The Ogbon-1 Stress Model

Part 8, Part 8: Calibration, the Ogbon-1 Mechanical Earth Model

Learning objectives

Assemble the six calibrated components into the complete Ogbon-1 stress model
Plot the stress state in the polygon and read the mobilized friction of 0.58 against the 0.6 limit
Show how a few MPa of added pore pressure pushes the field past the frictional edge
Cross-check the stress model against the Rock Physics velocity calibration of the same well

The Model Assembled

Six sections, six components: $S_v$ v 67.7, $P_p$ p 35.3, UCS 65, the elastic moduli, $S_{hmin}$ hmin 46.0, and $S_{Hmax}$ Hmax 62.0. This section puts them together into the Ogbon-1 stress model, the deliverable Part 8 set out to build. The dashboard shows the stress ladder on the left and the frictional polygon on the right, with the stress point, $S_{hmin}$ hmin 46 across and $S_{Hmax}$ Hmax 62 up, dropped into the normal-faulting triangle.

Read the verdict. The effective stresses are $S_v' = 32.4$ v′=32.4 and $S_{hmin}' = 10.7$ hmin′=10.7 MPa, and the mobilized friction they imply is 0.58, just under the 0.6 frictional limit of Section 5.2. The field is stable, but barely: it sits close to the edge of what friction allows. That is not an artifact of this well. The crust tends to sit near frictional failure everywhere it has been measured, the critically-stressed-crust observation, so a model that lands at 0.58 is telling the truth about the Earth, not flattering the example.

One Bad Decision from the Envelope

Now make the bad decision. Raise the pore pressure with the slider, as an injector or a pocket of overpressure would, and watch the mobilized friction climb while the polygon shrinks toward the fixed stress point. It takes less than half a MPa, only about 0.5 MPa of added pressure, to push the mobilized friction past 0.6 and the point outside the polygon: a well-oriented fault would slip. That is the whole argument of induced seismicity compressed into one motion, and it is why the Ogbon-1 stress model is not a static picture but a starting condition. The small effective $S_{hmin}'$ hmin′ of 10.7 MPa is the reason the field is so sensitive: there is almost no margin to spend. Part 9 makes the fault explicit and Part 10 depletes the field and walks the stress path, but both begin here, from a model poised right at the edge.

Two Courses, One Rock

One cross-check closes the calibration. The Rock Physics course calibrated the same Ogbon-1 sand's velocity in its Section 7.4 and found it too stiff to be a friable sand, its true home the stiff-sand line of a consolidated rock. Geomechanics, arriving from stress rather than velocity, finds that same rock moderately strong at UCS 65 and the field poised at mobilized 0.58. The two descriptions are consistent: a competent, consolidated sand, neither weak nor failing on its own, carrying a stress state near but inside the frictional limit. When two courses built on different physics agree about one rock, the calibration is trustworthy. With the Ogbon-1 mechanical earth model complete and cross-checked, the course turns to what the model is for: faults in Part 9, and the producing field in Part 10.

References

Zoback, M. D. (2007). Reservoir Geomechanics (chs. 9-12, the integrated stress model). Cambridge University Press.
Townend, J., & Zoback, M. D. (2000). How faulting keeps the crust strong. Geology, 28(5), 399-402.
Plumb, R., Edwards, S., Pidcock, G., Lee, D., & Stacey, B. (2000). The mechanical earth model concept. IADC/SPE Drilling Conference, SPE 59128.