I N NOVATI N G WH I LE DR I LLI N G
Industry seeks ways
to understand and fight
elusive downhole enemies
Dysfunctions like HFTO can be hard to detect, but companies are identifying
mitigation solutions through sensors, bit studies, predictive software
BY STEPHEN WHITFIELD, ASSOCIATE EDITOR
I n operators’ quest to maximize the value they get from each
well by drilling longer and more complex bores, they’re also
having to deal with a higher risk of encountering downhole
dysfunctions, such as high-frequency torsional oscillation (HFTO)
or lateral shock. Such events can potentially lead to damage in
the BHA and other downhole tools, adding time and cost if they
have to be pulled prematurely. Understanding the nature of these
dysfunctions has, therefore, become more important than ever.
“In the industry, we’re always chasing ROP, trying to get the best
drilling performance we can. But getting the best performance
is not always tied to ROP,” said Raju Gandikota, Co-Founder and
Chief of Innovations at MindMesh, an engineering software and
technology development company. “It’s about managing the tool
Highlights
Testing of various sensors and their
placement showed that HFTO cannot be
detected from the surface.
Bit wear and cutter placement were found
to be among drivers of HFTO vibration
magnitude.
Near-real-time data streams can be
integrated with predictive models in a
digital twin to see how dysfunctions occur
as the drill string moves downhole.
22 life, having vibration control and maintaining the effectiveness of
your tools so that you can actually reach total depth.”
To better anticipate downhole dysfunctions and stave off their
more destructive effects, some companies are testing different
sensor types and various placement to see how they can more
effectively detect HFTO, while others are studying the relationship
between different bit designs and HFTO. Predictive software is
also poised to provide the industry with more information needed
for better decision making to prevent or mitigate downhole dys-
functions. While these approaches differ, the common goal is to
understand the source of damaging vibrations so that downhole
tools can become more durable and stay in the hole longer.
“How many times does a customer come to us and say, hey, we
are losing expensive downhole tools?” asked Prabhakaran Centala,
Engineering Manager for PDC Cutter Technology at NOV. “If they
bring these failures to the table, then we have to figure out the best
way to mitigate that. There are so many intricate details that can
help us bring about a different level of performance altogether.”
Understanding and identifying HFTO
HFTO, which is characterized by torsional oscillations of the
drill string or BHA that fall within the 100-300 Hz frequency, can
be particularly difficult to measure and quantify compared with
other types of vibrations, like stick/slip and bit bounce.
If left undetected, HFTO can lead to downhole tool failures, such
as twist-offs, connection cracks in the BHA, washouts and prema-
ture bit dulling. “Nowadays, rotary steerable tools are much more
sensitive, with moving mechanical parts and electrical compo-
nents. Because of this sensitivity and their complexity in nature,
we’re more susceptible to tool failures because of HFTO,” said
John Rodriguez, Drilling Engineer at Occidental Petroleum (Oxy).
M A R C H/A P R I L 202 2 • D R I L L I N G C O N T R AC T O R
I N NOVATI N G WH I LE DR I LLI N G
Dysfunctions such as high-frequency torsional oscillation (HFTO) and lateral shock can cause costly problems like washouts,
twist-offs and connection cracks at the BHA. Through different approaches, companies are trying to better understand the
source of damaging vibrations so the life of downhole tools can be improved and they can stay in the hole longer.
To attain a better understanding of HFTO, Oxy conducted tests
in the Delaware Basin from 2018 to 2020 utilizing commercially
available high-frequency downhole sensors in various positions
along the BHA, as well as an anti-stick/slip tool. These tests also
aimed to determine the ideal sensor types and placement of the
sensors for identifying HFTO.
“When you’re losing the BHA downhole, that’s something that’s
worth millions of dollars. These types of serious NPT events
prompted us to investigate HFTO. We wanted to do some field
testing to get an understanding of what was going on downhole,”
Mr Rodriguez said.
Oxy has shared results from some of the tests that were con-
ducted. In these tests, it looked at three sensor types – a high-
frequency “puck” sensor with a vibration sampling frequency
of 800 Hz placed in the shank of a 6.75-in. bit; a high-frequency
sensor with a vibration sampling frequency of 1,024 Hz that was
designed to be run above the RSS on a directional BHA; and an
ultra-high-frequency sensor (vibration sampling frequency of
1,500 Hz) installed in a carrier sub run above the MWD.
The data from each sensor was measured against data from
the surface. However, the sensors were not necessarily measured
against each other in every test. Some tests were conducted to
measure the performance of downhole tools that could help miti-
gate HFTO.
One test compared the performance of the low-frequency and
high-frequency sensors on a 6.75-in. BHA with no anti-stick/slip
tool. While the low-frequency sensor in the RSS recorded consis-
tent RPM throughout the test, ranging between 100-200 RPM, the
high-frequency sensor that was placed above the RSS measured
significantly greater variations, between 0-400 RPM, on multiple
occasions. Each variation in RPM coincided with an HFTO event
recorded during the run, the last of which led to a tool failure.
Meanwhile, the surface drilling data stayed consistent throughout
the run. The mechanical specific energy (MSE) remained low, top
drive torque remained steady, and top drive RPM stayed constant
at 175 RPM.
The results from this test led to two understandings, Mr
Rodriguez said. First, vibration data from a standard sensor in
the RSS cannot capture HFTO events, and second, HFTO cannot
be detected from the surface. “HFTO happens within a pretty
short time frame and distance. It doesn’t travel all the way to the
surface like LFTO. Because it’s a high-frequency event, the travel
distance is just within the BHA. We saw in our data sets that you
can’t identify it in real time while drilling. Your MSE and your ROP
don’t really change. You could be drilling fast and see stable drill-
ing indicators at the surface, but downhole it could be an entirely
different picture.”
Another test measured the high-frequency “puck” sensor in
a 6.75-in. bit shank and compared it against surface data. For
this test, Oxy utilized a 4.75-in. motor above the RSS in two
separate runs, one with and one without an anti-stick/slip tool,
a device placed above the RSS/MWD and below the motor that
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