General Operational Considerations
There are a number of important factors to consider in obtaining a good primary cement job, most of which have been described in this course. Additional factors that special emphasis are discussed here.
Slurry Volume
Because of the difficulty in gauging large open holes, surface casing hole volumes are rarely known precisely. The volume of cement required to fill the annulus can be based on historical experience, but if this information is unavailable, an excess slurry volume of at least 50% of the surface hole callipered annular volume should be pumped. If there is evidence that the upper portion of the hole is widely out of gauge, this can be boosted to 100% or more. This number can exceed 300% for the top section of some deepwater wells. Even when the callipered volume is believed to be accurate, an excess volume of up to 50% is often pumped for the surface casing.
For casing strings run after the surface casing, where there is an accurate callipered hole volume, a 10% excess in slurry volume is sufficient. When one is estimating the hole volume based only on bit size, at least a 50% excess factor should be applied. As with the surface hole, regional experience can be a guide.
In some areas, regulatory requirements dictate how far the top of the cement must be above the uppermost pay zone, and this will determine slurry volume. Three hundred to five hundred feet of cement above the top of the pay is typical for many locations.
Displacement
It is important to achieve complete displacement of the cement slurry, not only to obtain good isolation but also to avoid the need to drill out a large volume of tail cement from the casing after the cement job. Complete displacement also provides the opportunity to effectively pressure test the casing shoe.
If the plug does not bump when expected, signaling complete displacement of the slurry, an additional volume of displacement fluid should be pumped before stopping the operation (generally equal to the capacity of the two joints of casing typically run between the shoe and float collar). The displacement volume calculation is based on casing capacity tables from casing manufacturers. Sometimes these capacities do not accurately account for the true internal diameter of the casing delivered. For example, the internal displacement volume of 9,843 feet of 9-5/8 inch, 47 
casing is 720.5 barrels according to the tables. The actual volume required would be 735 barrels, a 14.5 barrel difference, if the internal diameter of the casing run were only 1% larger than the nominal value.
Careful operators systematically gauge either all casing joints or a statistically significant number of joints while the casing is on the pipe rack.
Another potential cause of displacement volume error is failing to account for the compressibility of the displacing drilling fluid, especially when using oil-based muds. Increasing pressure acts to compress fluid volume with depth, while increasing temperature acts to expand fluid volume. The appropriate compressibility of oil-based drilling muds should be obtained from the mud company to make certain that what could be a several barrel error is not introduced.
Temperature
Knowing the bottomhole circulating temperature (BHCT) is vital. Temperature affects nearly everything about a cement slurry: density, viscosity, flow regime, U-tubing behavior, and friction pressure. The BHCT can be obtained through logging, circulation of temperature probes, or computer modeling, but its accurate determination is critical.
When knowledge of cement strength development time is key, it is important to know how long it will take for the cement slurry temperature to reach the static temperature. Only a temperature simulation model can provide this information. The surface ambient temperature, as well as the temperatures of the dry cement and mix water, influences the slurry mixing temperature and thus the slurry temperature profile as it is pumped downhole. Also, when dry cement is added to mix water the hydration process generates heat. If all of these variables are not taken into account, waiting-on-cement (WOC) times may be too long or too short, and the performance of additives, particularly accelerators, can be misjudged.
Casing Movement
Casing movement – reciprocation, rotation, or both – improves the chances of obtaining a good cement job. Reciprocation is difficult to carry out from floating rigs or in highly deviated wells. Rotation, however, is always possible if the wellhead and rig designs are chosen to accommodate it. Generally speaking, such choices are recommended.
When conventional casings are cemented, the highest torque load is generally reached when the leading end of the cement reaches the shoe. The casing connections must be designed to withstand such torque loads if casing rotation is to be implemented as it should.
Begin pipe movement and mud conditioning immediately after the casing is on bottom. If wall cake and cuttings in the mud returns have either virtually stopped or declined rapidly in volume, repeat scratching movements.
Casing movement should begin at the start of circulation and continue throughout the circulation period. With reciprocating scratchers, the pipe is commonly moved through a 20-ft stroke, with a two-minute interval for the cycle. If rotating scratchers are used, the pipe is rotated as slowly as possible, usually between 10 and 20 rpm.
Bonding
Good zonal isolation depends not only on effective placement of the cement within the annulus, but also on effective bonding of the cement to both the formations along the wellbore wall and the outer surface of the casing string, or in the case of an annulus between two casing strings, two casing surfaces. Achieving a certain compressive strength does not guarantee that a good bond has been achieved.
There are two types of bonds that must be considered: shear bonding and hydraulic bonding.
- The strength of the shear bond mechanically supports the pipe in the hole and is defined as the force required to initiate pipe movement in a cement sheath divided by the cement/casing contact surface area.
- Hydraulic bonding blocks the migration of fluids along the pipe/cement or formation/cement interface. The strength of the hydraulic bond is defined as the pressure required to initiate leakage at either of these interfaces.
Both of these bond strengths can be measured in the laboratory for a variety of cement formulations, casing surface conditions, and formation surfaces. For zonal isolation, hydraulic bonding is more important than shear bonding.
Cement-to-casing shear bond strength tends to increase non-linearly with cement compressive strength. Also, a thin coating of mud at the cement-pipe interface results in a loss of both shear bond strength and hydraulic bond strength. Oil-based mud is more detrimental than water-based mud. Higher shear bond strengths are observed when the pipe surface is roughened by rust, wire brushing, or sandblasting to remove mill varnish. Pressurizing the casing during the setting process is detrimental to shear bond development.
For cement-to-formation bonding, higher bond strengths are obtained against permeable formations because the cement is dehydrated via filter loss, resulting in a higher strength cement.
Avoiding cement contamination
To prevent cement slurry problems in critical situations, two bottom plugs may be required, one ahead of and one behind the spacer fluid pumped ahead of the cement. The spacer pumped ahead of the cement will not remove the thin film of mud on the inside of the casing. When the top plug is pumped behind the slurry, it will collect this mud film and push it ahead, contaminating the tail end of the cement (Figure 1), which is the portion that will be key to obtaining high strength cement at the casing shoe. If two bottom plugs are pumped to frame the spacer, this film can be effectively removed.
Casing Quality Control
Following a few guidelines regarding casing quality control can help to ensure that issues unrelated to the cement or pumping operation do not endanger the success of the cementing job.
- First, tally all casing on the rack, being careful to count, number, and “rabbit” (check the internal ID gauge) all casing joints.
- Check all casing threads for debris or damage. Additionally, check the threads on any crossover equipment for proper thread type and cleanliness.
- Identify all joints by weight and thread type, and place them in proper order for running into the hole. This is particularly important in combination strings.
- Space the landing joints out so that there is no trouble installing the cementing head from either the stabbing board or rig floor after the entire casing string is run.
- Verify that float collars and float shoes are in good working condition. Check the operational features if differential floating equipment is used.
- Measure dimensions and prepare stage cementing equipment separately. If necessary, use thread-locking compound or tack weld on all field makeup connections of the float equipment, plus one or two joints above.
General “Good Cementing Engineering Housekeeping”
Also important is following these general guidelines regarding engineering preparedness.
- Follow all safety procedures including pre-job safety checklists, job safety analysis, pre-job communications (toolbox talks).
- Provide all service personnel involved ample notice of the start of casing operations, so that the correct people are on the job site and prepared before this time.
- Ensure that the cementing team leader and the person on the pump throttle understand the importance of the pumping rate to the success of a job.
- Maintain a log of operations that includes time, density measurements, mixing rate, displacement rate, wellhead pressure, operation in progress, volume of fluid pumped pump strokes per minute and total strokes, in addition to what is recorded by sensors and displayed on the computer screen.
- Hold variations in cement density close to limits and calibrate density measuring equipment is calibrated before the job starts.
- Be aware that changing over from slurry pumping to displacement fluid pumping takes seconds, not minutes.
