FRACKING AND EARTHQUAKES: NOT ALL INDUCED SEISMICITY IS BAD

Hydrofracturing and waste injection induce seismicity. For hazard planning and liability purposes, the seismicity should be separated into four categories corresponding based on differences in the interaction of the fluid pressure and the tectonic environment. Hydrofracturing and waste injection differ in mainly P_f magnitude and duration. It turns out that most damaging seismicity is beneficial.

In modern high-volume hydrofracturing of gas- and oil-bearing shale formations, fluid is forced into the subsurface at pressures above lithostatic pressure, thereby lifting the overburden to create open bedding plane fractures for subsequent petroleum migration. The high P_f also forces open zones of weakness perpendicular to bedding. Although the fracturing occurs seismically, the magnitudes are far below the threshold for surface damage.

Seismic issues usually come when waste injection raises the P_f along an existing fault plane and the fault slips; hyrofracturing rarely creates a large enough zone of high P_f to affect faults. Raising P_f along a fault reduces the frictional resistance, allowing slip at lower deviatoric stress levels. Since magnitude of fault-slip seismicity is directly related to Δσ, lower Δσ mean the resulting seismicity has lower magnitudes than would have occurred naturally. Though these events can produce severe seismicity, the reduced damage produced by the lowered seismic magnitudes should shield injection operations from liability.

Only where an inactive fault slips, or unfaulted rock fails, should liability be assigned to the injection operation. Slip on inactive faults only occurs in settings where Δσ are relatively small and so seismic magnitudes will be small. Although failure of tectonically stressed rock could produce significant seismicity, the localization of the failure to the zone of raised P_f should limit the magnitudes to much less than that of active faults. Nevertheless, the occurrence of unnatural seismicity opens the door to liability.