The volume of mud used to displace the cement from the cement stinger or the casing during the cementing operation is commonly known as the displacement volume.
The displacement volume is depend on the way in which the operation is conducted.
a. Stinger Operation :
The displacement volume can be calculated from the volumetric capacity of the cement stinger and the depth of the casing shoe. The cement is generally under displaced by 1-2 bbls of liquid.
Displacement Volume = Volumetric capacity of stinger x Depth of Casing – 1bbl
b. Conventional Operation :
In a conventional cementing operation the displacement volume is calculated from the volumetric capacity of the casing and the depth of the float collar in the casing.
Displacement Volume. = Volumetric Capacity of Casing x Depth of Float
c. Two-stage Cementing Operation:
In a two stage operation the first stage is firstly displaced by a volume of mud, calculated in the same way as a single stage cement operation described above.
The second stage displacement is then calculated on the basis of the volumetric capacity of the casing and the depth of the second stage collar.
Ist Stage :
Displacement Vol. = Volumetric Capacity of Casing x Depth of Float Collar
2nd stage :
Displacement Vol. = Volumetric Capacity of Casing x Depth of Multistage collar
The amount of mud to be pumped during the displacement operation may be quoted in terms of a volume (bbls, cuft etc.) or in terms of the number of strokes of the mud pump required to pump the mud volume. It will therefore be necessary to determine the volume of fluid pumped with each stoke of the pumps (vol./stroke). The number of strokes required to displace the cement will therefore be calculated from:
Number of strokes = Volume of displacement fluid/Vol. of fluid per stroke