Balanced Cement Plug Calculation

You already learn about balance cement plug and I would like to show how to calculate balance cement plug.

 Let’s start with the concept so you can imagine how the calculation should be.

1. Determine volume of cement that you need.

2. Determine height of cement and spacer when pipe in hole.

3. Determine displace volume to balance the hydrostatic both sides. When the displacement is completed, you should have equal height of cement/spacer/mud. You can see the figure below for more understanding.

4. When you pull cement stinger, you will have the balance set properly like this.

Example for the balanced cement calculation

The operator plans to set abandonment plug in the open hole across the pay sand.

Well information is listed below:

• 7” casing shoe is set at 6000’MD/5500’TVD.

• 7” casing grade 23 ppf BTC, ID = 6.366 inch.

• Drilling fluid: 10.0 ppg oil based mud

• Hole size = 6.25” based on caliper log.

• Hole TD = 12,000’MD/10,000’TVD

• Pay sand is from 9000 – 9500 ft

• The planned cement length is 1,000 ft.

• The planned bottom of cement plug is 9,600 ft.

• Spacer: 50 bbl of spacer and weight of spacer 10.5 ppg.

• Cement stinger: 1200 ft of 2-7/8” tubing.

• 2-7/8” tubing ID is 2.44 inch.

• Drill pipe size: 4” DP and ID of drill pipe is 3.35 inch.

• Use 0% excess

• Cement weight 16.0 ppg

 

Determine how much cement that we should need and how the displacement plan should be.

 

1. How much cement do we need for this job?

Hole capacity of the well = 6.25² ÷ 1029.4 = 0.0379 bbl/ft

Volume of cement = length of cement x hole capacity

Volume of cement = 1000 x 0.0379 = 37.9 bbl

 

2. How height of cement while the cement string in the well?

Cement stringer (2-7/8” tubing) capacity = 2.441² ÷ 1029.4 = 0.0058 bbl/ft

Annular capacity between hole and cement stinger= (6.25² – 2.875²) ÷ 1029.4 = 0.03 bbl/ft

 

Since there is pipe in the hole, the top of cement (TOC) will be higher.

 

The TOC is calculated by the following formula:

Length of cement with pipe = Cement Volume ÷ (Drill pipe capacity + Annular capacity)

Where;

Length of cement with pipe is in ft.

Cement Volume in bbl.

Drill pipe capacity in bbl/ft.

Annular capacity in bbl/ft.

 

Length of cement with pipe = 37.9 ÷ (0.03+0.0058) = 1,059 ft

 

It means that height of cement is 1,059 ft, therefore TOC when pipe in hole is 8541 ft (9,600-1,059).

 

3. Height of spacer while the cement string in the well

 

The plan is to pump 50 bbl of spacer; therefore, there might be some spacer up above the cement stinger.

 

Volume of spacer from top of cement to top of cement stinger (Spacer volume1)

 

Spacer volume1 = annular capacity between hole and cement stinger x length of cement stinger from TOP to top of stinger.

 

Spacer volume1 = 0.03 x (8541-8400) = 4.23 bbl.

 

The rest of spacer which is 45.77 (50-4.23) bbl is in between drill pipe and hole.

 

Length of spacer between drill pipe and hole = spacer volume ÷ annular capacity between hole and 4” DP

 

Annular capacity between hole and 4” DP = (6.25²-4²) ÷ 1029.4 = 0.0224 bbl/ft

 

Length of spacer between drill pipe and hole = 45.77 ÷ 0.0224 = 2043 ft

 

So the top of spacer is equal to top of stinger minus length of spacer between drill pipe and hole.

Top of spacer = 8400 – 2043 = 6357 ft.

 

4. Volume of space to that has equal height of spacer in the annulus

The volume of spacer inside string is equal to volume of spacer in cement stinger plus volume of spacer in 4” DP.

 

Volume of spacer in cement stinger = cement stinger capacity x length from top of stinger to top of cement

 

Cement stringer (2-7/8” tubing) capacity = 2.4412 ÷ 1029.4 = 0.0058 bbl/ft

Volume of spacer in cement stinger = 0.0058 x (8541-8400) =0.8178 bbl.

 

Volume of spacer in 4” DP = 4”DP capacity x length from top of stinger to top of spacer

4” DP capacity = 3.352 ÷ 1029.4 = 0.0109 bbl/ft

Volume of spacer in 4” DP = 0.0109 x (8400 – 6357) = 22.27 bbl

 

Total spacer volume in the string = 22.27 + 0.8178 = 23.1 (round up figure) bbl.

 

5. Volume of mud displacement

In order to balance hydrostatic pressure, volume of mud displacement is volume from surface to top of spacer.

 

Volume of mud displacement = capacity of 4” DP x top of spacer

Volume of mud displacement = 0.0109 x 6357 = 69.3 bbl.

 

Pumping steps for balanced cement plug are as follows:

 

1. Pump 50 bbl of spacer.

2. Pump 37.9 bbl of cement.

3. Pump 23.1 bbl of spacer.

4. Pump 69.b bbl of drilling mud. ** You may under displace 2-3 bbl to create cement falling effect in drill pipe. **

5. Pull slowly above TOC

6. Circulate bottom up

7. Pull out of hole to surface.

 

Final cement in place after pulling the cement stinger out of hole is below.

 

Ref books: Lapeyrouse, N.J., 2002. Formulas and calculations for drilling, production and workover, Boston: Gulf Professional publishing.

Bourgoyne, A.J.T., Chenevert , M.E. & Millheim, K.K., 1986. SPE Textbook Series, Volume 2: Applied Drilling Engineering, Society of Petroleum Engineers.

Mitchell, R.F., Miska, S. & Aadny, B.S., 2011. Fundamentals of drilling engineering, Richardson, TX: Society of Petroleum Engineers.