Their are a variety of additives which may be added to cement.. These additives may be delivered to the rigsite as liquid or dry additives. The amount of additive is generally quoted as a percentage of the cement powder used. Since each sack of cement weighs 94 lbs, the amount of additive can be quoted in weight (lbs) rather than volume. This can then be related to the number of sacks of additive. The number of sacks of additive can be calculated from:
Number of sacks of additive = No. sxs Cement x % Additive
Weight of additive = No. sxs of Additive x 94(lb/sk)
The amount of additive is always based on the volume of cement to be used.
The mixwater required to hydrate the cement powder will be prepared and stored in specially cleaned mud tanks. The amount of mixwater required for the operation will depend on the type of cement powder used. The volume of mixwater required for the cement slurry can be calculated from:
Mixwater Vol. = Mixwater per sack x No. sxs
Although cement and other dry chemicals are delivered to the rig site in bulk tanks the amount of dry cement powder is generally quoted in terms of the number of sacks (sxs) of cement required. Each sack of cement is equivalent to 1 cu. ft of cement.
The number of sacks of cement required for the cement operation will depend on the amount of slurry required for the operation (calculated above) and the amount of cement slurry that can be produced from a sack of cement. The amount of cement slurry that can be produced from a sack of cement, known as the yield of the cement, will depend on the type of cement powder (API classification) and the amount of mix water mixed with the cement powder. The latter will also depend on the type of cement and will vary with pressure and temperature. The number of sacks of cement required for the operation can be calculated from the following.
No. of Sacks = Total Volume of Slurry/Yield of Cement
Sufficient cement slurry must be mixed and pumped to fill up the following (see Fig 29):
A – the annular space between the casing and the borehole wall,
B – the annular space between the casings (in the case of a two stage
C – the openhole below the casing (rathole)
D – the shoetrack
The volume of slurry that is required will dictate the amount of dry cement, mix water and additives that will be required for the operation.
In addition to the calculated volumes an excess of slurry will generally be mixed and pumped to accommodate any errors in the calculated volumes. These errors may arise due to inaccuracies in the size of the borehole (due to washouts etc.). It is common to mix an extra 10-20% of the calculated open hole volumes to accommodate these inaccuracies.
The volumetric capacities (quoted in bbls/linear ft or cuft/limear ft or m3/m) of the annuli, casings, and open hole are available from service company cementing tables.. These volumetric capacities can be calculated directly but the cementing tables are simple to use and include a more accurate assessment of the displacement of the casing for instance and the capacities based on nominal diameters.
In the case of a two stage operation (Figure 30) the volume of slurry used in the first stage of the operation is the same as that for a single stage operation. The second stage slurry volume is the slurry required to fill the annulus between the casing and hole (or casing/casing if the multi-stage collar is inside the previous shoe) annular space.