CEMENTING CALCULATIONS
The following calculations must be undertaken prior to a cementation operation:
• Slurry Requirements
• No. of sacks of Cement
• Volume of Mixwater
• Volume of Additives
• Displacement Volume Duration of Operation
These calculations will form the basis of the cementing programme. They should be performed in this sequence as will be seen below.
1. Cement Slurry Requirements :
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
cementation operation)
C — the openhole below the casing (rathole)
D — the shoetrack
The volume of slurry that is required will dictate the amount of dry cement, mixwater and additives that will be required for the operation.
. Casing/Casing Annulus
. Casing/Hole Annulus
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Figure 29 Single Stage Cementing 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 openhole 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.
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. 2nd. Stage Annulus. Multi Stage Collar |
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Figure 30 Two-Stage Cementing Operation |
