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A float shoe is used in the drilling of oil and gas wells. It is a single cylindrical piece that performs the functions of both a float collar and a guide shoe. Its main purpose is to guide drilling equipment to the bottom of the well without snagging, like a guide shoe. It also contains a check valve to allow fluids to pass in only one direction, like a float collar.
As operators drill a well, they line the hole with casing. The casing is a steel pipe of the same diameter as the hole. It prevents the walls of the hole from caving in during the drilling process. It also prevents fluid from seeping into the hole. In the event that petroleum is found at the drill site, the casing is used as a conduit to pump petroleum out of the well.
As each section of casing, or joint, is only 30 feet (9 meters) long, many are needed to reach the bottom of most wells. A series of casings screwed together is called a casing string. The float shoe is screwed onto the bottom of the casing string to guide the casing to the bottom of the well. It prevents mud from entering the casing. It also helps the casing to float during the descent, which decreases the load acting on the oil derrick and other topside drilling equipment.
The float shoe is a short cylinder made of steel. It is rounded at the bottom to allow easy insertion into the well hole. The cylinder is hollow in the middle to allow liquid to flow through. Most are equipped with check valves, which allow the liquid to flow in only one direction. This prevents water or mud from being forced upward through the casing.
The type of check valve most often used in a float shoe is called a ball check valve, which is basically a ball inside a tube. The wider end of the tube faces the bottom of the float shoe. As liquid moves up the float shoe, it forces the ball up the tube until the ball hits the narrower neck at the top of the tube. The pressure of the liquid against the ball seals it to the tube, preventing liquid from entering the rest of the casing string. If water or cement is pumped down the float shoe, the ball drops away from the neck, allowing the liquid to flow by.