Summary
The downhole pressure environment is one of the most important factors
affecting the rate of penetration (ROP). It is believed that impermeable rocks
experience high differential pressures because of shear dilatancy and become
stronger and, thus, more difficult to drill. However, recently performed
atmospheric and pressurized single-cutter experiments show that contrary to
this belief, even at low pressures (100–200 psig) and even with permeable
rocks, significant increase in mechanical specific energy (MSE) is observed
compared to atmospheric tests.
The experiments were carried out in a single-cutter high-pressure testing
facility refurbished with high-precision sensors and a data acquisition system.
In the experiments, a 13-mm polycrystalline-diamond-compact (PDC) cutter was
used to cut Carthage marble and Indiana limestone samples with depths of cut
ranging from 0.025 to 0.050 in. More than 70 high-precision tests were
performed on these two rock types under confining pressures ranging from 0 to
1000 psig. The confining fluids were either water or mineral oil.
Unexpectedly, analysis of the MSE consistently showed that increases in the
confining pressure as small as 150 psig can increase the MSE of the cutting
process significantly and reduce the cutting efficiency by half. These
reductions in the cutting efficiency that were even more dramatic in the
permeable and saturated Indiana limestone could not be explained by the
strengthening of the rock under confining pressures.
Upon analysis of the results of experiments (cutting forces, volume of cuts,
and visual inspections of cuttings), a new theory was proposed to explain this
unexpected behavior. This new theory, based on the frictional forces and the
cutting mechanism under pressure, gives useful insights into the physics of
cutter-rock interaction. Such insights are invaluable to the improvements of
drilling practices selection [weight on bit (WOB), type of drilling fluid, and
its properties] and the ROPs.
© 2010. Society of Petroleum Engineers
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History
- Original manuscript received:
26 November 2008
- Meeting paper published:
17 March 2009
- Revised manuscript received:
4 May 2009
- Manuscript approved:
11 June 2009
- Published online:
14 January 2010
- Version of record:
11 March 2010
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