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bit. Called “hybrid bits,” they combine natural diamonds, PDCs, TSPs, and even
tungsten carbide inserts.
Operators use natural diamond, PDC, TSP, and hybrid bits to drill soft,
medium, and hard formations. They are especially effective in abrasive formations.
These natural and synthetic diamond bits are the most expensive type of bit. When
properly used, however, they can often drill faster and last longer than steel-tooth or
carbide insert bits.

Bit Sizes and Attributes
Bits are available in many sizes, from 3M inches (95.25 millimetres) to 28
inches (711.2 millimetres) in diameter, depending on the diameter of the hole the
operator needs to have drilled. An operator or contractor can special order smaller or
larger sizes if required. Moreover, because formations of various hardnesses exist,
manufacturers also offer bits with cutters designed to drill formations of different
hardnesses. In general, they offer bits with cutters suited to drill soft, medium soft,
medium, medium hard, hard, and very hard and abrasive formations.

Weight on Bit and Rotating Speeds
Putting weight on a bit makes its cutters bite into the rock. Usually, drillers
apply weight on the bit by allowing some of the weight of the drill collars above the
bit to press down on it. The amount of weight depends on the size and the type of bit
and the speed at which the driller rotates it. The amount of weight also depends on
the type of formation being drilled. For a general idea, consider that a driller can
rotate bits anywhere from 70 to 180 revolutions per minute and can place anywhere
from 26,000 to 130,000 pounds (11,570 to 57,850 decanewtons) of weight or force on
them.

Circulating system
One unique characteristic of rotary drilling is the pumping of drilling fluid to
the bottom of the hole to pickup cuttings made by the bit and lift them to the surface
for disposal. At the same time, solid particles in the fluid plaster the sides of the hole

opposite porous and permeable formations and keep them from caving in. The ability
of a rotary rig to circulate drilling fluid has made it the drilling method of choice all
over the world.

Drilling Fluid
Drilling fluid – fluid – is usually a mixture of water, clay, weighting material,
and a few chemicals. During drilling it circulates in steel tanks. Some formations
swell in the presence of water and impede drilling, so the operating company requires
that the contractor use oil instead of water as a base for the mud. In a few cases, the
operator may choose air or gas as a drilling fluid. (A fluid can be either a gas or a
liquid. Air is a gaseous fluid; water is a liquid fluid.) Unlike mud, air or gas exerts
very little pressure on the bottom of the hole because it is so much lighter in weight
(it is less dense) than mud. Accordingly, air or gas can dramatically increase the

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