1. Casing Design Workshop
Course Introduction
Casing Design Workshop

2. Basic Calculations – Quick Review
You should already understand:
Basics of hydrostatics
Basic hydrostatic calculations in a wellbore
Hydrostatic pressure – a reminder
Uniform in all directions at a point
Can only act normal (perpendicular) to a surface
We will now review some hydrostatics briefly
This is where the “pre-work” option can start. JEB

3. Hydrostatic Pressure Equation
Newton’s second law leads to the common hydrostatic pressure equation
General form of hydrostatic pressure equation
(incompressible fluid)
(compressible fluid)
Use button to show derivation slide (in the rare instance someone actually asks)
Derivation?

4. Hydrostatic Pressure
A useful form for our course

5. SI Units in Pressure Equation
SI Units (System International): consistent units
For most oil field applications, g, is taken to be approximately equal to a standard gravity
See textbook for the “official” value of standard gravity. Local gravity varies enough that the approximate value here is accurate for our purposes.

6. USC Units (U.S. Customary)
Inconsistent units (require conversion)
The symbol gc is a conversion factor from mass to force based at standard gravity. It is not the gravitational acceleration as many mistakenly think, g is the local acceleration of gravity.

7. USC Units
With the conversion factors we normally write the hydrostatic pressure equation for USC units in the oilfield as:
And we assume h0 = 0 in this version
I include p0 here specifically to remind people that the surface pressure is not necessarily 0 in many applications.

8. Hydrostatic Pressure Example
Vertical, smooth tube at 10,000 ft
2 inch diameter
Seals on bottom free to move in packer
Same pressure below packers
Air in annulus A
8.4 ppg water in B
Which tube weighs more at the surface?
A? B? Same? A B

9. Answer
Same. There are no buoyancy effects on this vertical, smooth tube.
What if the tubes have external couplings on them (same size and number)?
A, why?
With couplings, A weighs more than B because the pressure difference between the bottom and top of each coupling in the water results in a buoyant force upward on string B A B

10. What if the wellbore is inclined?
Assume both wells are inclined at 30°
No couplings
Ignore friction and tubing sag (i.e., tubing is perfectly rigid)
A > B ?
A < B ?
A = B ?
This is the axial load at the top and will not be affected by buoyancy. (This is not the hook load).

11. Inclined Well
A = B
The hydrostatic force is still perpendicular to the axial load direction
But:
Both A and B are less than in the vertical well

12. Directional Wells
Always use vertical depths (rather than measured depths) for hydrostatic pressure calculations
Use vertical depths for determining axial loads (except when accounting for borehole friction and curvature loads)
Use measured depths for casing section lengths when purchasing and running casing
All of our calculations in this course will assume vertical depths only

13. Sample Well Bore Hydrostatics
A refresher in USC units
Learn the use of conversion factor
Derivation in textbook

14. Hydrostatic Pressure Calculate pressure at 10500 ft
Memorize the conversion factor

15. Hydrostatic Pressure Solution

16. Hydrostatic Pressure 2
Calculate hydrostatic pressure at ft

17. Hydrostatic Pressure 2 Solution

18. Hydrostatic Differential Pressure
Calculate static surface tubing pressure

19. Hydrostatic Differential Pressure Solution

20. The U-Tube Method
You can always use a U-tube schematic to visualize and calculate hydrostatic pressures
The column on the left must balance the column on the right

21. Gas Calculations Gas density depends on
Type of gas
Pressure
Temperature
Some use a constant gradient, e.g., 0.1 psi/ft, 0.15 psi/ft, etc.
Density varies with depth
We will use methane
Molecular weight 16
Compressibility factor, (approximately)
Methane is the least dense gas we will likely encounter. Some major companies specify methane for all casing design as the safest choice.

22. A Simple Gas Formula for Methane
T is in Fahrenheit and h is in vertical feet, SI version in manual and textbook
p1 is the known pressure, p2 is the unknown pressure (p1, p2 at depths h1, h2)
See textbook for derivation
T is in Fahrenheit, h in feet (SI version in text and manual).

23. Gas Calculation
Gas pressure is not linear between two points (it is exponential) but …
A two point calculation with methane is accurate enough for most casing design
For critical wells you may want to use a more sophisticated numerical procedure

24. Example Gas Pressure Deviation
METHANE
Two-point calculation
varies only 20 psi
in this example
Difference is insignificant for our purposes. This uses methane with an assumed constant compressibility factor of 1, other gases may vary significantly.

25. Do Not Be Confused by a Vacuum
Cannot by itself cause casing collapse
Cannot suspend a significant column of liquid in an annulus (maximum of 34 ft of fresh water)
A vacuum reduces atmospheric pressure by about 15 psi
15 psi
Everyone knows this, but in field operations you will sometimes run into some bizarre notions about the “mysterious power” of a vacuum.
We want to add a voice-over or “talking head” to tell a story about vacuums causing failures on pumping wells. The reality is that oxygen caused the failure and takes ppb, not even ppm for it to be an issue. Easily addressed with oxygen scavenger and/or corrosion inhibitor. Good observation, wrong conclusion. JEB

26. Advanced Topics Not Covered in Course
Advanced Loads:
Borehole friction* – in directional wells
Lateral buckling loads* – from axial loading
Bending stress magnification* – from coupling standoff in tension/compression
Thermal loads* – pipe expansion/contraction
Thermal loads – due to annular fluid expansion/contraction
Other advanced topics
Sub-sea operations
Casing wear*
Corrosion
* These topics are covered in your textbook

27. It looks easy.
I am ready!