In the drilling business, unscheduled events can be the bane of our existence. One of the most common – and most costly – is stuck pipe. When it comes to stuck pipe, there are two kinds of drillers: Those who have had it (and will admit it), and those who are going to have it.
There are, in general, three different mechanisms that cause stuck pipe. Each
has different causes and different solutions, but one thing they all have in
common is that prevention is the most important factor. A problem you have
prevented is one you don’t have to solve or pay for.
First, a pack-off is common in unconsolidated formations. It usually happens as
a combination of events that are easily prevented if the driller is paying
attention. The easiest way to pack off and stick the pipe is to rip through
soft, unconsolidated formations or sand, trying to set a new penetration record.
You might drill a joint down faster than anybody on the planet, but if you
spend 3 days getting it back, you haven’t proved anything. A hole only can be
drilled as fast as it can be cleaned and a decent wall cake built, and this
take time and good mud.
Fast hole will load up the annulus with cuttings, and increase the hydrostatic
pressure on the formation. When the pressure reaches the point that it
overcomes the natural pressure of the formation, lost circulation results. When
this happens, all the cuttings in the annulus will fall on the bit, and you’ll
be stuck. It is characterized by no movement, no rotation and no circulation.
The natural tendency of most drillers is to pull “all she’s got” to try to pull
free. In the case of a sand bridge above the bit, this probably will have the
effect of packing the bridge so tight that recovery may be impossible.
A better approach is to shut down the pump, bleed off the pressure in the drill
string, and very slowly – and gently – pull into the bridge. If you can rotate,
so much the better. This often will allow the sand to slowly creep around the
bit, eventually freeing the pipe. When you are above the bridge, you can
condition the hole – and the mud – and slowly work you way back to bottom. Keep
in mind that a hole that has packed off and broken down will be a potential
trouble spot until you have casing across it. Reduce your mud weight and solids
content as much as possible.
The next common problem with pipe sticking is “differential sticking,” commonly
known as wall-stuck. It almost always is a mud problem. What happens is that
the hydrostatic pressure in the well bore gets much higher than the formation
pressure, and forces the drill string into the wall of the hole. The drill
string then is subject to the formation pressure, rather than the annular
pressure. This problem is characterized by the inability to rotate or pull the
pipe, but free circulation.
The only way to free it is to reduce the pressure in the well bore to the pressure
of the formation. A straight pull seldom works, but a slight amount of torque
on the drill string sometimes will help roll the pipe out of the formation.
Once again, brute force is not the answer. Prevention is a matter of good mud
conditioning before you get in trouble.
The third common cause of stuck pipe is a well geometry problem. On smaller
rigs, this often is caused by excessive pull-down. Many drillers use their
pulldown, rather than using drill collars. Rig manufacturers will disagree with
me – and probably a lot of drillers – but, in my opinion, pulldown is a crutch
for drillers too lazy to pick up drill collars. The closer the weight is to the
bit, the better and straighter the hole. Excessive pulldown forces the drill
pipe into the walls of the hole and deflects the bit, causing key-seats.
The problem usually is seen when pulling out of the hole, and is characterized
by full circulation and the ability to go down, but not up. With a tophead rig,
it sometimes is possible to rotate up through the key-seat. It is much more
difficult with a kelly-drive rig as rotation without the kelly usually requires
some field-expedient methods. Properly drilled vertical holes usually don’t key
seat, but excessive pulldown occasionally will put the bit in the next county,
cause no end of problems, and make casing running difficult or impossible. In
horizontal drilling or directional drilling, drillers generally run packed-hole
assemblies to prevent the key seats. A good bottom hole assembly with the
proper stabilizers and drill collars, along with good drilling methods, will
prevent this in most cases.
Recognizing the cause of stuck pipe goes a long way toward ultimate recovery,
but prevention is the best alternative. Good low-solids mud with the right
properties and good drilling practices will prevent most stuck pipe events.
This saves money and increases profit in the long run.
A while back, my brother Willard got tickets to a NASCAR race. Problem was, he
had a dentist appointment in the morning of the same day. He went to the
dentist as early as he could, and told the dentist, “I’ve got my buddies
sitting in the car, we’re going to a race, and I don’t have time for the
anesthetic to kick in; just pull the tooth.”
The dentist thought this must be a pretty tough man to have a tooth pulled
without anesthetic. He said, “OK, which tooth is it?”
Willard turned to his wife, LeeAnn, and said, “Open your mouth, honey, and show
The World According to Wayne: Unscheduled Events
March 1, 2010