2. Basic Calculations

Table of Content

  1. Volumes and Strokes
  2. Slug Calculations
  3. Accumulator Capacity — Usable Volume Per Bottle
  4. Bulk Density of Cuttings (Using Mud Balance)
  5. Drill String Design (Limitations)
  6. Ton-Mile (TM) Calculations
  7. Cementing Calculations
  8. Weighted Cement Calculations
  9. Calculations for the Number of Sacks of Cement Required
  10. Calculations for the Number of Feet to Be Cemented
  11. Setting a Balanced Cement Plug
  12. Differential Hydrostatic Pressure Between Cement in the Annulus and Mud Inside the Casing
  13. Hydraulicing Casing
  14. Depth of a Washout
  15. Lost Returns — Loss of Overbalance
  16. Stuck Pipe Calculations
  17. Calculations Required for Spotting Pills
  18. Pressure Required to Break Circulation

2.17 Calculations Required for Spotting Pills

The following will be determined:
a. Barrels of spotting fluid (pill) required
b. Pump strokes required to spot the pill

Step 1 Determine the annular capacity, bbl/ft, for drill pipe and drill collars in the annulus:

Annular capacity, bbl/ft = Dh, in.2 − Dp, in.2 1029.4

Step 2 Determine the volume of pill required in the annulus:

Vopl bbl = annular capacity, bbl/ft × section length, ft × washout factor

Step 3 Determine total volume, bbl, of spotting fluid (pill) required:

Barrels = Barrels required in annulus + barrels to be left in drill string

Step 4 Determine drill string capacity, bbl:

Barrels = drill pipe/drill collar capacity, bbl/ft × length, ft

Step 5 Determine strokes required to pump pill:

Strokes = vol of pill, bbl ÷ pump output, bbl/stk

Step 6 Determine number of barrels required to chase pill:

Barrels = drill string vol, bbl − vol left in drill string, bbl

Step 7 Determine strokes required to chase pill:

Strokes = (bbl required to chase pill ÷ pump output, bbl/stk) + strokes required to displace surface system

Step 8 Total strokes required to spot the pill:

Total strokes = strokes required to pump pill + strokes required to chase pill

Example: Drill collars are differentially stuck. Use the following data to spot an oil based pill around the drill collars plus 200 ft (optional) above the collars. Leave 24 bbl in the drill string:
Data:
Well depth = 10,000 ft
Hole diameter = 8-1/2 in.
Drill pipe = 5.0 in. 19.5 lb/ft; capacity = 0.01776 bbl/ft; length = 9400 ft
Pump output = 0.117 bbl/stk
Washout factor = 20%
Drill collars = 6-1/2 in. OD x 2-1/2 in. ID; capacity = 0.0062 bbl/ft; length = 600 ft
Strokes required to displace surface system from suction tank to the drill pipe = 80 stk.

Step 1 Annular capacity around drill pipe and drill collars:

a. Annular capacity around drill collars:

Annular capacity, bbl/ft = [(8.52 − 6.52) ÷ 1029.4]
Annular capacity = 0.02914 bbl/ft

b. Annular capacity around drill pipe:

Annular capacity, bbl/ft = [(8.52 − 5.02) ÷ 1029.4]
Annular capacity = 0.0459 bbl/ft

Step 2 Determine total volume of pill required in annulus:

a. Volume opposite drill collars:

Vol, bbl = 0.02914 bbl/ft × 600 ft × 1.20
Vol = 21.0 bbl

b. Volume opposite drill pipe:

Vol, bbl = 0.0459 bbl/ft × 200 ft × 1.20
Vol = 11.0 bbl

c. Total volume bbl, required in annulus:

Vol, bbl = 21.0 bbl + 11.0 bbl
Vol = 32.0 bbl

Step 3 Total bbl of spotting fluid (pill) required:

Barrels = 32.0 bbl (annulus) + 24.0 bbl (drill pipe)
Barrels = 56.0 bbl

Step 4 Determine drill string capacity:

a. Drill collar capacity, bbl:

Capacity, bbl = 0.0062 bbl/ft × 600 ft
Capacity = 3.72 bbl

b. Drill pipe capacity, bbl:

Capacity, bbl = 0.01776 bbl/ft × 9400 ft
Capacity = 166.94 bbl

c. Total drill string capacity, bbl:

Capacity, bbl = 3.72 bbl + 166.94 bbl
Capacity = 170.6 bbl

Step 5 Determine strokes required to pump pill:

Strokes = 56 bbl ÷ 0.117 bbl/stk
Strokes = 479

Step 6 Determine bbl required to chase pill:

Barrels = 170.6 bbl − 24 bbl
Barrels = 146.6

Step 7 Determine strokes required to chase pill:

Strokes = 146.6 bbl ÷ 0.117 bbl/stk + 80 stk
Strokes = 1333

Step 8 Determine strokes required to spot the pill:

Total strokes = 479 + 1333
Total strokes = 1812

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