Casing Design Workshop Course Introduction Casing Design Workshop

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

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?

Hydrostatic Pressure A useful form for our course

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.

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.

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.

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

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

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).

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

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

Sample Well Bore Hydrostatics A refresher in USC units Learn the use of 0.052 conversion factor Derivation in textbook

Hydrostatic Pressure Calculate pressure at 10500 ft Memorize the 0.052 conversion factor

Hydrostatic Pressure Solution

Hydrostatic Pressure 2 Calculate hydrostatic pressure at 10500 ft

Hydrostatic Pressure 2 Solution

Hydrostatic Differential Pressure Calculate static surface tubing pressure

Hydrostatic Differential Pressure Solution

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

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.

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).

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

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.

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

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

It looks easy. I am ready!