The Three Pressures

Gradients, Not Pressures

Nobody who works with the subsurface carries pressures around as raw numbers; they carry gradients, pressure per unit depth, because to first order everything scales with depth and the gradient is what stays put. A density converts directly: multiply by the gravity factor, $0.00981$ MPa per meter per g/cc, and a fresh-water column comes out at $9.8$ MPa/km, a saline brine near $10.1$, and a sedimentary rock column near $22.6$ MPa/km for an average density of 2.3 g/cc. In the oilfield's imperial currency those same three numbers are the ones every drilling engineer can recite: 0.433 psi/ft for fresh water, about 0.465 psi/ft for the saturated brine used in Gulf of Mexico planning, and almost exactly 1 psi/ft for the overburden, a coincidence of units so convenient it functions as a mnemonic.

The Currency Exchange

Drillers add one more currency: mud weight in pounds per gallon. A mud of density $\rho$ ppg exerts about $0.052\,\rho$ psi/ft, so fresh water is 8.34 ppg, and any downhole pressure can be quoted as the mud weight that would just balance it at that depth, its equivalent mud weight (EMW). EMW is how this course will talk to the wellbore in Part 6: pore pressure sets the EMW floor, the fracture resistance sets the EMW ceiling, and the mud program must thread between them.

Play the exchange in the figure. With the canon densities, 1.03 g/cc brine and 2.3 g/cc overburden, the ladder at 3000 m reads $30.3$ and $67.7$ MPa, which the currency strip translates to 0.447 and 0.997 psi/ft, and the brine column balances at an EMW of about 8.6 ppg. Stiffen the brine to 1.10 or lighten the overburden to sandstone-only 2.0 and watch every currency reprice together: they are one number wearing four costumes.

Departures Are the Signal

The reason to internalize the reference lines is that information lives in departures from them. A measured pore pressure that plots on the hydrostat says the fluid column is connected to the surface, however deep and however old the rock. A pressure above the line says a seal has been holding load; how far above, divided by the depth of burial since sealing, hints at when. And a gradient steeper than lithostatic over any interval is geologically unsustainable for long: the fluid would lift the overburden. Part 4 turns this one plot into a full prediction discipline; for now, whenever you see a pressure quoted, place it on the ladder before you believe anything else about it.

References

• Zoback, M. D. (2007). Reservoir Geomechanics. Cambridge University Press.
• Fjaer, E., Holt, R. M., Horsrud, P., Raaen, A. M., & Risnes, R. (2008). Petroleum Related Rock Mechanics (2nd ed.). Elsevier.
• Bourgoyne, A. T., Millheim, K. K., Chenevert, M. E., & Young, F. S. (1986). Applied Drilling Engineering. SPE Textbook Series, Vol. 2.