Page 159 - Drilling Technology in Nontechnical Language

150 Drilling Technology in Nontechnical Language Second Edition

The rate of penetration of any drill bit is limited by the ability of the
mud to clean the bottom of the hole. Up to a point, increasing the flow

rate and increasing the speed with which the mud flow hits bottom will
increase the rate of penetration. In softer formations, this force can be

sufficient to remove rock by hydraulic force. Unfortunately, this often
causes an overgauge hole to be drilled as rock is eroded from the gauge
area of the bit.

Grading the Dull Bit

When a bit is pulled out of the hole after drilling, it is referred to as
being dull (as opposed to sharp). The dull bit will have various features
caused by downhole conditions encountered by the bit. If these features are
properly recognized, together with information recorded while drilling, an
accurate picture of downhole conditions can be built up. This then allows
a better choice of bit to be made for the same depth in the next well to
be drilled. These drilling records and dull bit analyses (called dull bit
gradings) are therefore very important to improving future performance.
It is also important to properly grade the bit in order for the next bit in the
hole to be properly selected and run.
For instance, tungsten carbide teeth are quite brittle, so they can break
when shock loads are encountered (fig. 6–8). If a dull TCI bit has a high

proportion of broken teeth, one possible cause may be excessive vibration
of the drillstring while drilling. Other causes could be hitting the bottom
of the hole too hard with the bit, excessive drilling parameters (high
WOB and high RPM at the same time), or some steel junk in the hole. If
tungsten carbide teeth are not properly cooled due to insufficient flow rate

or clogging up of the bit with formation cuttings, they go through many
cycles of heating and cooling as the bit rotates. This causes cracking of
the teeth (called heat checking), and it also leads to broken teeth. It is very
important that the true cause of these broken teeth is established before
decisions are made on the next bit to run in the same well and at the same
depth in the next well.

Dull bit gradings are recorded using a standard system of letters and
numbers. There are eight characteristics that are noted under this system.
Four relate to the cutting structure, one relates to the bearings, one to the
wear of the bit gauge, one to any other dull features, and the last shows
why the bit was pulled out of the hole.

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