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

2.5 Drill String Design (Limitations)

The following will be determined:
Length of bottom hole assembly (BHA) necessary for a desired weight on bit (WOB).
Feet of drill pipe that can be used with a specific bottom hole assembly (BHA).

1. Length of bottom hole assembly necessary for a desired weight on bit:

$\mathrm{Length,\; ft}=\frac{\mathrm{WOB}\times f}{\mathrm{Wdc}\times \mathrm{BF}}$

where WOB = desired weight to be used while drilling
f = safety factor to place neutral point in drill collars
Wdc = drill collar weight, lb/ft
BF = buoyancy factor

Example: Desired WOB while drilling = 50,000 lb
Safety factor = 15%
Drill collar weight 8 in. OD—3 in. ID = 147 lb/ft
Mud weight = 12.0 ppg

Solution:
a. Buoyancy factor (BF):
$\mathrm{BF}=\frac{65.5-12.0}{65.5}$
BF = 0.8168
b. Length of bottom hole assembly (BHA) necessary:
$\mathrm{Length,\; ft}=\frac{5000\times 1.15}{147\times 0.8168}$
Length= 479 ft

2. Feet of drill pipe that can be used with a specific BHA

NOTE: Obtain tensile strength for new pipe from cementing handbook or other source.

Determine buoyancy factor:

$\mathrm{BF}=\frac{65.5-\mathrm{mud\; weight,\; ppg}}{65.5}$

Determine maximum length of drill pipe that can be run into the hole with a specific BHA.:

${\mathrm{Length}}_{\max }=\frac{\left[\right(T\times f)-\mathrm{MOP}-\mathrm{Wbha}]\times \mathrm{BF}}{\mathrm{Wdp}}$

where T = tensile strength, lb for new pipe
f = safety factor to correct new pipe to no. 2 pipe
MOP = margin of overpull
Wbha = BHA weight in air, lb/ft
Wdp = drill pipe weight in air, lb/ft. including tool joint
BF = buoyancy factor

Determine total depth that can be reached with a specific bottom-hole assembly:

Total depth, ft = Length_max + BHA length

Example: Drill pipe (5.0 in.) = 21.87 lb/ft - Grade G
Tensile strength = 554,000 lb
BHA weight in air = 50,000 lb
BHA length = 500 ft
Desired overpull = 100,000 lb
Mud weight = 13.5 ppg
Safety factor = 10%

Solution:

a. Buoyancy factor:

$\mathrm{BF}=\frac{65.5-13.5}{65.5}$
BF = 0.7939

b. Maximum length of drill pipe that can be run into the hole:

${\mathrm{Length}}_{\max }=\frac{\left[\right(554000\times 0.90)-\mathrm{100000}-\mathrm{50000}]\times \mathrm{0.7939}}{\mathrm{21.87}}$
Length_max = 12,655 ft

c. Total depth that can be reached with this BHA and this drill pipe:

Total depth, ft = 12,655 ft + 500 ft
Total depth = 13,155 ft

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