Page 134 - Practical Well Planning and Drilling Manual

Section 1 revised 11/00/bc 1/17/01 2:56 PM Page 110

[ ] Well Design
1.5.2

mode) surrounding the borehole. In a tectonically relaxed area, a ver-
tical well has equal horizontal stresses acting along the cross section
compared to a deviated well that will have unequal stresses (vertical
and horizontal) acting along the cross section. The stress concentration
at the borehole wall is higher in inclined wellbores, making it more
prone to collapse, and since deviated holes are likely to be open for
longer, there is more time for the wellbore to become unstable.
In an area where the horizontal stresses vary with direction (tec-
tonically stressed), a deviated well tends to be more stable when drilled
in the direction of the highest horizontal stress, and least stable when
drilled perpendicular to it. The higher the wellbore inclination, the
more pronounced the effect. If the surface location is not fixed then the
surface location could be relocated to allow an azimuth that gives the
best stability.
The optimum mud density in a tectonically relaxed environment
tends to increase with hole angle. In general, field experience indicates
an approximate increase of the mud pressure gradient by 2 ppg (0.11
psi/ft) between vertical and horizontal. In a tectonically stressed envi-
ronment this relation can be different.

1.5.2. Dogleg Severity Limits—Combined Buildup and Turn Rate

Dogleg severities have to be considered at this stage. DLS has to be
restricted (especially higher up in the hole) to avoid drillpipe damage,
fatigue, and casing wear. This impacts on the directional well design
because you will specify a kickoff point depth to reach a known target
position. How quickly you build angle will determine the final inclina-
tion (i.e., a faster build rate gives a lower inclination) and measured
depths of the casings.
This becomes more of a problem the deeper the well is. Say your
planned kickoff depth was 1500 ft. You initially want a build rate of 3˚
per 100 ft of drilled hole to a 30˚ inclination. Deeper down, the build
section is cased off. Drilling at 10,000 ft TVD (that would be at 11,212
1
MD) in 12 /4 in hole with 0.65 psi/ft mud, the drillstring tension at the
kickoff depth could be in the order of 180,000 lbs, using 5 in grade S
drillpipe and 8 in collars. (Refer to “Tension due to weight in a deviat-

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