The Reactivation Pressure

Part 9, Part 9: Faults and Induced Seismicity

Learning objectives

Compute the reactivation pressure, the pore-pressure rise that brings a fault to slip, sigma_n effective minus tau over mu
Read the reactivation-pressure curve across dip: about 0.5 MPa at the optimal 60 degrees, 11.3 MPa for the canon dip-30
Watch faults slip in sequence as injection raises the pressure, optimal orientations first
Connect the reactivation pressure to the safe injection limit of a field

How Much Pressure to Slip

Section 9.1 said a fault slips when $\tau$ reaches $\mu\sigma_n'$ n′; 9.2 mapped which orientations sit closest. This section answers the operational question: how much added pore pressure does it take? Raising $P_p$ p lowers the effective normal stress $\sigma_n'$ n′ without touching the shear $\tau$ , so the Coulomb margin closes. The pressure that just closes it is the reactivation pressure, $\Delta P = \sigma_n' - \tau/\mu$ n′−tau/mu. It is the fault's budget, the pore-pressure rise it can absorb before it moves.

The curve is the reactivation pressure for every fault dip. It bottoms near 0.5 MPa at the optimal 60 degrees, the fault the field is poised to slip, and rises steeply for poorly-oriented faults: the canon dip-30 fault can take 11.3 MPa, more than twenty times as much. Move the injection slider and watch the faults slip in order, the optimally-oriented first, then progressively worse-oriented ones as the pressure climbs. That order is the signature of induced seismicity: the best-oriented faults go first, and they go early.

The Injection Budget

Flip the question and it becomes an operating limit. If every fault must stay stable, the injection budget is the smallest reactivation pressure among the faults that actually exist in the field, and for a field carrying an optimally-oriented fault that is barely any pressure at all. This is why induced seismicity is so hard to manage by pressure alone in a critically-stressed field: the margin is set by the worst fault, not the average one. Careful operators map their faults, find the most dangerous, and hold the injection pressure below its reactivation limit with room to spare. The reactivation pressure also depends on friction, $\mu$ , and clay-filled faults can be weaker than the Byerlee 0.6 of Part 3.3, which lowers the budget further, so an honest analysis uses the fault's own friction rather than an optimistic default.

From Pressure to Magnitude

The reactivation pressure says whether and when a fault slips. It does not say how big the resulting earthquake is. A fault can creep aseismically or rupture in a felt event, and the difference is how much area breaks and how much stress it drops. Knowing that the Ogbon-1 field is one bad decision from reactivating its optimal fault is only half the risk picture; the other half is the size of the event that decision would trigger. The next section puts a bound on it: given the volume injected, how large an earthquake can it drive?

References

Streit, J. E., & Hillis, R. R. (2004). Estimating fault stability and sustainable fluid pressures for underground storage of CO2 in porous rock. Energy, 29(9-10), 1445-1456.
Segall, P. (1989). Earthquakes triggered by fluid extraction. Geology, 17(10), 942-946.
Zoback, M. D. (2007). Reservoir Geomechanics (ch. 12). Cambridge University Press.