Page 323 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 299

2.9.1
Drilling Problems--Avoidance Planning [ ]

surrounding the borehole. In a typical case when the in-situ stresses are
S.Vertical > (S.Horiz max = S.Horiz min ), a vertical well has equal horizon-
tal stresses acting along the cross section, compared to a horizontal
well that has unequal stresses (S.Vertical and S.Horiz or S.Horiz )
max min
acting along the cross section. Consequently, the stress concentration
at the borehole wall is higher in case of the horizontal hole, making it
more prone to collapse. Field data indicate more problems in deviated
hole. Note that these are partly associated with increasing open-hole
time, dogleg severity, and hole-cleaning problems. Changing the stress
state around the wellbore certainly has an influence, but the magnitude
can only be ascertained if good estimates of the in-situ stresses (mag-
nitude and direction) are known. The optimum mud pressure in a tec-
tonically relaxed environment (S.Horiz = S.Horiz ) tends to
max min
increase with hole angle. In general, field experience indicates an
approximate increase of the mud pressure gradient by 2ppg (0.11
psi/ft) between vertical and horizontal. This relationship differs in a
tectonically stressed environment.
A deviated well tends to be more stable when drilled in the direc-
tion of the principal horizontal stress, and it is least stable when drilled
perpendicular to it. This may affect the surface location in severe cases.
Unequal formation horizontal stresses cause directionally prefer-
ential hole enlargement. This is why a 4-arm caliper should be rou-
tinely run; it allows evaluation of the severity of the directional
stresses. In severe cases, the major axis may be off-scale and the
minor axis in gauge.
Fracturing. Borehole fracture initiation pressure depends on the
borehole orientation. Therefore, leak-off pressure and formation
breakdown pressure are in part orientation dependent. The pressure
required for sustained fracture propagation and lost circulation is con-
trolled by the minimum stress and is therefore independent of hole
deviation and azimuth.
Drilling mechanics. The bottom hole assembly and drill pipe
scrape along the borehole wall possibly eroding (loose) damaged rock
material. A smooth assembly, such as a barrel shaped stabilizer, reduces
the scraping action.
During drilling, the BHA vibrates. In case of heavy lateral vibra-
tions, parts such as stabilizers and long unsupported drill collars may
hit the borehole wall, imposing substantial dynamic loads onto the

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