Sufficient cement slurry must be mixed and pumped to fill up the following (see Fig bellow):
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 cement slurry volume that is required will dictate the amount of dry cement, mixwater 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 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.
Below the formula to calculate cement slurry volume requirements
1. Volume annulus : volume between casing and hole, casing and casing
Some company calculate this volume normally with excess
What is excess on cement slurry volume design ?
Excess is safety volume cause by hole surface is not flat like casing on normally bigger than diameter bit, so this excess prepared for
Cement casing job 7″ , hole diameter 8 1/2″ long will cement 2000ft
a. W/O excess (basically for annulus casing and casing)
b. with excess 40% (basically for annulus hole and casing)
a. (0.785 x 8,5 x 8,5 / 144 ) – (0.785 x 7 x 7 / 144 ) x 2000
- Blue is capacity open hole W/O excess
- Red is capacity casing (always not include excess)
- 0.785 come from 3.14 / 4
- 8.5 Diameter hole / bit on inch
- 7 Diameter Casing
b. (0.785 x 8,5 x 8,5 / 144 x 1.4 ) – (0.785 x 7 x 7 / 144 ) x 2000
- 1.4 from 100% + 40% ” 100% normal same with 1 and 40% excess
2. Shoe track volume : Volume inside casing normally 1 or 2 joint between float shoe and float collar
shoe track = capacity volume casing x 1 or 2 joint
3. Rathole : cement below float shoe normally under 7 ft.
Capacity volume hole 8 1/2″ x height rathole
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- Number of Sacks of Cement
- Mixwater Requirements
- Additive Requirements
- Displacement Volume
- Duration of Operation