I’m going to try to keep this pretty straightforward this month, because my wife still is mad at me for getting accidentally locked in a port-a-john at the rig, so don’t tell Tom.
Almost all rigs have the capacity to drill a hole deeper or a large-enough
diameter to be unable to run the casing string without overloading the derrick
or the drawworks. Often, the driller will pull the rig off the hole and rig up
a crane of suitable weight capacity to run the casing. This works, but it is
time-consuming and expensive. It also relies on outside, third-party companies
that may or may not understand our objectives.
There is a better way. The casing can be “floated” into the hole, relieving
much of the weight on the rig. This is done with a float shoe, which acts as a
check valve at the bottom of the casing, and allows the volume inside to
displace enough fluid to balance part of the weight. Of course, a normal check
valve will not work; it must be drillable, to continue the well. It must be
made of drillable materials. Halliburton and other companies make float shoes
in most sizes, but, in the larger sizes, we would have to take out a second
mortgage to afford one. Actually, it’s pretty easy to build one
The easiest material out of which to build a float shoe is the casing you are
going to run in the hole. It is the right size, and it has the mill certs your
engineer wants to see, so that will shut him (or her) up.
Start with a piece of casing about 4 feet long. The float shoe is going to need
a rounded bottom so it will slide by any bridges in the hole, so you will need
something rounded to make a mold. On smaller sizes, a weld-on cap will work; on
the larger sizes, the stamped end-cap from a butane tank is
Set the rounded end cap up on a base, or bury it in the ground to get a stable
work platform. Liberally grease the inside of the cap. Set the casing stub up,
square and level on the cap. Duct-tape the two together. Now you are ready to
build your float shoe.
The first thing is the valve itself. It must seal securely and be able to
withstand the differential pressure, but still be drillable. I use a ball
valve. The ball itself is made of a baseball, liberally coated with the
urethane compound that we use to coat tool handles.
The body is made of common PVC pipe and fittings. I use a piece of 4-inch PVC
for the lower end, a 2-in.-by-4-in. tapered reducer for the seat, and a piece
of 2-inch PVC for the upper tube. The 4-inch must extend all the way to the
bottom, with the baseball inside, the reducer about halfway up, and the 2-inch
out the top. The baseball goes inside the 4-inch. When it floats, it will seal
the pressure, but when you pump cement, it will open allowing full flow. Run a
couple welding rods across the 4-inch to keep from pumping the ball all the way
Because cement tends to shrink when it sets, it is important to anchor the
cement in the shoe. In smaller sizes, I use several welding rods, inserted
through the casing, from side to side, to anchor the plug. In larger sizes, use
2-inch-long (or so) self-tapping screws, inserted from the outside of the
casing in several places, so they stick inside to provide an anchor point for
After the valve is built and the plug anchors are installed, stand the valve
assembly in the float shoe body, and fill it with neat cement. Let it set a
couple days, until you’re ready to run pipe and go for it.
On the first joint or two, you will have to fill the casing before it gets down
to the floor, but after that, you can run pipe, filling as needed to control
I have a couple precautions for you when running a float shoe: Since the casing
is not taking any of the displacement of the hole, you will create large surge
pressures on the formation. It is possible to knock the bottom out of the well
if you don’t run slowly. Carefully calculate the collapse pressure of your
casing, and make sure to fill the casing often enough to allow a safety margin
on casing collapse pressure. In a worst-case scenario, it is possible for the
plug to fail and come up the casing. This will cause the rig to take the full
weight of the string. It usually doesn’t happen too fast; you can see as a
continuous steady increase on the weight indicator. If this happens, set it
down on the slips, right now!
Usually, everything goes well, and you can land your casing where you want it,
at a weight your rig can handle. When you drill out, all you will be drilling
will be cement, PVC and a baseball. I usually watch for the remains of the
baseball in the returns; it gives me a good indicator that I am just about
through the shoe.
I’ve included a sketch of the float shoes we make. If I can help, drop me a
The World According to Wayne: The Skinny on Floating Casing
March 1, 2009