Often, we don’t get to pick the locations we drill on or the formations we may encounter. The customer hires us to be able to drill any formation, anywhere, so we’ve got to be prepared for whatever Mother Nature throws at us.
One of the most troublesome formations to drill can be plastic, water-sensitive
clays. It can be like drilling on a rubber mat; the formation is just stiff
enough that the teeth of the bit won’t penetrate into it enough to provide a
gouging action, and instead just slowly scrape the surface of the borehole
face. Since impact is easily absorbed by the plastic clay, ROP (rate of
penetration) is decreased, and the cuttings quit looking like drilled cuttings,
and start dissolving into the mud because of their hydroscopic properties. More
WOB will sometimes help if the mud is right, and the formation isn’t too thick
– sorta the blast-right-through-it method.
For thicker formations and ongoing problems, there are other ways to deal with
bit balling. The first is identification. “How do I know if the bit is balled
up?” If you’ve missed the initial clues by not watching the cuttings closely,
or are sitting in the doghouse talking to your girlfriend on the cell phone
while the joint drills down, you may notice that penetration has pretty much
quit. The second is a marked decrease in torque.
On some shaft-drive rotaries, there is no easy way to gauge the drill pipe
torque. Secondary clues are all you will have to work with. Has the rig changed
sound or feel? Or, with the bit on bottom, disengage the rotary, and watch how
long it takes to spin down to a stop. When the torque is high, and the bit is really
“biting,” it’ll come to a stop pretty quickly. Be very careful not to backlash
the string. If the torque has dropped off because the bit is balled up, it’ll
spin longer before stopping, indicating less torque on the bit. On hydraulic
rotaries or top-head rigs, the gauge will show a decrease.
This combination of fine, mushy cuttings, decreased ROP, and lowered torque are
good indicators of a balled bit. The question then is: “Now what do I
If you are running a tricone bit, and the balling has just occurred, you
sometimes can set the weight on bottom, and “rock” the bit a few times to free
the cones and continue. If the balling is more severe, try picking up off
bottom and rapidly spinning the bit while reciprocating the string up and down
to dislodge the ball. High flow rates are the key here, so run the pump as hard
as you’ve got nerve. If all else fails, before you trip out of the hole, you
might pump a walnut-hull sweep. It will tend to sandblast the bit and remove
the ball, and won’t hurt the mud. Don’t try this if you are running small jets
in the bit, as plugging can be an issue.
As with many other things, such as fishing jobs, prevention is much easier than
cure. Plastic and hydroscopic clays are best treated before they are
encountered. Once a clay has absorbed water, there is no cure. The first thing
is to have enough flow to create turbulent flow, at least at the bit and around
the BHA (bottom-hole assembly). While hydraulic horsepower is important through
proper bit jetting, sometimes a compromise must be reached. I consider
hole-cleaning very important.
A low water-loss mud is very important to minimize damage to the formation
outside the borehole and prevent sloughing. Encapsulation with polymers also is
useful to mitigate balling and formation damage.
In a nutshell: Good mud with low water-loss properties, low viscosity to
increase turbulent flow, and close attention to the cuttings will get you to TD
quicker than all the cures in the book.
After telling ya’ll about my brother Willard’s
moose-hunting expedition in Alaska, I got several e-mails from readers,
wondering how he was doing. One was from the Alaska DNR, asking me not to send
him up there anymore; they were running out of bush planes. So, this year, he
went hog-hunting in Arkansas. While out in the woods hunting, Willard came upon
a huge hole in the ground (probably a sinkhole). He couldn’t see the bottom, so
he chunked a rock in to listen. Nothing, it just went and went.
Willard looked around for something bigger he could throw in the hole, and
spied an old transmission in the brush. He dragged it over, and heaved it in
the hole to listen for bottom. About that time, a goat came running through the
brush full speed, and without even looking, dove in the hole.
As Willard was contemplating this odd event, an old farmer came up and asked
him, “Have you seen my goat?” Willard said, “Funny you should ask, I was
standing here by this hole, and a goat ran by me and jumped straight in the
“Couldn’t have been mine,” said the farmer, “I had him chained to a
The World According to Wayne: Bit Balling
January 4, 2010