The Mechanical Earth Model
Learning objectives
- Define a mechanical earth model as a depth-continuous profile of the stresses, pressures, and rock properties along a well
- Name the six components it assembles: Sv, Pp, Shmin, SHmax, rock strength, and the elastic moduli
- Trace how each component was built in Parts 0 to 7 and which decision it feeds
- State the Ogbon-1 datum: the shared well placed at the 3 km reference so the model ties to the pressure ladder and the velocity calibration
One Well, Every Mechanism
The first seven parts built the mechanisms one at a time: the stress tensor and Mohr circle, effective stress, rock failure, pore-pressure prediction, the frictional polygon, the wellbore and its mud window, and the fracture. Each was a piece studied in isolation. A mechanical earth model, or MEM, is the assembly: a single depth-continuous description of the mechanical state along a well, the vertical stress , the pore pressure , the two horizontal stresses and , the rock strength, and the elastic moduli, every one of them a curve against depth. You build it once, calibrated to the well's own measurements, and then you reuse it for every decision the field asks: the safe mud weight, the sanding onset, the fracture gradient, the fault-reactivation pressure, the depletion forecast. It is the deliverable a geomechanicist is paid for, the counterpart to the rock-physics template that closed Part 7 of the Rock Physics course.
The figure lays out the architecture. Six inputs feed a central stress model, and the stress model feeds the applications. Build each input, drop it into place, and watch the model assemble; the readout carries each component to the Ogbon-1 value the next five sections will actually measure. Nothing here is new physics, it is the accounting that turns seven parts of mechanism into one usable model.
The Ogbon-1 Datum
The rest of Part 8 builds this model on Ogbon-1, the synthetic well shared across the Petrophysics and Rock Physics courses. Those courses read its logs for water saturation and for velocity, questions that do not care about absolute depth. Geomechanics does care: every stress is a density integrated over a depth, so the datum is not a detail. This course places the Ogbon-1 log suite at its 3 km reference datum, the same depth as the Part 4 pressure ladder and the handshake, so the model we build is directly comparable to everything already computed. The reservoir sands, the shales, and the tight carbonate keep their real log character, their densities, porosities, and sonic slownesses; only the datum is set. That is exactly what a real MEM does when logged density is spliced into a regional compaction trend and integrated to total depth, because no well logs density from the surface down.
Why Build It Once
The payoff is reuse under one honest description. The next five sections calibrate the components in turn: from the density log (8.2), rock strength from the sonic and porosity logs (8.3), from the leak-off ledger (8.4), from the wellbore's own breakouts (8.5), and the whole thing assembled with a verdict (8.6). Build it carelessly and every downstream decision inherits the error; build it with each component fit to the data and its misfit read out loud, and the field has one model to plan against. The discipline is the calibration discipline of the Rock Physics course, choose the method, fit it to the well, read the misfit, then trust the prediction, applied now to stress instead of velocity. With the architecture in view, Section 8.2 lays the first course of the model: the vertical stress, integrated from the density log.
References
- Plumb, R., Edwards, S., Pidcock, G., Lee, D., & Stacey, B. (2000). The mechanical earth model concept and its application to high-risk well construction projects. IADC/SPE Drilling Conference, SPE 59128.
- 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.