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Casing Setpoint for a Variable Gradient Pore Pressure Case

Contrary to the last well you drilled, the in-situ conditions now present a pore pressure gradient that varies with depth. Your task as drilling engineer is to apply what you have learned thus far to make decisions to safely and economically drill this new well, again to a target depth of 15,000 ft.  

The pressure vs depth plot in Figure 3a shows that the pore pressure and fracture pressure curves are not straight lines, indicating changes in slope with depth. The pressure gradient plot is shown in Figure 3b, indicating that pore pressure gradients start around 8.5 ppg but increase to 15 ppg by 15,000 ft depth. Fracture gradients increase with depth as well, but not as fast, showing that the mud weight window narrows with depth, suggesting we will need to be more precise in our mud design in this case to be successful.

Figure 3a. Variable pressure gradient case. Pore pressure and fracture pressure vs depth.

Figure 3b.Variable pressure gradient case. Gradient vs depth plot for pore pressure and fracture pressure.

Compared to the pressure gradients in Figure 2 (on the previous page) down to 5,000 ft, this case is identical.  Consequently, what would you use for our starting mud weight?

8 ppg

Incorrect. 

9 ppg

Correct. 

9 ppg is the best answer, just as before, given the conditions are the same. 

11 ppg

Incorrect. 

14 ppg

Incorrect. 

If we drilled at 9 ppg down to 5,000 ft, would we encounter any problems?

No, the well can be drilled safely to 5,000 ft.

Correct. 

The mud weight falls within the mud weight window to this depth, so 9 ppg is  good choice. 

Yes, the well will encounter underbalanced conditions if drilled at 9 ppg to 5,000 ft.

Incorrect. 

Yes, the well will encounter overbalanced conditions if drilled at 9 ppg to 5,000 ft.

Incorrect. 

If drilling continues to 6,000 ft with a mud weight of 9 ppg, would problems be encountered?

No, the well can be drilled safely to 5,000 ft.

Incorrect. 

Yes, the well will encounter underbalanced conditions if drilled at 9 ppg to 5,000 ft.

Correct. 

Drilling beyond 5,800 ft will result in underbalanced conditions and make it likely that the well will take a kick. To overcome these problems, the mud weight must be increased before continuing with drilling. 

Yes, the well will encounter overbalanced conditions if drilled at 9 ppg to 5,000 ft.

Incorrect. 

Which mud weight would allow safe drilling to 8,000 ft?

8 ppg

Incorrect. 

10.5 ppg

Incorrect.

11.6 ppg

Correct. 

11.6 ppg is safe to 8,000 ft because it is equal to or greater than the pore pressure gradient but less than the fracture gradient throughout the well (from 0 to 8,000 ft). 

 

13 ppg

Incorrect.

Figure 4 shows the possible conditions to drill to 9,000 ft with a mud weight of 13 ppg. What is your engineering assessment of these conditions?

Drilling is safe because 13 ppg is within the mud weight window at 9,000 ft.

Incorrect. 

Drilling is safe because 13 ppg is greater than the pore pressure gradient at all depths exposed to the wellbore pressure.

Incorrect. 

Drilling is unsafe because 13 ppg exceeds the fracture gradient at depths shallower than 6,000 ft.

Correct. 

Drilling is not safe under these conditions because the wellbore is likely to fracture, causing lost circulation. 

Figure 4. Mud weight of 13 ppg drilling at 9,000 ft.

The shallow section of this hole, at depths less than 5,000 ft, cannot sustain mud weights greater than 12 ppg without risking fracturing and lost circulation. Yet, to drill to the bottom of the well requires a mud weight greater than 15 ppg in order to prevent kicks.

Which of the following assessments makes the most sense? 

The well cannot be drilled safely and must be abandoned.

Incorrect. 

The well can be drilled while taking kicks and producing gas as long as everyone is careful. Blowouts are not a serious safety concern.

Incorrect. 

The shallow parts of the wellbore with low fracture pressure gradients can be protected from higher mud weights by running an intermediate string of casing.

Correct. 

Put the low fracture gradient formations behind pipe, and drilling can continue at higher mud weight until the conditions are unfavorable again and another casing string must be run. 

The green line in Figure 5 shows the mud weight conditions that can drill to 8,000 ft safely, the maximum depth that can be reached without running an intermediate string of casing.

Figure 5. Casing set at 8,000 ft to prevent lost circulation conditions at shallow depths when drilling deeper.

How many more strings of casing must be run to get safely to 15,000 ft? Include the final production casing string that would be set at 15,000 ft.

1

 Incorrect. 

2

Correct. 

Mud weight must be continually increased to maintain overbalanced conditions, and 2 more casing strings must be installed to get to the target depth of 15,000 ft without risking lost circulation problems. 

3

Incorrect. 

Figure 6 is a solution to drilling this well successfully. The green lines indicate the mud weight conditions when a casing set point was reached.  The vertical black lines with triangles at their bases represent the depth range over which each casing string was set.

Figure 6. Final casing design for the variable gradient case. Conductor casing string not shown.

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