The following example describes in detail how to calculate the cement needed for a particular well design.
Annular capacity is calculated simply by multiplying a constant times the difference between the hole diameter and the pipe diameter. The result is the number of cubic feet of volume per foot of depth.
Annular Capacity in ft³/ft =0.005454(Dh²-Dp²)
Look at the pipe drawn here.
The diameter of the hole is 7-7/8 inches.
The diameter of the pipe is 5-1/2 inches.
Annular capacity 0.17325 ft³ / ft
To calculate the number of sacks of cement needed, first multiply the annular capacity by the number of feet of depth of the annulus. Then divide the total volume of the annulus by the yield of a single sack of cement.
For example, if the casing section drawn above is 1,000 feet deep, then the annular volume is 173 ft3. If the yield of a sack of cement is 1.18 ft3 then 173 ft3 divided by 1.18 ft3 equals 146.6. Therefore at least 147 sacks of cement are required for this particular cementing job.
Images: “Cement bags” by apidach via Shutterstock; “Graphic” by Top Energy Training