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.