I N NOVATI N G WH I LE DR I LLI N G
Finding links between HFTO, bit features
Oxy conducted a field test comparing the performance of a
low-frequency sensor in the RSS and a high-frequency sensor
placed above the RSS in a 6.75-in. BHA. During the HFTO events
recorded in the test, the high-frequency sensor measured
significant variations in RPM, while the low-frequency sensor
measured consistent RPM. This indicated that HFTO could not
be captured by RSS vibration data.
is designed to prevent excess torque from traveling up the drill
string. Mr Rodriguez noted some additional findings from this test.
First, the high-frequency sensor in the drill bit detected instances
of HFTO early in the first run, although the tangential accelera-
tions were less than 20G-acceleration. The bit was pulled for slow
ROP and was damaged beyond repair once tangential accelera-
tions exceeded 100G-accelerations.
Second, the anti-stick/slip tool used in the second run reduced
the level of tangential accelerations measured at the bit, where
tangential accelerations averaged less than 20G-acceleration for
the entire run. This indicated that the anti-stick/slip tool could
help reduce the level of tangential accelerations at the bit. While
this does not eliminate HFTO, it can help mitigate its most dam-
aging effects.
Oxy said it paused this series of HFTO testing in March 2020
following the oil price downturn but plans to resume this project
in the future.
24 For the past couple of years, NOV has been exploring the
relationship between bit design and HFTO, hoping to eventually
create a new bit design that could help minimize the downhole
dysfunction. In particular, the company worked to identify what links
observable HFTO with bit design metrics or features, develop-
ing tests based on four hypotheses that came out of previous
field observations and other studies on HFTO. The first theory
NOV considered was around worn cutters. As worn cutters are
more susceptible to low-frequency torsional vibration, they could
potentially be more susceptible to HFTO, as well.
Second, tracking PDC cutters are less efficient than non-
tracking cutters, primarily because of the shape of the cut that
the PDC makes when interacting with the rock. “Tracking” and
“non-tracking” cutters refer to the placement of a backup row of
PDC cutters directly behind the primary cutters. These secondary
cutters, which help enhance the durability of fixed cutter bits, can
be placed either at the same radius as the primary cutters (track-
ing) or in between (non-tracking).
A tracking cutter will engage with the formation over a long
portion of the cut shape with a small depth of cut (DOC); a small
DOC is associated with a high MSE. A large shear length to shear
area ratio indicates an inefficient cutting shape with a small DOC
and potentially contributes to HFTO.
Third, diamond-impregnated secondary components could
help reduce vibratons. NOV noted that overly sharp, freshly
ground PDC cutters are more likely to cause the vibrations indica-
tive of HFTO compared with cutters that have been slightly used.
However, the company noted no instances of HFTO when using
impregnated bits, even though they commonly drill at very small
DOC in hard rock. This was because rock fails through grinding,
rather than shearing, as the cutting face of the impregnated bit
slides against the rock with little risk of overengagement.
Part of NOV’s testing studied whether it was possible to manu-
facture cylindrical components shaped like a cutter made from
the same material as the impregnated bits and placed in a sec-
ondary cutting location on the face of a PDC bit. This combination
of shearing and grinding rock failure could mitigate HFTO.
The fourth hypothesis pertains to the effective back rake angles
of the cutter placement on the bit. As the effective back rake angle
increases, it tends to limit the DOC and can lead to a greater risk
of HFTO occurring.
To test these theories, NOV conducted several experiments at
its pressurized drilling laboratory in Conroe, Texas, over the past
year. Two 216-mm bits with seven blades and 16-mm cutters were
used. The bits were tested on rock cores under different drilling
conditions to simulate field tests.
Bit A had 32 face cutters and 18 tracking secondary cutters.
Each element of the bit design was chosen to test the theoretical
connections between bit design and HFTO: cutter wear, the ratio
of shear length to shear area, the choice of secondary cutter mate-
rial and effective back rake angles. The back rake angles on the
face of the cutter were between 19° and 20°. When first used, this
bit had wear as received from field testing; the cutters were worn,
and the gauge pad had suffered some wear.
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I N NOVATI N G WH I LE DR I LLI N G
In lab testing, NOV examined the relationship between bit wear and HFTO. Configuration 1 (dull cutter) showed greater average
HFTO amplitude than Configuration 2 (new cutter), with the difference becoming more pronounced at lower RPMs.
Bit B, a new design, had 30 face cutters with the shape alter-
nating between chisel and full round. Further, the cutters were
laid out in a star spiral order, with the shape alternating between
chisel and full round. The alternating shape and star layout was
chosen to improve the ratio of shear length to shear area.
There were 10 secondary cutters in Bit B made from the same
material as a diamond impregnated cutter. These secondary cut-
ters were placed in a tracking location. The back rake angles of
the face cutters varied from 15° in the cone to 20° near the gauge.
The bits were tested in five separate configurations. The first
configuration was used to examine the effect of bit wear on HFTO,
with successive configurations changing different elements of
the bit to isolate which parameters had the most significant
impact. Configuration 1 involved the use of Bit A directly from the
field, already having dull and broken round cutters on its primary
and secondary blades. Configuration 2 involved the replacement
of the dull and damaged cutters on Bit A with undamaged 16-mm
round cutters on the primary and secondary blades. Configuration
3 involved the use of non-planar cutters to increase back rake
angles. Configuration 4 involved replacing the secondary cutters
with cylindrical diamond impregnated material. Configuration 5
involved the use of Bit B.
Each configuration was tested at 10 separate combinations of
weight on bit (WOB) and RPM. Before testing a configuration, the
hole was prepared by drilling 130 mm into the rock to generate
the bottomhole pattern, ensuring that all tests start with similar
formation engagement.
Testing of Configuration 1 compared with Configuration 2
showed that worn cutters increase the risk for HFTO. At 100 RPM,
the new cutters in Configuration 2 led to a 53% reduction in the
amplitude of HFTO. The disparity in amplitude increased as RPM
decreased – Configuration 2 saw an 82% drop at 75 RPM and a 97%
drop at 50 RPM. This indicated that the PDC cutters should be cho-
sen for longevity and durability in areas where HFTO is a concern.
Configuration 3 had a 39% decrease in HFTO amplitude com-
pared with Configuration 2, indicating that higher back rake
angles – the angles of the face away from the end cutting edge
of the drill bit – can reduce HFTO for a given WOB and RPM.
However, the average ROP also increased 23% from Configuration
2 to Configuration 3. The use of diamond impregnated cutters
(Configuration 4) in place of the secondary cutters showed a 12%
reduction in amplitude and an 11% increase in ROP.
The differences in back rake angle proved less significant than
the other factors tested in determining the risk for HFTO. The data
did not support the theory that increased back rake angle would
worsen HFTO – in fact, Configuration 3 showed the 39% decrease
in HFTO amplitude despite having an increased back rake com-
pared with Configuration 2.
The results from the study indicated that the optimal bit design
to limit HFTO vibration magnitude requires the use of durable cut-
ters with either non-tracking secondary PDC cutters or diamond-
impregnated cutters and a low back rake angle.
While NOV said it has no imminent plans to turn the learnings
from this study into a new product, the company’s ReedHycalog
business unit does hope to have a roadmap by the end of this year
for developing a product line of bits designed specifically for miti-
gating HFTO. “Our next goal is to develop an agenda where we can
go to our customers and tell them how we’re going to bring about a
bit that will help the industry reduce HFTO at the source. We don’t
have a product line solely dedicated to HFTO right now, but we do
have an agenda,” Mr Centala said.
Predicting dysfunctions with a digital twin
Computer-based well planning and drilling dynamics model-
ing is a standard practice for improving drilling performance.
However, it has its limitations.
Primarily, conventional well planning software produces static
models of the downhole, which are not useful in analyzing down-
hole behavior over a period of time. That is key to identifying the
risk of dysfunctions, said Mr Gandikota of MindMesh. “What we
are missing with the static model is the interaction of the drill
bit cutting rock, and how the BHA interacts with the hole while
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