CEMENT ADDITIVES
Most cement slurries will contain some additives, to modify the properties of the slurry and optimise the cement job. Most additives are known by the trade-names used by the cement service companies. Cement additives can be used to:
• Vary the slurry density
• Change the compressive strength
• Accelerate or retard the setting time
• Control filtration and fluid loss
• Reduce slurry viscosity
Additives may be delivered to the rig in granular or liquid form and may be blended with the cement powder or added to the mixwater before the slurry is mixed. The amount of additive used is usually given in terms of a percentage by weight of the cement powder (based on each sack of cement weighing 94 lb). Several additives will affect more than one property and so care must be taken as to how they are used (Figure 4).
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It should be remembered that the slurry is mixed up and tested in the laboratory before the actual cement job.
Accelerators are added to the cement slurry to shorten the time taken for the cement to set. These are used when the setting time for the cement would be much longer than that required to mix and place the slurry, and the drilling rig would incur WOC time. Accelerators are especially important in shallow wells where temperatures are low and therefore the slurry may take a long time to set. In deeper wells the higher temperatures promote the setting process, and accelerators may not be necessary.
The most common types of accelerator are:
• Calcium chloride (CaCl2) 1.5 — 2.0%
• Sodium chloride (NaCl) 2.0 — 2.5%
• Seawater
It should be noted that at higher concentrations these additives will act as retarders.
In deep wells the higher temperatures will reduce the cement slurry’s thickening time. Retarders are used to prolong the thickening time and avoid the risk of the cement setting in the casing prematurely. The bottom hole temperature is the critical factor which influences slurry setting times and therefore for determining the need for retarders. Above a static temperature of 260 — 275 degrees F the effect of retarders should be measured in pilot tests.
The most common types of retarders are:
• Calcium lignosulphanate (sometimes with organic acids) 0.1 — 1.5%
• Saturated Salt Solutions
(c) Lightweight additives (Extenders)
Extenders are used to reduce slurry density for jobs where the hydrostatic head of the cement slurry may exceed the fracture strength of certain formations. In reducing the slurry density the ultimate compressive strength is also reduced and the thickening time increased. The use of these additives allows more mixwater to be added, and hence increases the amount of slurry which is produced by each sack of cement powder (the yield of the slurry). Such additives are therefore sometimes called extenders.
The most common types of lightweight additives are:
• Bentonite (2 — 16%) — This is by far the most common type of additive used to lower slurry density. The bentonite material absorbs water, and therefore allows more mixwater to be added. Bentonite will also however reduce compressive strength and sulphate resistance. The increased yield due to the bentonite added is shown in Table 4.
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• Pozzolan — This may be used in a 50/50 mix with the Portland cement. The result is a slight decrease in compressive strength, and increased sulphate resistance.
• Diatomaceous earth (10 — 40%) — The large surface area of diatomaceous earth allows more water absorption, and produces low density slurries (down to 11
ppg).
Heavyweight additives are used when cementing through overpressured zones. The most common types of additive are:
• Barite (barium sulphate) — this can be used to attain slurry densities of up to 18ppg. It also causes a reduction in strength and pumpability.
• Hematite (Fe2O3) — The high specific gravity of hematite can be used to raise slurry densities to 22 ppg. Hematite significantly reduces the pumpability of slurries and therefore friction reducing additives may be required when using hematite.
• Sand — graded sand (40 — 60 mesh) can give a 2 ppg increase in slurry density.
Fluid loss additives are used to prevent dehydration of the cement slurry and premature setting. The most common additives are:
• Organic polymers (cellulose) 0.5 — 1.5%
• Carboxymethyl hydroxyethyl cellulose (CMHEC) 0.3 — 1.0%
(CMHEC will also act as a retarder)
(f) Friction reducing additives (Dispersants)
Dispersants are added to improve the flow properties of the slurry. In particular they will lower the viscosity of the slurry so that turbulence will occur at a lower circulating pressure, thereby reducing the risk of breaking down formations. The most commonly used are:
• Polymers 0.3 — 0.5 lb/sx of cement
• Salt 1 — 16 lb/sx
• Calcium lignosulphanate 0.5 — 1.5 lb/sxg)
As well as the compounds deliberately added to the slurry on surface, to improve the slurry properties, the cement slurry will also come into contact with, and be contaminated by, drilling mud when it is pumped downhole. The chemicals in the mud may react with the cement to give undesirable side effects. Some of these are listed below:
Mud additive Effect on cement
barite increases density and reduces
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caustic |
compressive strength acts as an accelerator
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calcium compounds diesel oil thinners |
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decrease density decrease density act as retarders |
The mixture of mud and cement causes a sharp increase in viscosity. The major effect of a highly viscous fluid in the annulus is that it forms channels which are not easily displaced. These channels prevent a good cement bond all round the casing.
To prevent mud contamination of the cement a spacer fluid is pumped ahead of the cement slurry.