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3.8 Risks 157

P mud >P pf

Borehole Borehole
Pipe Pipe

Filter cake

Formation Formation
(a) (b)

Figure 3.24 (a) Situation without filtercake and (b) stuck
pipe along the borehole wall in a permeable formation with
filter cake. (Modified after Schlumberger Oilfield Glossary
2009.)

overbalanced and the drilling mud forms a thick filtercake at the borehole wall.
The differential pressure presses the DCs into the filtercake, and if the contact
area is large enough the string gets stuck due to the high-contact forces. However,
overbalanced drilling may be necessary in permeable sediments, for example, if
gas influx needs to be kept out, but bears a high potential of differential sticking
(Figure 3.24a and b). Differential sticking mainly occurs during stagnancy of
rotary, for example, if a new pipe joint is added to the drillstring.
• Thermally induced stress on borehole wall and/or casing/cement (due to drilling
process): A mud which is colder (hotter) than the formation, will reduce (increase)
both pore pressure and hoop stresses (that are the tangential stresses on the
borehole wall). Consequently, cooling the mud should have beneficial effects on
borehole stability, whereas heating would have negative effects. As regards the
temperature/pore pressure coupling, this becomes negligible for rocks having
permeability greater than 10 µD (microDarcy).

3.8.4
Geological Risks

Geological risk arises from geological uncertainties or if the rock mechanical
reaction of certain lithologies to the drilling process is insufficiently considered.
In terms of well integrity as part of the well planning, the potential geologic risks
should be specified for each geologic unit of the well profile before drilling. Some
general risks as observed in different lithologies of boreholes are listed in Table 3.2.
Wells may encounter major unexpected structural geologic/stratigraphic changes
during drilling. Therefore, real-time correlation must be systematically planed in
advance, and must identify key marker beds and decision points. A systematic
and careful cutting sampling belongs to the key issues in recognizing changes