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?
If we drilled at 9 ppg down to 5,000 ft, would we encounter any problems?
If drilling continues to 6,000 ft with a mud weight of 9 ppg, would problems be encountered?
Which mud weight would allow safe drilling to 8,000 ft?
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?
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 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.
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.