Cement Calculations

In the previous topic, we introduced how to perform cement calculations to fill an annular space for a particular well design. Why don’t you take a stab at making a calculation yourself.

You will likely notice that drillers have streamlined a number of simple mathematical formulas to make key calculations during the drilling of a well. These formulas are geared toward taking typical oilfield units (in this example, inches for hole or pipe diameter) as input, with the solution given in the proper units for ease of processing or procuring materials.

Cement job calculations

Here we examine the calculations needed to determine the amount of cement required to fill 1,000 ft of annular space in a particular well (diagramed below). The particular cement to be used has a yield of 1.18 ft3 for each sack. Yield is the volume attained when a sack of cement is mixed with water.

Go through the guided steps below to calculate the number of sacks of cement required to complete this cementing job.

  • The length of section to cement is 1,000 ft.
  • The yield of each sack of cement is 1.18 ft3.

Calculate the annular capacity

Annular capacity (in this case, the volume between the outer edge of the hole and the outer edge of the pipe) is calculated simply by multiplying a constant times the difference between the hole diameter (Dh) and the pipe diameter (Dp). The result is the number of cubic feet of volume per foot of depth in the well.

Annular Capacity in ft³/ft=0.005454(Dh²-Dp²)

Look at the pipe drawn here and make note of the following two measurements:

  • The diameter of the hole (Dh) is 7-7/8 inches.
  • The diameter of the pipe (Dp) is 5-1/2 inches.

Your first step is to use the formula given above to determine the annular capacity. Make your calculation and then check your answer below.

Solution to determine the annular capacity in volume / depth interval ( ft³ / ft):

Annular Capacity
=0.005454(7.875²-5.5²)
=0.005454(62.016-30.25)
=0.005454(31.766)

Annular capacity = 0.17325 ft³ / ft

Calculate the annular volume of cement needed

Your next step is to calculate the annular volume to fill with cement. Make your calculation and then check your answer below.

Annular Volume = Annular Capacity x Length of Section to Cement

Solution to determine the annular volume (ft³):

Annular Volume
= (0.17325 ft³ / ft)(1,000 ft)

Annular Volume = 173 ft3

Calculate the number of sacks of cement needed

Finally you must calculate the number of sacks of cement needed for this specific cement job. A cement with a particular yield per sack has been selected and is noted above. Make your calculation and then check your answer below.

# of Sacks of Cement = Annular Volume / Yield per Sack of Cement

Remember: Yield is the volume attained when a sack of cement is mixed with water.

Solution to determine the number of sacks of cement:

# of Sacks of Cement
= 173 ft³ / 1.18 ft³ per sack
= 146.6 sacks

# of Sacks of Cement = 147

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