Subsurface Cementing Equipment: Casing Hardware
The cementing equipment used on or within the casing string is commonly referred to as “casing hardware.” These items fall generally into three basic categories:
- Guide shoes and float equipment
- Wiper tools
- Centralization and hole conditioning equipment
A number of companies manufacture a range of casing hardware and there are a wide variety of sizes and modifications available for specific cementing situations. This discussion provides an overview of the main categories and designs of casing hardware.
Note: Equipment for liner cementing or squeeze cementing is distinct from the casing hardware discussed in this section. Those equipment components are discussed in the subject Cementing Casing Liners and Remedial Cementing Equipment: Squeeze Packers, Bridge plugs, Inflatable tools.
Guide Shoes and Float Equipment
Guide shoes (Figure 1) are screwed onto the bottom of the casing string. They help guide the casing to the bottom of the hole and keep it from getting hung up on irregularities in the wellbore. The outer part of a guide shoe is generally made of steel and matches the casing diameter while the inner part is made of a drillable material like concrete or composite.
A modified guide shoe (Figure 2) with side ports ensures that approximately 60% of the fluid is pumped through the ports versus the nose. This can help to wash away obstructions or help to get the casing to bottom if drill cuttings have settled out in the hole.
A float shoe (Figure 3) is slightly more complicated than a guide shoe in that it contains a check valve that prevents backflow of fluid into the casing from the annulus as the casing is lowered into the well. This serves several purposes:
- It allows the casing to be “floated” into the well, reducing the hook load. Fluid is added to the casing at the surface at a rate that maintains the desired degree of buoyancy.
- When the casing is in place, the backflow valve permits fluid to be circulated down the casing, around the shoe and up the annulus, conditioning the hole prior to pumping the cement.
- Float shoes allow the pumping of cement slurries that are heavier than the mud they are displacing. Should pumping be interrupted, the check valve prevents the heavier cement slurry from “u-tubing” from the annulus back into the casing.
The internal components of float shoes, including valves that come in a number of designs, are designed to be drillable once the casing is cemented and ready to be drilled out.
A float collar (Figure 4) is similar to a float shoe, except that it is designed for placement higher in the casing string, between adjacent casing joints – usually one to three casing joints above the float or guide shoe. A float collar contains a backpressure valve similar to a float shoe and also a smooth surface or latching profile for the wiper plug.
Because the cement immediately below the bottom wiper plug that seats in the float collar may be contaminated, the float collar should be positioned far enough above the bottom of the casing to minimize the amount of contaminated cement pumped out around the shoe (Figure 5).
Inner String Cementing
With large diameter casing strings, drill pipe may be used as the conduit for pumping cement. This is accomplished using a “stinger” on the end of the drill pipe that mates into a special receptacle within the casing string – it can simply stab into the receptacle or screw into it (Figure 6).
The cement is pumped down the drill pipe and up around the casing as it would if the cement were being pumped directly down the casing.
Wiper Plugs
Cementing wiper plugs (Figure 7) are barriers shaped to fit within a casing string and separate the cement slurry from the drilling mud ahead of it and the displacing fluid behind it.
Wiper plugs wipe the inside of the casing and, when seated in a tool such as a float collar, provide a positive indication – a sudden increase in pump pressure – that the cement has been properly displaced. These plugs are made from flexible and rigid plastic or aluminum elements and are designed to be drillable. Most are also designed to not rotate once seated, making them easier to drill up.
The bottom plug that precedes the cement slurry has an internal bypass so that once it has pushed all of the drilling mud out of the casing it ruptures and lets the cement slurry through.
The top plug that follows the cement slurry and separates the slurry from the displacing fluid is designed to withstand the force of being landed abruptly and does not rupture.
Top and bottom plugs are often color coded to avoid being launched out of sequence. Pumping the top plug first would leave the casing full of cement, requiring it to be drilled out.
Two-wiper plug system Animation 1
When the two-plug system is used, the operator should verify that the bottom plug is, in fact, placed in the bottom position in the cementing head.
Displacement of the top plug must be carefully monitored. The volume of displacing fluid pumped behind the plug should be determined from calibrations on cementing-unit tanks or by measuring out of a storage tank. Another method is to count pump strokes per minute and convert to volume pumped as a function of time, based on the piston displacement of the pump.
If the top plug does not bump at the appropriate calculated volume (allowing for displacement-fluid compressibility), displacement should be stopped. Troubleshooting the situation requires that the operator have accurate measurements of the volume of surface lines and equipment, the capacity of the casing, the depth to the top of the float collar, and the amount of fluid pumped. Surface cementing equipment capacity should be calculated even though this volume will be small compared to the hole volume.
External Casing-Mounted Tools: Centralizers, Scratchers, and Wipers
Two important elements of achieving a successful primary cement job are:
- Ensuring that the casing is properly centered in the hole and not laying against the side of the wellbore or external casing string
- Ensuring that the mud cake on the wellbore wall has been removed so that the cement can bond fully to the formation
Devices mounted on the outside of the casing as it is run into the well help to achieve these objectives.
Centralizers
Casing centralizers are designed to position the casing more centrally in the hole, reduce drag and differential sticking (where casing is held against the wellbore wall by fluid pressure), and keep casing from becoming stuck in grooves in the wellbore wall of deviated wells (“key seats”). By keeping the casing centered they also help to promote more efficient displacement of the mud in the annulus by the advancing cement slurry. Centralizers come in a variety of designs and sizes (Figure 9).
- Rigid centralizers are built with a fixed bow height
- Semi-rigid styles are built with a bow that can collapse without destroying the integrity of the centralizer
- Spring-bow centralizers (the most commonly used) have steel bows that are designed to flex inward. API Specification 10D provides specifications for bow-spring centralizers
- Single piece centralizer
- Roller-equipped centralizers are used to reduce drag in long horizontal wellbores
Most centralizers are kept in place on the casing with stop collars.
Proper spacing of centralizers on the casing depends on casing size, hole size, wellbore deviation, and the various factors that relate to drag. Cementing companies use software programs to determine optimal centralizer placing. Some practical guidelines for vertical wells are:
- Surface and intermediate casing. One centralizer should be placed immediately above the shoe and one at the top of each of the bottom six joints, to ensure centralization and uniform placement of cement in this critical section. Centralizers may also be installed to improve cement placement around zones where good cement bond is critical, such as water-bearing sandstone.
- Production casing. Place one centralizer immediately above the shoe and one at the top of each of the bottom six joints. They should be placed on every joint through the producing zones and extend 100 feet above (and below, if applicable). Other potential problem zones, such as key seats and sticking areas should also be protected with centralizers.
- Liners. Use centralizers if clearance and hole conditions permit.
- Stage cementing. Centralizers should be spaced over the cemented interval above the stage collar and one joint below, since there is no casing movement in such jobs. When used, the external packer acts as the lower centralizer.
The degree to which a casing string is centered in the hole is quantified by either the value of eccentricity or pipe standoff ratio (Figure 10).
- Eccentricity is the ratio of the distance between the centers of the cylinders to their radial difference.
- Pipe standoff ratio, expressed as a percentage, is 1 minus eccentricity times 100.
When a casing string is concentric, the pipe standoff ratio is 100% and the eccentricity is zero. Conversely, when the inner casing string is touching the wall of the borehole or outer casing string, eccentricity is 1 and pipe standoff ratio is 0%.
The proper placement of centralizers to maximize standoff depends on the well’s deviation from vertical over its length as well as the degree of variance from the drill bit diameter.
For example, Figure 11 shows pipe standoff versus depth for a well that is 6,562 feet deep.
The plus signs indicate the placement of centralizers. The hole profile (A), shows areas where the borehole diameter is larger than gauge. Graph B shows the extent of the well’s departure from vertical in meters.
In graph C, 51 centralizers are proposed at intervals of one every three joints of casing. Note that standoff declines significantly in the areas of severe washout and towards the bottom of the hole where the departure from vertical is greatest.
Obtaining a good cement job around 1,100 to 1,200 m will be difficult with standoff as low as 30%. A centralizer placement optimization program can be used to identify optimal placement of additional centralizers.
In graph D, the number of proposed centralizers was increased from 51 to 70, placing them at two per casing joint in the washed out sections and two every five or one per every two joints near the bottom of the hole. This centralizer optimization program dramatically minimizes the portions of the hole where standoff is less than 80%.
Scratchers
Scratchers (Figure 12) placed on the casing aid in the removal of wellbore mud cake prior to cementing, which improves the cement bond. They are designed for casing reciprocation (up and down movement) and rotation before and during cementing. When using these devices, after about every 10 to 15 joints of casing are run, the driller must stop and circulate the annulus clean of displaced mud solids.
- Rotating-type wipers or scratchers are run across the zone of interest plus an additional 20 ft above and below the zone.
- Reciprocating-type scratchers are generally spaced at 5 to 15 ft intervals throughout the zone and an additional 20 ft above and below the zone.
When reciprocating, the vertical casing movement should always exceed the distance between wipers or scratchers. If reciprocal movement equals the spacing, the removed mud cake and cuttings can accumulate at the end of each stroke.
When wipers or scratchers are used, mud circulation should always be started before the pipe is moved, and initially, the pipe should be moved slowly. If no pipe movement is planned, these devices should not be used.
Cementing Casing Liners
A casing liner is not a piece of cementing equipment, but its installation involves equipment features specifically related to the cementing process.
A conventional casing string is run the entire length of the well and “hung off” within the casing head at the surface. Each successive string is set within the previous, larger diameter string. A casing string that does not extend to the top of the wellbore, but instead is suspended from inside the bottom of the previously run casing string, is called a liner (Figure 13).
The casing joints making up a liner are identical to a conventional string, only the devices for anchoring the liner within the previous string are different.
The overlap of a liner with the casing from which it is hung can vary from about 50 ft for drilling liners to as long as 500 ft for production liners.
- A drilling liner is set to case off and isolate lost circulation zones, overpressured zones or formations that can expand into the wellbore.
- A production liner is run as the last casing to total depth as a replacement for an entire production casing string.
One advantage of a liner is a substantial savings in casing cost. To save casing, however, additional tools and risks are involved. A liner can be fitted with components so that it can be connected (“tied back”) to the surface at a later time if necessary. Liners are also run to enable the use of larger diameter production tubing within a portion of the wellbore, to reduce the required casing weight capability of the rig, to quickly deal with drilling problems, or to avoid lost circulation.
Liners are run into the hole on drill pipe and cemented through the attached drill pipe. The cement is circulated around the bottom of the liner as with any other casing string, but instead of hanging off the casing in the wellhead, the shorter liner is hung from the previous string of casing. A packoff seals the annulus above the hanger, protecting the setting cement outside the liner. The drill pipe is retrieved to the surface after cementing. The liner setting tool is designed to enable all of these tool manipulations during and after the cement displacement process.
Remedial Cementing Equipment: Squeeze Packers, Bridge Plugs, and Inflatable Tools
Remedial cementing tools are devices associated with the placement of cement downhole to either fill an uncemented void behind the casing (squeeze cementing) or to set a cement plug within an existing casing string. These devices are designed to be retrievable, permanent, or drillable, depending on the situation and well configuration (Figure 14).
For example, squeeze packers are run on a drill pipe work string that can be expanded against the inside of the casing to seal off the space above or below the space to be remediated. These tools are typically set using a combination of rotation and up-and-down motion of the work string. They are not designed to be parted from the work string and are retrieved with along with it after use.
A drillable squeeze packer is more commonly referred to as a cement retainer. Cement retainers are generally made of cast iron rather than steel – strong but brittle and therefore drillable—and are set within the casing using either a wireline or a work string of drill pipe, tubing, or coiled tubing. A cement retainer prevents flow in either direction. A retainer can be drilled out or remain as a permanent plug, depending on the situation.
A bridge plug is a drillable/retrievable device that is set on either a wireline or a work string. This device is designed to contain pressure from above or below, and is often used to protect the casing below the zone being squeezed.
Inflatable packers contain a rubber packing element that can be expanded against the inside of a casing string or against the open hole wellbore wall. They are useful when it is necessary to packoff a zone beneath a wellbore restriction (they have smaller outer diameters than mechanical packers) or within an open hole section of uncertain diameter.
Examples
- To squeeze off some perforations or perhaps a hole in a casing string, one option is to set a cement retainer via wireline just below the squeeze zone, and then go into the hole with a work string and a retrievable squeeze packer. After setting the packer, the cement is pumped at high pressure into the perforations. The packer is then released, the work string is pulled, and a drill string drills out the cement remaining inside the casing as well as the drillable cement retainer. The well is then ready for tubing and production.
- To plug a well, a simple approach may be to carry out the same type of operation on the producing perforations. Depending on the location of the producing zone or zones, cement retainers or bridge plugs may be added and additional cement pumped into the casing to permanently prevent any flow or pressure communication within the casing over time.
The specific design of squeeze or plugging procedures will vary widely depending on the circumstances and well configuration.
