drill bit use guide

1. ©2006ENSPMFormationIndustrie-IFPTraining
CHAPTER 6
BIT SELECTION

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Bit selection
• This Chapter aims at selecting the bit that will lead to
the lowest cost per foot, taking into account angle
corrections and reamed sections.. Both Technical
and Economical parameters will be involved in this
selection.
• Due to the great variety of formations to drill and due
also to the relative misunderstanding of the
characteristics of these formations, of the bits
currently available and of the various drilling
environments (material and costs..), it is impossible
to point out a few bits that would do a great job
everywhere.

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Bit selection
• In exploratory wells, geological data regarding the
formations to drill come from seismic surveys,
regional geology maps and some samples collected
here and there, where the corresponding rocks reach
the surface.

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Bit selection
• In development wells, lithology, formation thickness
and rock characteristics are usually well known.
• Cuttings samples, cores, plugs taken on cores, direct
measurements on cuttings, gives indications about
the nature of the rocks, their compressive strengths,
porosity & permeability.
• Electric logs, recording of drilling parameters and
MWD logs sometimes merged in one single document
called Master Log or Composite Log, allows direct
correspondence between the parameters, the bit
behavior and the rocks being drilled.

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Bit selection

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Bit selection
• Another set of information available concerns the bits used
in the surrounding wells (called offset wells) under the
form of morning reports or bit records.
• To optimize the choice of the bit, it is necessary to take into
account:
– Data concerning the formation to be drilled.
– Parameters applied to the bits used in the pevious wells, the
performances carried out by these bits and their wear.
– Basic equipment and surface installations used (hydraulic
power available, capacity of the equipment of mechanical
treatment of the solids, composition of the drill stem, etc)

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Bit selection
• Availabilities and of the delivery times of the various
types of bits, the surrounded costs of the apparatus
and the tools used.
• Characteristics of mud used (type, density, content of
solids, etc).
• Specificities of the well to be drilled (trajectory, slope,
etc).

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Parameters linked to the formation
• Drilling performance will be influenced by various
parameters linked to the formations to drill, they can be
divided in two sets:
– The intrinsic properties of the rocks (mineralogy, grain
size and fabric of the deposits, matrix strength, porosity
..)
– The environment at the time of drilling (confinement
pressure, pore pressure, reactions with the drilling fluids,
stresses on the rocks..)
• The two sets of parameters will interact to modify the
rock strength in compression and shearing, abrasivity
and behavior at the moment it is destroyed by the bit.

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Parameters linked to the formation
• To have an idea of the formation drillability, it is
therefore important to know:
– The Rock Composition, by direct observations (cuttings
and cores) or by in-situ measurement through electric
logs. Both studies are complementary.
– The in-situ compaction that is related to the pore
pressure and the local stresses (vertical and
horizontal).
– The value of the stresses that prevail in the area around
the well (overburden pressure, rock tensile strength..)
These values can be determined by leak off tests, sonic
and caliper logs, seismic interpretations (under
compacted zones).

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Information from bits used on previous wells
• Information supplied by the dull grading.
– Dull grading is a direct and fast way for selecting both
the bit and the appropriate parameters for the man on
the rig, only if it is properly made.
• Cost per Foot.
– That is in fact the most important one, as bits are rarely
reused (except the diamond bits), looking for the lowest
cost per foot, even at the detriment of the bit life is what
matters. The longest distance drilled within the shortest
time, according to rig power available.

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Other selection criteria
• The capacities of the rig must be taken into account.
The power of pumping and rotation must be sufficient
to make it possible to use the bit with the parameters
envisaged.

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Conclusion
• It is very difficult to choose the best adapted bit to
drill a formation for many reasons.
• First of all, the technology and the possibilities
offered by certain types of bits evolve/move very
quickly. Such type of bit which today is not adapted
for such type of rock can be it tomorrow.
• It also should be remembered that rocks having the
same name can have very different characteristics.
• The best way to select the best adapted bit to a
formation or a phase of drilling is still to establish
correlations between the tools used, the parameters
applied, the wear of the tools and the characteristics
of the rocks to be drilled.