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

2.14 Depth of a Washout

Method 1

Pump soft line or other plugging material down the drill pipe and notice how many strokes are required before the pump pressure increases.

Depth of washout, ft = strokes required × pump output, bbl/stk ÷ drill pipe capacity, bbl/ft

Example:
Drill pipe = 3-1/2 in. 13.3 lb/ft
Capacity = 0.00742 bbl/ft
Pump output = 0.112 bbl/stk (5-1/2 in. by 14 in. duplex @ 90% efficiency)

NOTE:A pressure increase was noticed after 360 strokes.

Depth of washout, ft = 360 stk × 0.112 bbl/stk ÷ 0.00742 bbl/ft
Depth of washout = 5434 ft

Method 2

Pump some material that will go through the washout, up the annulus and over the shale shaker. This material must be of the type that can be easily observed as it comes across the shaker. Examples: carbide, corn starch, glass beads, bright coloured paint, etc.

Depth of washout, ft = strokes required × pump output, bbl/stk ÷ (drill pipe capacity, bbl/ft + annular capacity, bbl/ft)

Example:
Drill pipe = 3-1/2 in. 13.3 lb/ft capacity = 0.00742 bbl/ft
Pump output = 0.112 bbl/stk (5-1/2 in. × 14 in. duplex @ 90% efficiency)
Annulus hole size = 8-1/2 in.
Annulus capacity = 0.0583 bbl/ft (8-1/2 in. × 3-1/2 in.)

NOTE: The material pumped down the drill pipe was noticed coming over the shaker after 2680 strokes.

Drill pipe capacity plus annular capacity:
0.00742 bbl/ft + 0.0583 bbl/ft = 0.0657 bbl/ft
Depth of washout, ft = 2680 stk × 0.112 bbl/stk ÷ 0.0657 bbl/ft
Depth of washout = 4569 ft

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