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All diamond impregnated, surface-set
diamond, PCD (TSP)-set and tungsten-carbide set bits
require the use of circulating fluid during normal operation.
The circulating fluid has several functions including:
- To flush the bit’s
cuttings up and out of the drill hole.
- To clear the cuttings
from the bit face so that the diamonds (or other cutting
media) do not re-grind the resulting sludge.
- To cool the bit
crown during operation.
- To lubricate the
bit and to improve the cutting action.
- To reduce the friction
of the rotating drill string.
- To monitor the behaviour
of the bit and the formation being drilled. The behaviour
and change of properties of the circulating fluid
is an important source of information for the operator
by considering: return fluid loss, system pressure
increases or decreases, and changes from being acidic
to alkaline.
- To reduce vibration. Wherever
feasible, the use of liquid polymer viscosifiers and
lubricants are recommended to enhance bit performance.
As circulating fluid additives, these materials tend
to reduce rod torque and vibration particularly in
extremely deep or severely angled drill holes.
Typically, the circulating fluid is
pumped from the surface through the drill string, between
the inner and outer tubes of the core barrel assembly,
across the bit face and then returns to the surface through
the annular gap between the drill string and the drill
hole wall. The basic rule is that the ascending velocity
of the circulating fluid should be greater than the precipitation
velocity of the largest cuttings in the fluid. Consequently,
the larger the cuttings, the more fluid is needed.
As conditions vary
considerably from one drilling operation to another,
it is difficult to define absolute parameters for in-hole
fluid circulation. Some on-site experimentation may
be required. While bit hydraulics are controlled to
some extent by regulating the drill’s circulating
pump and circulating fluid viscosity, the actual amount
of circulating fluid to apply is largely determined
by the bit face waterway configuration as well as the
annular gap between the drill hole wall and the drill
string.
Annular velocity
is defined as the rate at which the circulating fluid
and cuttings are returned to the surface through the
annular gap between the drill hole wall and the drill
string. Excessive annular velocity may cause hydraulic
erosion of the drill hole wall in soft formations while
insufficient annular velocity will cause cuttings to
remain in suspension and will result in stuck drill
rods, equipment wear, low penetration rates and hole
cave-ins. As an alternative to running with higher annular
velocities, the drill operator may increase the viscosity
of the circulating fluid for effective hole cleaning.
The normal recommendations for annular
velocity are:
Maximum: 165 feet/minute (or 50 metres/minute)
Minimum: 60 feet/minute (or 18 metres/minute)
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