Diamond Bit Selection
Choice of bit style, diamond size and diamond quality can mean the
difference between an economical bit run or a costly bit run.
Some formations are more drillable with diamond bits than others, but
these formations and their drillability change from area to area. Diamond
bits normally perform better in hard formations, because it is easier to keep
the bit clean, the cuttings are smaller, and individual diamonds cut with a
plowing action rather than by chipping and tearing.
Diamond bits require hydraulics equivalent to, or greater than other bits in
order to stay clean and run cooler in softer, stickier formations. The smaller
the diamond bit, the better it performs - mainly because of hydraulics.
Since the cutting surface of a diamond bit runs very close to the formation,
the cuttings move from the center of the hole across the face of the bit to
the outside of the borehole. The larger the bit, the greater amount of
cuttings to be moved across the face, which may result in partial clogging
of the flow area and a decrease in penetration rate unless hydraulics are
maintained at high energy levels.
Special Designs
Standard bit styles can be used in most cases. Special designs, or standard
bits with special features, are manufactured for unusual applications. For
example:
1. Low pressure drop bits for downhole motors
2. Flat bottom, shallow cone designs for sidetracking with
downhole motors
3. Deep cone, short gauge bit design for whipstock jobs or
sidetracking
4. Core ejectors can be built into most styles where cone wear is a
problem or where larger cuttings are desired
5. Deep cones having a 70° apex angle are normally used to give
built-in stability and greater diamond concentration at the cone
apex. In certain formations, a deep cone could fracture the
formation horizontally, leaving a plug in the bit cone. Thereafter,
the formation plug would be ground and splintered away beneath
the bit face, inducing diamond breakage and premature failure.
In fracturing type formations, a shallower cone angle of about
90° or 100° may be used.
Selection Guideline
Because formations of the same age and composition change in character,
with depth, and drill differently, no universal bit selection guide can be
prepared. However, general guidelines include:
Soft formations
Sand, shale, salt, anhydrite or limestone require a bit with a radial fluid
course set with large diamonds. Stones of 1-5 carats each are used,
depending on formation hardness. This type of bit should be set with a
single row of diamonds on each rib and designed to handle mud velocities
ranging from 300-400 fps to prevent balling.
Medium formations
Sand, shale, anhydite or limestone require a radial style bit with double
rows of diamonds on each blade or rib. Diamond sizes range from 2-3
stones per carat. Mud should be circulated through these bits at a high
velocity. Good penetration rates can be expected in interbedded sand and
shale formations.
Hard, dense formations
Mudstone, siltstone or sandstone usually require a crowsfoot fluid course
design. This provides sufficient cross-pad cleaning and cooling and allows
a higher concentration of diamonds on the wide pads. Diamond sizes
average about 8 stones per carat.
Extremely hard, abrasive or fractured formations
Schist, chert, volcanic rock, sandstone or quartzite require a bit set with
small diamonds and a crowsfoot fluid course to permit a high concentration
of diamonds. The diamonds (about 12 per carat) are set in concentric
“metal protected” ridges for perfect stone alignment, diamond exposure
and protection from impact damage.
Choice of bit style, diamond size and diamond quality can mean the
difference between an economical bit run or a costly bit run.
Some formations are more drillable with diamond bits than others, but
these formations and their drillability change from area to area. Diamond
bits normally perform better in hard formations, because it is easier to keep
the bit clean, the cuttings are smaller, and individual diamonds cut with a
plowing action rather than by chipping and tearing.
Diamond bits require hydraulics equivalent to, or greater than other bits in
order to stay clean and run cooler in softer, stickier formations. The smaller
the diamond bit, the better it performs - mainly because of hydraulics.
Since the cutting surface of a diamond bit runs very close to the formation,
the cuttings move from the center of the hole across the face of the bit to
the outside of the borehole. The larger the bit, the greater amount of
cuttings to be moved across the face, which may result in partial clogging
of the flow area and a decrease in penetration rate unless hydraulics are
maintained at high energy levels.
Special Designs
Standard bit styles can be used in most cases. Special designs, or standard
bits with special features, are manufactured for unusual applications. For
example:
1. Low pressure drop bits for downhole motors
2. Flat bottom, shallow cone designs for sidetracking with
downhole motors
3. Deep cone, short gauge bit design for whipstock jobs or
sidetracking
4. Core ejectors can be built into most styles where cone wear is a
problem or where larger cuttings are desired
5. Deep cones having a 70° apex angle are normally used to give
built-in stability and greater diamond concentration at the cone
apex. In certain formations, a deep cone could fracture the
formation horizontally, leaving a plug in the bit cone. Thereafter,
the formation plug would be ground and splintered away beneath
the bit face, inducing diamond breakage and premature failure.
In fracturing type formations, a shallower cone angle of about
90° or 100° may be used.
Selection Guideline
Because formations of the same age and composition change in character,
with depth, and drill differently, no universal bit selection guide can be
prepared. However, general guidelines include:
Soft formations
Sand, shale, salt, anhydrite or limestone require a bit with a radial fluid
course set with large diamonds. Stones of 1-5 carats each are used,
depending on formation hardness. This type of bit should be set with a
single row of diamonds on each rib and designed to handle mud velocities
ranging from 300-400 fps to prevent balling.
Medium formations
Sand, shale, anhydite or limestone require a radial style bit with double
rows of diamonds on each blade or rib. Diamond sizes range from 2-3
stones per carat. Mud should be circulated through these bits at a high
velocity. Good penetration rates can be expected in interbedded sand and
shale formations.
Hard, dense formations
Mudstone, siltstone or sandstone usually require a crowsfoot fluid course
design. This provides sufficient cross-pad cleaning and cooling and allows
a higher concentration of diamonds on the wide pads. Diamond sizes
average about 8 stones per carat.
Extremely hard, abrasive or fractured formations
Schist, chert, volcanic rock, sandstone or quartzite require a bit set with
small diamonds and a crowsfoot fluid course to permit a high concentration
of diamonds. The diamonds (about 12 per carat) are set in concentric
“metal protected” ridges for perfect stone alignment, diamond exposure
and protection from impact damage.
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