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3.8 Risks 155
components and resulting loss of parts in the borehole. Typical risks of downhole
failures are
• Breakage of drillstring components: A breakage of drillstring components (such
as DC, HWDP, DP, and mud motor) generally results in leaving all parts below
the broken part in the hole. Depending on the specific situation a fishing
operation with suitable special tools will be done in order to retrieve the lost
part (fish) as described previously. If these operations are not successful within
a certain time/certain attempts a plug-back and a sidetrack may be taken into
consideration.
• Failure or breaking of MWD system: If an MWD system fails normally it can be
retrieved and substituted by a backup tool so that there will be only some delay.
Only in very few situations a breakage occurs leading to fishing operation.
• Failure/loss of drill bit parts: A typical drill bit failure is the loss of a cone from
a tricone bit due to excessive wear in the bearing. In most of the cases the parts
which remained in the borehole can either be caught by special fishing tools
(e.g., junk basket) or by running a flat-bottom mill to the bottom of the hole and
destroy the junk by milling.

3.8.3
Geological–Technical Risks

Geological–technical risks cover drilling technical problems during operation that
are caused by reaction of the geological formation to applied drilling technology.
Such problems mostly arise from unexpected geomechanical behavior of known or
unknown formations or if drilling technology is insufficiently matched to known ge-
ological conditions (Figure 3.23). A modification of drilling parameters – especially
mud technology – will commonly solve geological–technical problems. Basically, a
rock mechanical understanding and geomechanical modeling will help to identify
risk zones and to quantify rock failure under certain circumstances. Hence, drilling
technical parameters like mud weight can precisely be specified by geomechanical
models minimizing wellbore stability problems.
• Borehole wall breakouts/cavings/washouts: Breakouts are generated by high
stress concentrations on the borehole wall, induced by too low mud pressures in
weak brittle rock.
• Squeezing or swelling formations: Squeezing and creeping formations are salt
rocks, creeping is increased by higher temperatures and thick overburden. From
the Northeast German Basin it is known that particularly salt rock below 2000 m
depth belong to risky zones in terms of casing failure. Swelling formations like
clay or subordinately shale can be controlled by higher mud weight and additives
like inhibitors which prevent clay minerals to assimilate water from drilling
mud. If the mud weight is not adapted to the presence of swelling formations, a
drillstring can be stuck within seconds.
• Circulation losses (e.g., in fractured or karstic rocks): Circulation losses are the
reduced or total absence of fluid flow up to the annulus when fluid is pumped