Lesson 18 Casing Design Example PETE 411 Well Drilling Lesson 18 Casing Design Example

Casing Design Example Example Problem API Design Factors “Worst Possible Conditions” Effect of Axial Tension on Collapse Strength Iteration and Interpolation Design for Burst, Collapse and Tension

Read: Applied Drilling Engineering, Ch.7 HW #9 - Velocity Profiles Due 10-18-02 PETE 411 Lessons can be found at: http://pumpjack.tamu.edu/~juvkam-wold/ Multimedia Programs can be found at: Network Neighborhood / juvkam-wold2 / Multimedia

Casing Design Example Design a 9 5/8-in., 8,000-ft combination casing string for a well where the mud wt. will be 12.5 ppg and the formation pore pressure is expected to be 6,000 psi. Only the grades and weights shown are available (N-80, all weights). Use API design factors. Design for “worst possible conditions.”

Casing Design - Solution Before solving this problem is it necessary to understand what we mean by “Design Factors” and “worst possible conditions”. API Design Factors Design factors are essentially “safety factors” that allow us to design safe, reliable casing strings. Each operator may have his own set of design factors, based on his experience, and the condition of the pipe.

Casing Design These are the API design Factors: In PETE 411, we’ll use the design factors recommended by the API unless otherwise specified. These are the API design Factors: Tension and Joint Strength: NT = 1.8 Collapse (from external pressure): Nc= 1.125 Burst (from internal pressure): Ni = 1.1

Casing Design What this means is that, for example, if we need to design a string where the maximum tensile force is expected to be 100,000 lbf, we select pipe that can handle 100,000 * 1.8 = 180,000 lbf in tension. Note that the Halliburton Cementing Tables list actual pipe strengths, without safety factors built in.

Casing Design Unless otherwise specified in a particular problem, we shall also assume the following: Worst Possible Conditions 1. For Collapse design, assume that the casing is empty on the inside (p = 0 psig) 2. For Burst design, assume no “backup” fluid on the outside of the casing (p = 0 psig)

Worst Possible Conditions, cont’d Casing Design Worst Possible Conditions, cont’d 3. For Tension design, assume no buoyancy effect 4. For Collapse design, assume no buoyancy effect The casing string must be designed to stand up to the expected conditions in burst, collapse and tension. Above conditions are quite conservative. They are also simplified for easier understanding of the basic concepts.

Casing Design - Solution Burst Requirements (based on the expected pore pressure) The whole casing string must be capable of withstanding this internal pressure without failing in burst. Depth Pressure

Casing Design - Solution Collapse Requirements For collapse design, we start at the bottom of the string and work our way up. Our design criteria will be based on hydrostatic pressure resulting from the 12.5 ppg mud that will be in the hole when the casing string is run, prior to cementing.

Collapse Requirements, cont’d Casing Design Depth Collapse Requirements, cont’d Pressure

Casing Design Req’d: Burst: 6,600 psi Collapse: 5,850 psi

Casing Design Note that two of the weights of N-80 casing meet the burst requirements, but only the 53.5 #/ft pipe can handle the collapse requirement at the bottom of the hole (5,850 psi). The 53.5 #/ft pipe could probably run all the way to the surface (would still have to check tension), but there may be a lower cost alternative.

Casing Design Depth To what depth might we be able to run N-80, 47 #/ft? The maximum annular pressure that this pipe may be exposed to, is: Pressure

Casing Design First Iteration At what depth do we see this pressure (4,231 psig) in a column of 12.5 #/gal mud?

Casing Design This is the depth to which the pipe could be run if there were no axial stress in the pipe… But at 6,509’ we have (8,000 - 6,509) = 1,491’ of 53.5 #/ft pipe below us. The weight of this pipe will reduce the collapse resistance of the 47.0 #/ft pipe! 6,509’ 8,000’

Casing Design Weight, W1 = 53.5 #/ft * 1,491 ft = 79,769 lbf This weight results in an axial stress in the 47 #/ft pipe

Casing Design 4,680 psi (with 5,000 psi stress) The API tables show that the above stress will reduce the collapse resistance from 4,760 to somewhere between 4,680 psi (with 5,000 psi stress) and 4,600 psi (with 10,000 psi stress)

Casing Design Interpolation between these values shows that the collapse resistance at 5,877 psi axial stress is: With the design factor,

Casing Design This (4,148 psig) is the pressure at a depth Which differs considerably from the initial depth of 6,509 ft, so a second iteration is required.

Casing Design Second Iteration Now consider running the 47 #/ft pipe to the new depth of 6,382 ft.

Interpolating again, This is the pressure at a depth of Casing Design Interpolating again, This is the pressure at a depth of

Casing Design This is within 13 ft of the assumed value. If more accuracy is desired (generally not needed), proceed with the: Third Iteration Pcc3 = ?

Third Iteration, cont’d Casing Design Third Iteration, cont’d

Casing Design Third Iteration, cont’d This is the answer we are looking for, i.e., we can run 47 #/ft N-80 pipe to a depth of 6,369 ft, and 53.5 #/ft pipe between 6,369 and 8,000 ft. Perhaps this string will run all the way to the surface (check tension), or perhaps an even more economical string would include some 43.5 #/ft pipe?

Casing Design At some depth the 43.5 #/ft pipe would be able to handle the collapse requirements, but we have already determined that it will not meet burst requirements.

Burst? N-80 43.5 #/ft? Depth = 5,057? 5,066? 5,210? N-80 47.0 #/ft 6,369 6,382 6,509 N-80 53.5 #/ft 8,000

Tension? N-80 53.5 #/ft? N-80 47.0 #/ft Depth = 6,369 6,369 6,382 6,509 N-80 53.5 #/ft 8,000

Tension Check The weight on the top joint of casing would be With a design factor of 1.8 for tension, a pipe strength of

Tension Check The Halliburton cementing tables give a yield strength of 1,086,000 lbf for the pipe body and a joint strength of 905,000 lbf for LT & C.

Casing Design Review We have 4 different weights of casing available to us in this case: 1. Two of the four weights are unacceptable to us everywhere in the string because they do not satisfy the burst requirements. 2. Only the N-80, 53.5 #/ft pipe is capable of withstanding the collapse requirements at the bottom of the string

Casing Design Review 3. Since the 53.5 #/ft pipe is the most expensive, we want to use as little of it as possible, so we want to use as much 47.0 #/ft pipe as possible. 4. Don’t forget to check to make sure the tension requirements are met; both for pipe body, and for threads and couplings (T&C).

Casing Design Review The collapse resistance of N-80, 47 #/ft will determine to what depth it can be run. Two factors will reduce this depth: Design Factor Axial Stress (tension) “Halliburton” collapse resistance: 4,760 psi Apply design factor:

Casing Design Review To determine the effect of axial stress requires an iterative process: 1. Determine the depth capability without axial stress 2. Determine axial stress at this point

Casing Design Review 3. Determine corresponding collapse resistance 4. Determine depth where this pressure exists 5. Compare with previous depth estimate 6. Repeat steps 2-6 using the new depth estimate 7. When depths agree, accept answer (typically 2-4 iterations) (agreement to within 30 ft will be satisfactory)

Linear Interpolation

Linear Interpolation

Linear Interpolation With design factor: