Determining the formations to be drilled is the critical step in bit selection. Once an understanding is reached as to what type and density of cutting structure is required to actually remove the rock, then considerations can take place for other aspects such as stability, steerability, matching aspects to drive types, etc. The primary purpose though, is to select the appropriate drill bit that has sufficient durability to remove formation at a reasonable penetration rate.
1. Rock Classification 4. Roller Cone IADC 2. Bit Selection Properties 5. Fixed Cutter Selection 3. Roller Cone Selection 6. Problematic Formations
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Formation Factors
Rock Classification
a) Argillaceous rocks
• Claystone – flat, microscopic clay particles that form a loose and disordered assembly. Soft, sticky, and water absorbent. Certain clay minerals are reactive with water and cause swelling
• Shale – Claystone that has become compacted. The clay particles become ordered and lie horizontal. As above, particular clay minerals are reactive to water causing swelling shales
• Siltstone – This is an intermediate between sandstone and claystone / shale, and is classified basically on grain size alone. Similar properties to sandstone though the grains are less than 1/16mm in size.
• Marl – Semi consolidated clay or siltstone. In some regions it is a calcareous claystone. Relatively soft to drill
b) Arenaceous
• Sandstone – Consolidated sand size particles (2mm – 1/16mm) that are cemented usually with silica or calcareous cement. The particles are generally quartz but may also consist of feldspar, mica, and glauconite as it is derived from igneous rocks. The properties of the sandstone are dependent on particle size, sorting, shape, and strength of the cement. This means that sandstones can be very diverse in nature in terms of drillability. They can also be very abrasive due to the high quantity of quartz. The hardness is usually dependant on the cement. Note that unconsolidated sandstone is basically loose quartz grains and known as sand (generally easy to drill)
• Conglomerate – These consist of coarse material in a soft clay / silt matrix.
Usually present in top-hole sections, where the coarse material can be boulder size. This is problematic for PDC drill bits as the boulders may ‘rattle’ between the blades causing heavy cutter breakage. Roller cones are best suited to these formations due to their crushing action, though there is still the risk that loose boulders may lodge between the cones and lock them.
c) Carbonates
• Limestone – Formed by calcium carbonate deposits and may also contain shell fragments. The hardness of the rock will vary and depend on the quantity of other sedimentary rocks (clay / sand) and the cementation
• Dolomite – As above but is formed from magnesium carbonate instead of calcium
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d) Evaporites
• Salt – Formed by the evaporation of seawater to leave behind the salt minerals.
Due to the relatively soft and light nature of the salt, overlying sediment deposition usually deforms the salt beds to bulge towards the surface, forming what is know as a salt dome or diapir
• Anhydrite – This is resultant from the deposition of calcium sulphate. It is usually present in massive form, though may be crystalline. Similar to limestone in drillability
• Gypsum – A hydrated variant of Anhydrite, thus softer to drill
(Note: Ensure that the local formation name reflects the actual lithology. For example some shales such as the ‘Laffan Shale’ and the ‘Wolfcamp Shale’ are in fact carbonate rocks. This also occurs with some ‘Sandstone’ groups. Confirm on the mud logs or with the geologist the actual lithology instead of the actual local name for the formation.)
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Formation Factors
Bit Selection Properties
Following determination of the lithologies to be drilled, an assessment of the formation characteristics must be performed. This can be carried out by a number of models, though at base level, the best approach is analysis of offset records for dull conditions (see section on dull grading interpretation) and drilling reports.
The six primary characteristics for bit selection are:
1) Stickiness – Formations that are sensitive to water become sticky and pose balling issues. Consideration should be placed on maximising HSI and JIF of the bit and selecting designs with large face volume and open cutting structures.
2) Elasticity – Elastic formation have a tendency to deform rather than fail. As such, elastic formations should be drilled with large cutting elements with high depth of cut e.g. long milled teeth or 19mm PDC bits.
3) Porosity – Rocks with high porosity will fail easily. You can determine porosity by using sonic data. Values lower than 60 ms/ft indicate a tight rock that will require either impreg or heavy set inserts bits to drill.
4) Pressure – Differential pressure is the difference between the hydrostatic pressure in the annulus and the formation pressure. The general scenario is that the
hydrostatic will be greater than the pressure in the formation. This has a negative effect on potential penetration rates, with the higher the difference resulting in lower ROP. This is because it adds to the confining force and thus compressive strength is greater. It also holds down rock chips generated by the cutting action of roller cone bits, thus slowing rock removal.
5) Abrasiveness – Generally sand formations. Bit considerations should relate to enhanced cutting structure (abrasion resistance cutters or hardfacing), body protection, and extra gauge protection, particularly in directional wells.
6) Compressive strength – This is a measure of the force per unit area that a formation can withstand before it fails in compression, thus the lower the value, the easier it is to drill. Compressive strength is measured in psi. Values vary considerably depending on formation type and as such are commonly used for classifying formation strength. This in turn is often used for evaluating bit requirements. The list below provides approximate compressive strength values.
• Very Soft – Formation strengths less than 4,000 psi. High drillability.
Typical lithologies include clay, soft shales, marl, gumbo (sticky) clays, and unconsolidated sand, or poorly cemented, sands
• Soft – Formation strengths in the range of 4-8,000 psi. Dominated by the majority of shales and claystones. Other formations include soft
evapourites (such as salt) and soft siltstones
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• Soft to Medium – Formation strengths in the range of 8-12,000 psi.
Formations include soft limestones, marls, soft - medium sandstones, medium shales, chalk and medium anhydrites
• Medium – Compressive strengths in the range of 12-15,000 psi. Typical lithologies include sandy and chalky limestones, medium sandstones, hard shales, anhydrites
• Medium to Hard – Formation strengths in the range of 15-20,000 psi.
Hard formations that are not highly abrasive. Lithologies include hard shale, siltstone, low abrasive hard sandstones, limestone, dolomite, and anhydrite
• Hard – Formation strengths up to approximately 35,000 psi, often abrasive. These formations include hard dolomites, crystalline limestone, hard, brittle shales, hard abrasive sandstone and siltstones
• Very Hard – Very hard formations such as quartzite, very fine grained, well-cemented sandstones and siltstones, igneous, and metamorphic rocks with strong crystalline lattices. Formation strengths often in the range of 35-80,000 psi
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Formation Factors
Roller Cone Selection
The table below is a rough guide based on rock classification / compressive strength. The numbers indicate the IADC code of the bit design. The IADC code is used heavily for rock bit classification, with the primary number increasing from 1 to 8 with increasing hardness of rock. A more detailed table can be viewed at the rear of this section that ties in the correlation between formation strength, IADC number, and actual product types from the major bit manufacturers. For further information regarding the IADC
classification, please refer to the IADC section within the guidelines.
Milled Tooth Insert
Soft 111 - 137 415 - 447
Soft - Medium 515 - 547
Medium 615 - 637
Medium - Hard 211 - 217
Hard 331 - 337 717 - 737
Very Hard 817 - 835
12MFY, F10D, F10T, F12T, F14T
MF1, MF10T, MF12