The Diamond Bit
A diamond bit (either for drilling or coring) is composed of three parts:
diamonds, matrix and shank. The diamonds are held in place by the matrix
which is bonded to the steel shank. The matrix is principally powdered
tungsten carbide infiltrated with a metal bonding material. The tungsten
carbide is used for its abrasive wear and erosion resistant properties (but far
from a diamond in this respect). The shank of steel affords structural
strength and makes a suitable means to attach the bit to the drill string.
Diamond bits are sold by the carat weight (1 carat = 0.2 grams) of the
diamonds in the bit, plus a setting charge. The price will vary depending
upon classification (or quality) and size. The setting charge is to cover the
manufacturing cost of the bit. A used bit is generally returned to salvage
the diamonds and to receive credit for the reusable stones (which
materially decreases the bit cost). This credit is frequently as much as 50%
of the original bit cost.
Uses of Diamond Bits
As with any bit selection, the decision to run a diamond bit should be based
on a detailed cost analysis. There are, however, certain drilling situations
which indicate the likelihood of an economical application for diamond
bits.
• Very short roller cone bit life: If roller cone bit life is very short due to
bearing failure, tooth wear, or tooth breakage, a diamond bit can
increase on-bottom time dramatically. Diamond bits have no bearings
and each diamond has a compressive strength of 1,261,000 psi
(approximately 1.5 times that of sintered tungsten carbide). The
relative wear resistance is approximately 100 times that of tungsten
carbide.
• Low penetration rates with roller cone bits: Frequently, when roller
cone bits drill at slow rates (especially 5 ft/hr or less), due to high mud
weights or limited rig hydraulics, diamond bits can provide a savings.
The “plowing” type cutting action of diamond bits generally produces
higher penetration rates when using heavy muds. Since the drilling
fluid is distributed between the bit face and the formation in a smooth
uniform sheet, it takes less hydraulic horsepower per square inch to
clean under a diamond bit than under the same size roller cone bit.
• Deep, small holes: Roller cone bits that are 6-inch and smaller have
limited life due to the space limitations on the bearing, cone shell
thickness, etc. Diamond bits being one solid piece often last much
longer in very small boreholes.
• Directional drilling: Diamond side tracking bits are designed to drill
“sideways” making it a natural choice for “kicking off” in directional
drilling situations.
• Limited bit weight: Diamond bits drill at higher rates of penetration
with less weight than normally required for roller cone bits in the
same size range.
• Downhole motor applications: Roller cone bits generally have bearing
failures on motor applications due to high rotary speeds. Diamond bits
will have a very long life under these conditions.
• Cutting casing windows: Window cutting through casing using
diamond bits is now an effective, field-proven method for re-entering
older wells to increase production, to apply directional drilling
techniques, or to sidetrack. Using permanent casing whipstocks and
specially designed diamond bits, wider and longer windows are cut
when sidetracking.
• Coring: The use of diamond bits for coring operations is essential for
smooth, whole cores. Longer cores are possible with increased onbottom
time and cores “look better” because of the cutting action of
diamond bits as compared to those of roller cone bits.
There are some drilling situations which should be avoided when using
diamond bits:
• Very hard broken formations: Broken formations can cause severe
shock loading on diamond bits resulting in diamond breakage and a
short bit life.
• Formations containing chert or pyrite: Chert and pyrite tend to break
apart in large pieces and “roll” under a diamond bit, causing diamond
damage.
Reaming long sections in hard formations: Since the “nozzles” of a
diamond bit are formed by the formation on one side and the bit
matrix on the other side, hydraulic cooling and cleaning are extremely
poor during reaming. This can result in diamond “burning” or
breakage in the gauge area
A diamond bit (either for drilling or coring) is composed of three parts:
diamonds, matrix and shank. The diamonds are held in place by the matrix
which is bonded to the steel shank. The matrix is principally powdered
tungsten carbide infiltrated with a metal bonding material. The tungsten
carbide is used for its abrasive wear and erosion resistant properties (but far
from a diamond in this respect). The shank of steel affords structural
strength and makes a suitable means to attach the bit to the drill string.
Diamond bits are sold by the carat weight (1 carat = 0.2 grams) of the
diamonds in the bit, plus a setting charge. The price will vary depending
upon classification (or quality) and size. The setting charge is to cover the
manufacturing cost of the bit. A used bit is generally returned to salvage
the diamonds and to receive credit for the reusable stones (which
materially decreases the bit cost). This credit is frequently as much as 50%
of the original bit cost.
Uses of Diamond Bits
As with any bit selection, the decision to run a diamond bit should be based
on a detailed cost analysis. There are, however, certain drilling situations
which indicate the likelihood of an economical application for diamond
bits.
• Very short roller cone bit life: If roller cone bit life is very short due to
bearing failure, tooth wear, or tooth breakage, a diamond bit can
increase on-bottom time dramatically. Diamond bits have no bearings
and each diamond has a compressive strength of 1,261,000 psi
(approximately 1.5 times that of sintered tungsten carbide). The
relative wear resistance is approximately 100 times that of tungsten
carbide.
• Low penetration rates with roller cone bits: Frequently, when roller
cone bits drill at slow rates (especially 5 ft/hr or less), due to high mud
weights or limited rig hydraulics, diamond bits can provide a savings.
The “plowing” type cutting action of diamond bits generally produces
higher penetration rates when using heavy muds. Since the drilling
fluid is distributed between the bit face and the formation in a smooth
uniform sheet, it takes less hydraulic horsepower per square inch to
clean under a diamond bit than under the same size roller cone bit.
• Deep, small holes: Roller cone bits that are 6-inch and smaller have
limited life due to the space limitations on the bearing, cone shell
thickness, etc. Diamond bits being one solid piece often last much
longer in very small boreholes.
• Directional drilling: Diamond side tracking bits are designed to drill
“sideways” making it a natural choice for “kicking off” in directional
drilling situations.
• Limited bit weight: Diamond bits drill at higher rates of penetration
with less weight than normally required for roller cone bits in the
same size range.
• Downhole motor applications: Roller cone bits generally have bearing
failures on motor applications due to high rotary speeds. Diamond bits
will have a very long life under these conditions.
• Cutting casing windows: Window cutting through casing using
diamond bits is now an effective, field-proven method for re-entering
older wells to increase production, to apply directional drilling
techniques, or to sidetrack. Using permanent casing whipstocks and
specially designed diamond bits, wider and longer windows are cut
when sidetracking.
• Coring: The use of diamond bits for coring operations is essential for
smooth, whole cores. Longer cores are possible with increased onbottom
time and cores “look better” because of the cutting action of
diamond bits as compared to those of roller cone bits.
There are some drilling situations which should be avoided when using
diamond bits:
• Very hard broken formations: Broken formations can cause severe
shock loading on diamond bits resulting in diamond breakage and a
short bit life.
• Formations containing chert or pyrite: Chert and pyrite tend to break
apart in large pieces and “roll” under a diamond bit, causing diamond
damage.
Reaming long sections in hard formations: Since the “nozzles” of a
diamond bit are formed by the formation on one side and the bit
matrix on the other side, hydraulic cooling and cleaning are extremely
poor during reaming. This can result in diamond “burning” or
breakage in the gauge area
