Casing Accessories

Casing accessories are used to: • Guide casing into the hole • To center casing in the hole

• To scrape mud off the wall of the hole 10.4.2.6.1 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 and for cement to exit.

Figure 85 9-5/8” casing guide shoes.

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 float and guide shoe • Automatic fill-up guide shoe

Combination float and guide shoes are often used, particularly on long, expensive strings of pipe, because the float device is actually a backpressure valve and it sometimes desirable to have another aside from the float collar. The valve does not allow mud to enter the shoe but allows mud or cement to exit the casing into the annulus. The casing partially filled with mud to keep it from collapsing from the external pressure of the mud in the annulus. A partially empty casing tends to float thereby easing some of the strain on the derrick. The amount of flotation depends on the amount of fluid placed inside the casing string as it is filled from the surface. The idea is to keep enough mud inside the casing to prevent collapse, but not so full as to lose flotation. Floating casing into a well is only done with very long and very heavy strings of casing.

Figure 86 A float shoe.

A variation of the guide and float shoe is the differential, or automatic, fill-up shoe. This device permits a controlled amount of fluid to enter the bottom of a casing string

while it is being run into the hole. The valve in this shoe keeps mud from getting into the casing at first, but only to a point. As the casing and differential fill-up shoe gets deeper into the hole, mud pressure causes the valve to open and mud to enter the casing. Most automatic fill devices can be made inoperative by filling the casing from the top and applying pressure. Automatic shoes and collars then revert to function as regular backpressure valve devices.

The use of automatic fill-up equipment tends to eliminate one of the causes of lost circulation because it reduces the piston effect of casing in the hole and thereby lowers surge pressure as the casing is lowered into the hole. Automatic fill-up equipment eliminates the need for filling the casing as each joint is made up

10.4.2.6.2 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 casing's being partially empty. 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. This provides space inside the casing for contaminated cement.

When the casing has been run to the desired depth, circulation is established through the float valve. It 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 are attached to the casing string using thread-locked joints. They withstand greater torque before breaking out than tack-welded casing. The welding of shoes, couplings, and collars may weaken the casing and is avoided whenever possible.

10.4.2.6.3 Multistage Cementing Devices

Multistage cementing device is a tool that consists of a ported coupling placed at the proper point in the string. It is used for cementing two or more separate sections behind a casing string. The stage collar is run with long casing strings wherein there is a danger of a formation breakdown if the cement were displaced from the bottom of the string.

Cementing of the lower section of casing is done first in the usual manner, using plugs that will pass through the stage collar without opening the ports. After the first cement stage job has been performed, special plugs are dropped to open the multistage tool hydraulically, and fluid is circulated through the stage tool to the surface. After the cement slurry volume has been pumped, a final plug is dropped and the cement is displaced through the ports. The ports are subsequently closed when the final plug reaches the stage collar.

to form a uniform sheath around the casing to effectively prevent migration of fluids from permeable zones. The force exerted by a centralizer against the borehole to keep the pipe away from the borehole wall is called the restoring force.

Figure 87 A casing centralizer.

Several styles of centralizers are available for matching different well specifications and hole sizes. One type of centralizer is hinged and latches over the casing couplings. Other centralizers are held in position by stop collars. Stop collars keep centralizers and other casing attachments in place on the casing. There are also turbulence generating designs that help clean the annulus and distribute the cement more evenly and uniformly. All the different types offer ample clearance for fluid passage and are extra effective in centering the casing, even in highly deviated holes. 10.4.2.6.5 Cement Scratchers and Wipers

Scratchers and wipers help remove filter cake and gelled mud from the well as the casing is moved. This action provides a better bonding for the cement and helps isolate one zone from another in thecemented area.

Wipers have looped cables to help clean the wellbore. They have large flow areas between the loops to permit the filter cake to pass through the wellbore. There are two styles of wipers—reciprocating and rotating.

Cement scratchers are mechanical wall cleaning devices attached to casing. They help remove filter cake and gelled mud from the borehole wall by abrasion as the casing string is reciprocated or rotated. This action provides a better cement bond and helps isolate one zone from another in the cemented area. Most scratchers may be attached to the casing using mechanical stop collars or clamps. Reciprocating scratchers are normally spaced at 15-to 20-foot intervals throughout the section to be cemented, but rotating scratchers are usually placed opposite the pay zones only.

Figure 88 A casing scratcher.

Strength is added to the cement sheath by using scratchers constructed of a continuous pattern of closely spaced loops. This pattern provides reinforcement in the annular space by lacing the cement with steel cable.

Figure 89 A looped casing scratcher or wiper.

Most operators work the pipe whether it is fitted with scratchers or not. Casing equipped with reciprocating cement wipers is worked up and down for a distance of 5 to 35 feet, depending on the spacing of the devices on the pipe.

Casing fitted with rotating scratchers is turned at a speed of about 20 rpm to prevent damage to the casing from torsional stress. Rotation of the casing string in deviated holes may be hazardous due to flexing and coupling damage during rotation.