Normal Pressure and the Ladder

The Reference, and the Departures

This part is about one line and how rock departs from it. The normal, or hydrostatic, pore pressure is what a fluid column connected to the surface would weigh: at our shared 3 km depth, $30.3$ MPa for the canon brine, climbing at about 10.1 MPa per kilometer. Everything interesting is a departure. A pore pressure above the line is overpressure, the dangerous kind, made when a seal traps fluid and forces it to help carry the rock load. A pressure below is underpressure, rarer, made by uplift or depletion. The Rock Physics course reached this same ladder from the velocity side in its Part 8.1; here the identical numbers, overburden 67.7, hydrostatic 30.3, effective 37.4, become a drilling decision instead of a velocity prediction, and the reservoir's 5 MPa of overpressure lifts the pore line to the 35.3 MPa that the whole course carries.

The figure adds the driller's working line: mud weight in pounds per gallon. The mud in the hole must be heavy enough to hold the pore pressure back, or the well kicks and takes an influx, and light enough not to exceed the rock's fracture resistance, or the well loses circulation into fractures it opens. Drag the mud weight between those bounds. At the canon reservoir the pore pressure sits at 35.3 MPa, an equivalent mud weight of about 10 ppg, and the fracture limit is up near 13 ppg, giving a comfortable window. Push into the overpressured ramp and the two bounds squeeze together; the window that felt roomy at the surface can pinch to nothing at depth, which is the entire problem Part 6 will draw in full.

Why Prediction, Not Just Measurement

A distinction that organizes this whole part: pressure can be measured once you are there, with the wireline testers the Petrophysics course covers in its Part 15, but by then the bit has already passed through the rock that mattered. Geomechanics' job is to predict the pressure ahead of the bit, from seismic velocities before drilling and from logs and drilling data while drilling, so the mud weight and casing plan are right before the well finds trouble. That is the difference between a controlled well and a blowout. The next sections build the prediction toolkit: how overpressure is generated (undercompaction and unloading), how it is read from velocity (Eaton and Bowers), and how it is detected in real time at the bit. First, though, the generating mechanism that makes most of the world's overpressure: burial too fast to drain.

References

• Zoback, M. D. (2007). Reservoir Geomechanics. Cambridge University Press.
• Swarbrick, R. E., & Osborne, M. J. (1998). Mechanisms that generate abnormal pressures: An overview. AAPG Memoir 70, 13-34.
• Mouchet, J.-P., & Mitchell, A. (1989). Abnormal Pressures While Drilling. Elf Aquitaine.