CASING ACCESSORIES Casing accessories and their functions are given below. CASING GUIDE SHOE A guide shoe is a heavy collar that is attached to the first length of casing or liner to be lowered into the hole. It has a rounded nose to guide the casing around obstructions. Casing shoes are provided with an inside bevel to guide tools into the casing after the cement or plastic material forming the nose is removed. It has an opening in the bottom to allow drilling mud to enter the casing as it is lowered. Later, cement exits through this opening. A guide shoe is always employed whether or not other casing accessories are used. Three kinds of guide shoes may be used: • Plain guide shoe, • Combination floats and guide shoe • Automatic fill-up guide shoe FLOAT COLLAR In addition to the guide shoe, a float collar is often used. A float collar is similar to a float shoe. Float collars permit the casing to float into the hole, by virtue of the casings being partially empty. The back-pressure valve is closed by the pressure of the outside fluid column, thereby preventing entry of the fluid as the casing is lowered into the hole. The amount of flotation depends on the amount of fluid placed inside the casing string as it is filled from the surface. Mud and cement can be pumped through the valve because it opens with pressure from above. When the casing has been run to the desired depth, circulation is established through the float valve. The float collar may be installed on top of the first joint or on top of the second or third joint to go into the hole. Most operators employ a float collar at a distance of one or more lengths above the casing shoe in order to provide space inside the casing for contaminated cement. The valve in the float collar also serves as a check valve in the string to prevent backflow of cement after it is pumped outside the string. The float collar serves as a stop for the top plug when cement is displaced, enabling a quantity of slurry to stay inside the string at the casing shoe, so the operator has reasonable assurance of having good quality cement outside the casing at that point. Some operators employ a float shoe and a baffle collar arrangement instead of a float collar. The purpose of the baffle collar, which resembles a float collar without a backpressure valve, is to stop the wiper plug and leave one or more joints of casing filled with cement. Float collars and shoes formerly were attached to the casing by welding, but this practice has generally been discontinued in favor of thread-locking compounds to prevent unscrewing while drilling out. Welding of shoes, couplings, and collars may weaken the casing and should be avoided if possible. Thread-locked joints, when properly prepared, withstand greater torque before breaking out than tackwelded casing. CENTRALIZERS Centralizers are often used on casing for two main purposes in connection with cementing: • To ensure a reasonably uniform distribution of cement around the pipe • To obtain a complete seal between the casing and the formation Centering the casing in the hole is necessary for the cement to form a uniform sheath around the casing to effectively prevent migration of fluids from permeable zones. Centralizers must, • Have sufficient restoring force (the force exerted by a centralizer against the borehole to keep the pipe away from the borehole wall) to center the casing in the hole and prevent differential sticking of the casing • Have enough space for free passage of the circulating fluid. • Be spaced close enough together to prevent the casing from contacting the formation wall even in deviated holes • Also allow a concentric sheath of cement around the pipe Centralizers may help to remove wall cake and to prevent cement channeling in the annulus. One type of centralizer is hinged and latches over the casing couplings. Other centralizers are held in position by stop collars. LANDING CASING Landing the casing involves the transfer of the casing’s string weight to the wellhead by means of a casing hanger that seats in the casinghead and seals the annulus between the outer and inner casing strings. Posted By #NoMiRaZa
Posted on: Sun, 06 Jul 2014 12:25:07 +0000
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