Directional Design [ ]
1.5.1

Wellbore profile. In the past, well profiles often built up, held
tangent for a while, then dropped off into the target (S profile). A
simple build and hold to target (J profile) is often better than an S
profile because:

1. An S profile is more complex with more directional work (rigtime,
rental tools expense).
2. More hole needs to be drilled.
3. The drop-off part will restrict your WOB, therefore, ROP will be
less. Also, rotary drop-off BHAs will lose some directional control
(due to their flexibility) and motor assemblies with bent subs
sometimes tend to “flip over” when orientated to drop. This is
worse when run lower down due to the length of flexible drillstring
above the BHA.
4. Maximum inclination is higher and there are more doglegs, with
increased hole drags. Good cement jobs are harder to achieve.
Wellbore stability becomes more of a problem at higher inclina-
tions. Wear on casings and drillstrings is increased. Fatigue may
become a problem.

The S well may be called for if:

1. Intermediate targets are specified, which force you to adopt this
path.
2. The target is offset and the wellbore needs to be close to vertical
through the reservoir.

The wellbore directional profile has a major influence on the
torques and drags encountered during drilling, as well as the drags that
occur while running logs and casing. In high-angle wells or those with
significant changes of azimuth as well as inclination, the frictional
forces can cause serious problems. Mud lubricating additives and non-
rotating protectors can be used to somewhat reduce torques and drag.
A computer program can also help to tune the wellpath to reduce fric-
tional forces; the directional company should have access to this kind
of software so as to optimize the wellpath.
In-situ field stresses. The combination of in-situ stresses and hole
orientation govern the tendency to destabilize the rock (collapse

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