PRODUCTION CASING SELECTION FOR COLLAPSE, BURST AND AXIAL DESIGN

FACTOR LOADS EXERCISE Instructions

Use the example well data from this document or the powerpoint notes handout to complete the following graphs.

Production Casing – Collapse Loads

Production Casing – Burst Loads

Production Casing – Axial Loads

When complete, scan or photograph your work and upload it. Post questions to the discussion board.

Production Casing – Collapse and Burst Design

Finally we come to the production casing. The production casing is the final string of casing. We expect that it should be capable of containing full well pressure throughout the producing life of the well as a backup for the tubing. It should not collapse should the well be depleted significantly, nor under any operations conducted in the wellbore during workovers or stimulations. And since this well is a gas well of fairly high pressure we expect it to not fail in the event of a near surface tubing leak.

We will use the following design factors:

· Collapse – 1.125

· Burst – 1.2

· Tension – 1.6 or 100,000 over pull

Now we look at the 7 inch casing available to us for this production string.

Table 6 - 3 7" Casing Inventory

Wt

(lb/ft)

Grade Conn. ID

(in.)

Collapse

(psi)

Burst

(psi)

Joint

Strength

(1000 lb)

29 N-80 LT&C 6.184 7030 8160 597

32 N-80 LT&C 6.094 8600 9060 672

29 P-110 LT&C 6.184 8530 11220 797

32 P-110 LT&C 6.094 10780 12460 897

35 P-110 LT&C 6.004 13030 12700 996

We will start with the collapse load for the evacuated production string.

Figure 6-21 Preliminary collapse selection

If we look at the burst loading chart below we see that it is not possible to use the casing inventory above.

Figure 6-22 Burst Loading

At this point we must make a serious decision as to whether we must purchase a very expensive string of casing to satisfy the maximum burst loading due to a shallow tubing leak or if we will use the conventional tubing backup load. We cannot adequately address such a choice here so we will show both. First the conventional tubing backup design.

Figure 6-23 Preliminary burst selection

We see that there is a section of 29 lb/ft pipe in the middle of the section that should definitely be eliminated. The result is shown below. In practice we might want to simplify this design further, but we are going to leave it as it is so that we can see how the combined loads affect it in the next chapter.

Figure 6-24 Modified burst design

Next we check the axial loading For the various cases.

· Installation – running

o Inside: mud, 15.3 ppg

o Outside: mud, 15.3

· Installation – plug bump

o Inside: 15.3 ppg mud + 1200 psi above differential displacement pressure

o Outside: tail slurry 16.6 ppg, lead slurry 15.9 ppg, spacer and mud 15.3 ppg

· Installation – post plug bump

o Inside: 15.3 ppg mud

o Outside: same as plug bump

Figure 6-25 Axial schematic of production string

Preliminary Calculations

· Cross Section Areas

· Section Weights

RUNNING CASE

· Calculate Pressures

· Calculate Section Forces

· Calculate Pressures

· Calculate Section Forces

· Calculate Pressures

· Calculate Section Forces

Plot the axial loads.

Figure 6-26 Axial loads for burst/collapse selection

The plug bump load is the highest so we will apply the design factors to it and then check our burst/collapse selection to see if it is adequate in axial load design.

Figure 6-27 Axial design check using alternate production casing string

Next we show a copy of the spread sheet calculations for this design.

Figure 6-28 Spread sheet axial load calculations for intermediate casing as tubing backup

And finally we summarize the results of the this tubing backup design.

Figure 6-29 Summary of alternate production casing design

Once again, in practice we would likely reduce this design to two or three sections instead of four, but there is a point we want to make about bi-axial design in the next chapter and this selection lends itself better to that than a simpler design.

Shallow Tubing Leak Case

Because now we are assuming this is a critical high pressure gas well, it is highly recommended that we use some sort of proprietary connection to avoid serious gas leaks. There are many types equally suited as we discussed in Chapter 2. For our example we have chosen the Tenaris Blue connection. It is a modern coupled and shouldered connection with a metal-to-metal seal and has 100% joint strength and a significantly higher internal yield (burst) rating than API LT&C couplings.

Table 6-4 High strength casing inventory

Production Casing Burst Design

In designing for burst and collapse of a production string collapse usually dominates the selection for the bottom of the string and burst dominates the selection near the surface. But in this case where we are considering a high pressure gas well, the inclusion of a surface tubing leak above a weighted packer fluid this is often not the norm. So, we will first consider burst in this production string.

First consider the load curves we developed in the previous chapter. We are going to have to make a serious decision. You can see immediately that the burst load near the bottom is extremely high and over 16,000 psi when the design factor is applied. Regular API LT&C casing cannot work with this design and that is why we chose a proprietary connection as in the above chart. As a first step we can eliminate the plug bump curve since it is not relevant.

Figure 6-30 Production casing burst loads

We will now consider the most severe loading that accounts for a near surface tubing leak.

Figure 6-31 Production casing preliminary selection based on burst design loads

This is not a practical string, and we would want to modify it. There are so many possibilities here that we could take into account all sorts of calculations on cost transport, availability and so forth. For this example we are going to arbitrarily limit our selection to three sections total. The only thing that is certain is that we must have 38 lb/ft Q-125 on bottom.. If we start with that as a given and then look at the rest of the string we can see that if we start there and alternate we can select 38 # Q, 38# P, and 32# P and have three sections of similar length. That will be our choice, but remember this is strictly arbitrary.

Figure 6-32 Production casing, a more practical burst selection

Let us now check this casing selection for collapse. Our collapse loads are straight forward.

Figure 6-33 Production casing collapse loads

The dominate collapse load is the production load, so we will apply the 1.125 design factor and plot the collapse strengths of our burst selection to see how if it meets our collapse design requirements.

Figure 6-34 Production casing collapse design and preliminary selection from burst design

This selection is more than adequate for the collapse loading. We will now check the axial loading.

Production Casing – Axial Load Design

The mud densities and load cases are as before.

· Installation – running

o Inside: mud, 15.3 ppg

o Outside: mud, 15.3

· Installation – plug bump

o Inside: 15.3 ppg mud + 1200 psi above differential displacement pressure

o Outside: tail slurry 16.6 ppg, lead slurry 15.9 ppg, spacer and mud 15.3 ppg

· Installation – Post plug bump

o Inside: 15.3 ppg mud

o Outside: same as plug bump

Figure 6-25 Alternate production string

In this string there are three sections, but two have the same inside diameter so that will simplify our manual calculations somewhat.

RUNNING CASE

· Calculate Cross Section Areas

· Calculate Section Weights

· Calculate Pressures

· Calculate Section Forces

PLUG BUMP CASE

· Calculate Pressures

· Calculate Section Forces

Here is a plot of the axial loads

Figure 6-36 Axial loads for alternate production string

And a copy of the spread sheet axial loads and design.

Figure 6-37 Axial loads and design for alternate production casing string

Check to see how the burst design fits the axial design and see that it is more than adequate.

Figure 6-38 Axial design check for burst selection

And lastly, we must keep in mind bit and tool sizes to be used in completion and remedial applications as some of these heavier weights will not allow the use of most standard size tools normally used in 7 inch casing.

This completes our production casing basic design.

Figure 6-39 Summary of alternate production casing design

That particular design is possibly an extreme for many, but this is the only truly safe approach for high pressure gas wells where a near surface tubing leak might occur (i.e., all high pressure gas wells!). The question is, are there alternatives to a near surface tubing leak like this one? Yes, there are alternatives. Some designs simply ignore the possibility with the assumption that if the casing ruptures it will be far enough below the surface that it can be controlled and remedied with a workover (maybe). In some areas it is possible to put a relief valve on the tubing head so that the leak pressure can be bled off and possibly flared in safety. This is not a particularly good solution, but it has been done often. There are cases where instead of a casing rupture, the tubing string has collapsed in the lower well bore and allowed communication as the propagating buckle opened the tubing to the casing when the buckle reached a connection (called a wet buckle) and thus relieved the high pressure in the lower casing. All of these things would require a complicated workover that in many cases would exceed the cost of the casing string.

Whenever we design a string like this one there are some things that differ from a conventional production string. First the burst pressure load and design lines have a negative slope instead of a positive slope as is normally the case. So the maximum burst loads occur at the bottom of each section rather than at the top. This can be seen in Figure 6 - 32. Hence the safety factor in the casing summary is calculated at the bottom of each section rather than the top. It also turns out that the increasing tensile strength of the lower sections means that the critical tensile load at any section transition point is in the bottom of the higher section rather than the top of the lower section. The tension safety factors listed in the summary were calculated at the top of the string, the bottom of the top section and the bottom of the second section.

Do not get accustomed to always checking tension and burst at the top of sections and collapse at the bottom of sections. You must always look at the load curve when checking safety factors at section transition points. The casing strength point closest to the load curve is always the critical point to check.