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INTERMEDIATE CASING LOAD DETERMINATION EXERCISE
Use the example well data from this document or the powerpoint notes handout to complete the calculations for each of the following. You may write your calculations on the Exercise Template, or use your own graph paper.
When complete, scan or photograph your work and upload it. Post questions to the discussion board.
String Load Phase Case
Intermediate Burst Installation Cementing
Intermediate Burst Installation Plug Bump
Intermediate Burst Install Pressure Test
Intermediate Burst Drilling Severe Gas Kick
Intermediate Collapse Installation Post Plug Bump
Intermediate Collapse Installation Lost Circulation
Intermediate Collapse Installation Cement Placement
The intermediate casing loading is often straight forward like the surface casing, except that the magnitude of the loads is generally greater. In many strings of intermediate casing the maximum pressure from a gas kick is not sufficient to fracture any formation below the shoe and hence the maximum surface pressure will be from a solid column of gas. In addition to that we will look at a procedure that has been used often in cases where the BOP installed at the surface is the weakest link in the pressure control and we must account for it in our casing design.
Our Example Well:
Depth Frm Press Mud Dens Frac Press Kick Marg Temp Ft Ppg equiv Ppg Ppg equiv Ppg equiv °F 0 8.4 8.5 — — 74 3000 8.7 9.2 12.3 11.8 128 10,500 11.3 11.8 15.7 15.2 263 14,000 14.8 15.3 16.2 15.7 326
Intermediate Casing Data
· Size: 9-5/8 · Depth: 10,500 ft · Mud Density: 11.8 ppg at 10,500 ft · Frac Pressure at shoe: 15.7 ppg equiv at 10,500 ft · Temperature at Surface and Shoe: 74 °F, 263 °F · Maximum Mud Density and temperature before next casing string: 15.3 ppg, 326 °F · Cementing Data:
o Cement to 500 ft inside surface casing, 500 ft tail slurry at 15.9 ppg, 7500 ft lead slurry w\50% excess at 14.2 ppg, 1000 ft spacer at 12.0 ppg, displace plug w/11.8 ppg mud, bump plug with 1000 psi above final displacement pressure. (Note this cementing program will prove inadvisable and will be revised in the next section.)
Intermediate Casing, Installation – Collapse Load
There is no collapse situation in the running process unless you consider failure to fill the casing. That is usually negligence or mistake rather than it is a design consideration. In the surface casing design we made the calculation for the empty string in our drilling collapse phase, but we may not do that for this deeper intermediate string so we will do it here.
o o i
p p p 0 0 0
p 0.052(11.8 )(10500 ) 0 6443
Before calculating the post plug bump collapse we will verify our cement hydrostatics, such that the cement pressure is greater than the hydrostatic formation pressure but less than the formation fracture pressure. Here we must be careful because we are considering an open hole and there is no certainty in the height of the cement. We want a minimum column length of 7500 ft, and we have specified 50% excess in the lead slurry to achieve that minimum length and from experience in this area we are comfortable that we can achieve the minimum length using that amount of excess. But for purposes of calculating pressures we do not know how long that column will actually be. The longest it could be is realized if the hole is actually gage size, in other words exactly 9-7/8 in. diameter. If the hole is exactly that diameter then the column will be 1.5(7500) = 11250 ft in length. That means we will have cement all the way to the surface. So we must calculate both the minimum cement pressure and the maximum:
fm min c
0.052 11.3 10500 6170 psi
0.052 15.7 10500 8572 psi
0.052 15.9 1000 12.0 7000 12.0 1000 11.8 1500
6739 psi 0.052 15.9 1000 12.0 9500 6755 psi
p p mt max cmt frac OKp p
After the plug has bumped and the pressure released the collapse loads are:
0 o i
0 0 0 psi
0.052 15.9 1000 12.0 9500 11.8 10500 312 psi
p p p
Notice that we used the worst case of cement pressure because we cannot know ahead of time the actual borehole capacity so we choose the worst case. This concludes our collapse loads for installation.
Figure 5-15 Intermediate casing, collapse, installation, post plug bump
Intermediate Casing, Drilling – Collapse Load
Since this string does not cover any high pressure zones the only drilling collapse loading should occur during a lost circulation event. At and below this shoe the lowest formation pressure is 13.6 ppg equivalent and for the sake of our example we will say that our data are very reliable and we can assume this is the lowest pressure we could have in a lost circulation event so no matter what mud weight we are using the casing should never be empty. The only remote possibility of that happening is a gas blowout that eventually bridges off at the shoe and then bleeds off completely at the surface. That sort of thing has happened, but we will not design for it here. The primary question we have to ask in our design situation is what is the fluid in the hole? In our case the maximum mud density we plan to use in the well is 15.3 ppg and that is what we will assume is in the hole when circulation is lost at the shoe which has a formation pressure equivalent to 11.3 ppg. What is the depth of the top of the 15.3 ppg mud supported by a 13.6 ppg formation?
0.052 15.3 0.052 11.3 10500
11.3 10500 7755 ft
15.3 10500 7755 2745 ft
The depth of the top of the mud is 10500 – 7755 = 2745 ft air down to that depth and mud inside the casing with a greater density than the mud outside. Not a likely collapse scenario, but for the sake of practice let us make the calculations and see how it looks.
Figure 5-16 Intermediate casing, collapse, drilling, lost circulation
For the fluid outside the casing we will use the mud density in which the casing is run, 11.8 ppg in this case. We could assume that the pressure is more nearly formation pressure over time, but we have no idea when it might reach that value. There is also a possibility of eventual gas migration behind the casing and in that case we might use the formation fracture gradient. However, this casing will be shortly superseded by the
production casing so it should not be of great concern what transpires over a period of months or even years.
0 0.052 11.8 2745 0 1684 psi
0.052 11.8 15.3 7755 1684 273 psi
So we see that in this scenario the collapse pressure is a maximum at 2745 ft and less than zero at the shoe which means that the pressure inside the casing at the shoe still exceeds the outside pressure by 273 psi.
Another possibility is that a depleted zone exists below the casing shoe and the possibility of lost circulation is a definite possibility. If we know the depleted pressure of the zone we can proceed as above, however, where we do not have that information we must make some assumptions. This gets us into a gray area where many times we rely on “company policy”. One common approach is to assume the worst we can expect is perhaps a zone that will just balance a column of salt water or even fresh water. Ignoring the consequences of an underground blowout that will probably accompany such an event, we will just concentrate on the collapse possibilities for now.
Another common approach is to assume the casing is empty (as a possibility during the running/installation process we postponed from earlier). For now we will look at both before we make a decision because in some cases the difference in casing selection is not too much different.
Figure 5-17 Intermediate casing, collapse, drilling, lost circulation with water or air inside.
The external fluid in these cases as already explained is usually taken to be the mud density in which the casing was run. It exceeds all the formation pressures that will be cemented behind the pipe.
In the case of fresh water in the pipe:
0 psi 0.052 11.8 8.3 10500 1911 psi
For empty casing:
0 psi 0.052 11.8 10500 6442 psi
We now plot these three possibilities.
Figure 5-18 Intermediate collapse load lines
Intermediate Casing, Installation – Burst Load
In the intermediate casing the installation burst possibilities are the same as in the surface casing. First we look at the possibil