Plug Cementing | What is Plug Cementing?
Plug Cement Slurry Design
Cement slurry design is determined by the objectives of the plug and depends on well depth, bottom hole circulating temperature (BHCT), and drilling-fluid properties. Key design considerations are:
- Rheology
- Density
- Compressive strength
- Thickening time
- Waiting on cement (WOC) time
For lost-circulation plugs, slurry density should be controlled to reduce bulk slurry loss. Bentonite and silicate extenders are useful slurry-density-reduction additives, but they lower the ultimate cement compressive strength. Gilsonite, a natural asphalt, is also a lightweight cement additive used in lost-circulation plugs. Low compressive strength is desirable in a lost-circulation cement plug because it will need to be drilled through, and a high-compressive-strength plug is more likely to cause the bit to deviate. Slurries used to correct lost circulation are also designed to have thixotropic properties to help prevent total cement loss into the lost-circulation zone.
Success of a sidetracking or whipstock plug depends to a large extent on high compressive strength. A densified cement (API Class A, G, or H) is recommended. Densified cements:
- Provide high strength
- Tolerate drilling-fluid contamination
- Expand upon setting
- Decrease fluid loss
- Have negligible permeability when set
- Provide strong bonding
Table 1 shows typical compressive strengths of densified Class G and H cements used for setting sidetracking cement plugs. Additives such as calcium chloride, dispersants, or retarders can be used in densified slurries. However, because slurries with dispersants have lower consistency and viscosity, the amount of dispersant should be kept to a minimum for better plug stability. Laboratory testing to ensure maximum viscosity, ample thickening time, and appropriate compressive strength should be performed before each job.
| Table 1: Compressive strengths of densified Class G and H cements | |||||
|---|---|---|---|---|---|
| Slurry Density | 100° F and 1,600 psi | 140° F and 3,000 psi | 170° F and 3,000 psi | 200° F and 3,000 psi | |
| After 12 hrs. | |||||
| 16.5 | 2,075 | 4,000 | 7,800 | 9,035 | |
| 17.0 | 2,850 | 6,535 | 8,375 | 10,025 | |
| 17.5 | 3,975 | 6,585 | 8,550 | 10,675 | |
| After 24 hrs. | |||||
| 16.5 | 5,475 | 8,985 | 9,750 | 10,460 | |
| 17.0 | 6,035 | 9,060 | 11,075 | 12,660 | |
| 17.5 | 7,025 | 10,125 | 11,860 | 12,875 | |
Some operators using whipstock plugs claim that higher compressive strengths and better success ratios are achieved when 10 to 20% sand and/or silica flour are included in the cement composition. A possible explanation of this phenomenon is that sand may improve drilling-fluid removal by its scouring action, and thus may reduce drilling-fluid contamination, affecting compressive strength in a way unrelated to any silica/cement chemical reaction.
To further reduce the risk of cement slurry contamination with drilling mud, it is advisable to pump a spacer fluid ahead of and behind the cement. The density of the spacer fluid should be equal to or greater than that of the drilling fluid. Because of the density difference between mud and cement, the cement will tend to migrate downward in the well. This is another reason for including a preflush in the plugging treatment.
Because of the small cement volumes associated with plug cementing, the slurry should be batch mixed or at least mixed at a rate that will ensure uniform slurry density.
