STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

Drilling Bits and Downhole Tools 789
This becomes
APb = 12.0(300)* = 370.7 psi
743qO. 95)'[ 3( 0.4688)']*
The velocity of the nozzle flow is found by equation 481.
The total area of the nozzle openings is
n:
a, = 3-(0.04688)' = 0.5178in2
4
Equation 4-81 becomes
-
v, = 300 = 185.9 ft/s
3.1 17( 0.5 178)
Diamond Bits
Diamond bits are being employed to a greater extent because of the advancements
in mud motors. High rpm can destroy roller rock bit very quickly. On
the other hand, diamond bits rotating at high rpm usually have longer life since
there are no moving parts.
Diamond Selection. Diamonds used as the cutting elements in the bit metal
matrix has the following advantages:
1. Diamonds are the hardest material.
2. Diamonds are the most abrasive resistant material.
3. Diamonds have the highest compressive strength.
4. Diamonds have a high thermal conductivity.
Diamonds also have some disadvantages as cutting elements such as: they are
very weak in shear strength, have a very low shock impact resistance, and can
damage or crack under extremely high temperatures.
When choosing diamonds for a particular drilling situation, there are basically
three things to know. First, the quality of the diamond chosen should depend
on the formations being drilled. Second, the size of the diamond and its shape
will be determined by the formation and anticipated penetration rate. Third,
the number of diamonds used also is determined by formation and the anticipated
penetration rate.
There are two types of diamonds, synthetic and natural. Synthetic diamonds
are man made and are used in PDC STRATAPAX type bit designs. STRATAPAX
PDC bits are best suited for extremely soft formations. The cutting edge of
synthetic diamonds are round, half-moon shaped or pointed.
Natural diamonds are divided into three categories. First are the carbonate
or black diamonds. These are the hardest and most expensive diamonds. They
are used primarily as gage reinforcement at the shockpoint. Second are the West
African diamonds. These are used in abrasive formations and usually are of
gemstone quality. About 80% of the West African diamonds are pointed in
shape and, therefore, 201 are the desirable spherical shape. Third are the Congo
or coated diamonds. These are the most common category. Over 98% of
these diamonds are spherical by nature. They are extremely effective in soft