drilling bit 6
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©2006 EN
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Formation Industrie -IFP
Training
CHAPTER 6 BIT SELECTION
©2006 EN
SPM
Formation Industrie -IFP
Training
SEC
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.
©2006 EN
SPM
Formation Industrie -IFP
Training
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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.
©2006 EN
SPM
Formation Industrie -IFP
Training
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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.
©2006 EN
SPM
Formation Industrie -IFP
Training
SEC
Bit selection
©2006 EN
SPM
Formation Industrie -IFP
Training
SEC
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)
©2006 EN
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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.
©2006 EN
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Formation Industrie -IFP
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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.
©2006 EN
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Formation Industrie -IFP
Training
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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.