Page 10 - Offshore Well Construction

CHAPTER 6
Drill Bits
Petroleum Extension-The University of Texas at Austin

BIT SELECTION
Bit performance is measured by the total length and time drilled before the bit has to be pulled and replaced. Minimum
cost per metre (or foot) is the primary objective. Careful review of offset well data must be undertaken when selecting
a bit for any particular hole section.

Primary considerations when selecting a bit type are:
• Geology
• Formation properties
• Compressive strength
Refers to the intrinsic strength of the rock which is based upon its composition, method of deposition,
and compaction. It is important to consider the ‘confined’ or ‘in situ’ compressive strength of a
given formation. Many bit manufacturers now provide a supplementary rock strength analysis
service as an aid to bit selection.
• Elasticity
Affects the way in which a rock fails. A rock that fails in a plastic mode will deform rather than
fracture.
• Abrasiveness
• Overburden pressure
Affects the amount of compaction of sediments and therefore the rock hardness.
• Stickiness
• Pore pressure
Affects mud weight requirements which, in turn, can affect penetration rates.
• Porosity and permeability
• Formation changes within a given hole section
Changes in formation during one bit run can have a significant effect on bit performance. The formations
to be drilled and the predicted depths of formation changes will be given in the drilling program and
will form the basis of bit selection. It is important to remember the difference between exploration and
appraisal/development drilling in that:
• For appraisal/development drilling, much will be known about the properties of the predicted
formations and bit selection will be based upon offset bit performance along with electric log data
(sonic, gamma ray, mud log data, core samples, etc.).
• For exploration drilling, little may be known of the drillability of the formations that are likely to
be encountered and so a more conservative bit program will be developed. In such situations, it
is prudent to load out a wider variety of bit designs to cover all eventualities.
• Hole size and casing program
• Directional profile of well path and steerability of bit design
• Drive type (rotary/rotary steerable/mud motor/turbine)
• Drilling fluid properties
• Hydraulics
• Rig capabilities

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