Page 213 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 189

2.4.3
Drill Bit Selection, Parameters, and Hydraulics [ ]

below maximum at section TD. Calculating exact nozzles and flow rate for
optimum hole cleaning under the actual conditions at the time cannot be
done due to the large number of variables, some of which are unknown.
Note that it will not always be possible to fully optimize bit
hydraulics since other considerations may take precedence. For
instance, if the use of LCM is anticipated, then use nozzles of at least
16 /32 in size, even if the optimization calculations suggest smaller noz-
zles. Pressure drops through downhole motors and motor seals may
restrict the minimum bit pressure drop.
Apart from calculating the nozzle total flow area (TFA), the type
and arrangement of nozzles can make a significant difference to
bit/bottom cleaning and therefore ROP. In a rotationally symmetrical
bit (such as a conventional tricone drill bit) without a center jet, plac-
ing three equally sized nozzles may lead to a dead zone of no flow at
the center of the bit. Asymmetric nozzle sizing (i.e., using one large
and two smaller nozzles for the desired TFA) can cause a cross-flow
effect across the bit face, ensuring that no dead zone exists. Experience
and lab results indicate improved ROP from cross-flow configuration.
The large nozzle also creates turbulent pressure fluctuations which
helps lift rock chips off the bottom, reducing redrilling of cuttings and
effectively increasing bit efficiency and ROP. Some operators also use
one blank nozzle to give a cross-flow effect from the two remaining
nozzles and this has demonstrated some ROP improvements.
Laboratory experiments indicate ROP improvements from asymmetric
nozzle configurations of around 20% (see references in Section 2.4.9,
“References for Drill Bit Selection”).
Extended or mini-extended nozzles can also be used (in combina-
tion with asymmetric sizing if required). By decreasing the distance
between the nozzle and formation, less dissipation of energy takes
place before the jet impinges on bottom. In softer formations this can
give a worthwhile increase in ROP, probably due to a combination of
better bottom cleaning and some erosion of the formation. Extended
nozzles have to be handled carefully; if the bit breaker is worn, then
makeup torque can be transmitted between the nozzles and the bit
breaker, which may lead to downhole loss of the nozzle.
A relatively recent nozzle design works in theory by creating both
positive and negative pressure regimes (i.e., below mud hydrostatic) on
bottom. The Vortexx nozzle has an asymmetric inside profile which
also produces a directional flow; on a PDC bit these nozzles are aligned
in such a way as to produce cross-flow effects along the blades by using
a template which has been worked out for a particular bit design. To

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