What is the reason for placing a joint between guide shoe and float collars in drilling?
It is hard to say without seeing the design in question, but is the guiding shoe also a float-shoe? The casing joint in question could be kept empty until cementing starts, to reduce derrick stress
Generally when you place a joint between the guide/float shoe and float collar.It creates a buffer to avoid displacement of the cement outside. The shoe which can result in a “wet shoe” meaning there is no good cement around your shoe and will therefore most like fail a leak off test. During displacement the wiper plug is landed on the float collar leaving the last joint of casing filled with cement. The rule of thumb is you should not displace more than half of the shoe joint (shoe track) capacity to safeguard over displacement which pushes the cement too far outside the shoe.
What is the purpose of casing shoe in oil and gas well drilling?
Casing shoe is a casing accesory. These days there is a float (non return valve) built in it so its known as Float shoe. Float shoe along with float collar and with a couple of casing joints in between forms the shoe track.
Primary uses of CS
Floats serve as well control devices during Running in hole. Float shoe is first and float collar is above, as a back up. It is rounded at the bottom so it helps guide the casing string while running in hole.Allows casing fillup and circulation through casing, while RIH. Sometimes auto fill up floats are used which allow fill up automatically but by dropping a ball you turn it into proper floats.
During cement job the volume of the shoe track, i.e. volume between float shoe and float collar serve as a buffer volume to pump additional displacement fluid without ending up with a wet shoe. in case you don’t bump the plug with the theoretical displacement volume during the cement job.
Post cement job the floats serve as a barrier preventing the heavy cement from U-tubing.
Sometimes a reaming shoe is run at the bottom, which is used for reaming casing down in tight formations.It can also serve as a barrier for temporary abandonment of the well. So the shoe is a critical part of the well.
A leaking or wet shoe is a recipe for distaster. If you read about the macondo incident, the major reason for the screwed up cement job is suspected to be the failure of floats.
After the well has been drilled for a particular section ( say 12.1/4″ ), the well is needed to be cased off to isolate it from from water zone and formation. Accordingly casing of particular size ( here 9.5/8″) is run.
But at the very first casing, a casing shoe or casing guide shoe is make up. The main purpose of this shoe is to guide the shoe along the drilled profile. And prevent lateral forces on casing bottom and smooth landing.
In some shoe, float is there which prevents the back flow of Drilling fluid into the casing string.