Weight on Bit
A certain minimum WOB is required to overcome the compressibility of the formation. It has been found experimentally that once this threshold is exceeded, penetration rate increases linearly with WOB (Figure 26). There are however certain limitations to the WOB which can be applied:
a. Hydraulic horsepower (HHP) at the bit
If the HHP at the bit is not sufficient to ensure good bit cleaning the ROP is reduced either by:
i. bit balling where the grooves between the teeth of the bit are clogged by formation cuttings (occurs mostly with soft formation bits), or
ii. bottom hole balling where the hole gets clogged up with fine particles (occurs mostly with the grinding action of hard formation bits).
If this situation occurs no increase in ROP results from an increase in WOB unless the hydraulic horsepower (HHP) generated by the fluid flowing through the bit is improved (Figure 27). The HHP at the bit is given by:
HHPb = PbXQ Г714
where:
Pb = pressure drop across the nozzles of the bit (psi)
Q = flow rate through the bit (gpm)
To increase HHP therefore requires an increase in Pb (smaller nozzles) or Q (faster pump speed or larger liners). This may mean a radical change to other drilling
factors (e. g. annular velocity) which may not be beneficial. Hole cleaning may be
improved by using extended nozzles to bring the fluid stream nearer to the bottom of the hole. Bit balling can be alleviated by using a fourth nozzle at the centre of the bit.
b. Type of formation
WOB is often limited in soft formations, where excessive weight will only bury the teeth into the rock and cause increased torque, with no increase in ROP
c. Hole deviation
In some areas, WOB will produce bending in the drillstring, leading to a crooked hole. The drillstring should be properly stabilised to prevent this happening.
d. Bearing life
The greater the load on the bearings the shorter their operational life. Optimising ROP will depend on a compromise between WOB and bearing wear.
e. Tooth life
In hard formations, with high compressive strength, excessive WOB will cause the teeth to break. This will become evident when the bit is retrieved. Broken teeth is, for example, a clear sign that a bit with shorter, more closely packed teeth or inserts is required.
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