We know a few things about what conditions must be present in order for an injection operation to induce seismicity. Namely, a slip-prone fault, shear forces on that fault, and a way for the increase in pore pressure caused by the injection to have an impact on the fault. One method to prevent induced seismicity is to prohibit injection wells where unfavorable conditions exist.
But rather than a sweeping, one-size-fits-all rule, a case-by-case approach has been taken in some areas. The National Research Council recommends, and several agencies have adopted, a “traffic light system” (TLS).
Injection operations without associated seismicity levels or with seismicity levels at or below baseline, “background” levels are considered green-light status and continue operations without special monitoring.
Wells near which seismic events begin to occur enter yellow-light status, whereby pumping continues, perhaps at a slower rate, and any new-well permits or changes to existing permits require more information about operations and geology until seismicity stops, at which time the area returns to green-light status.
Continued or increased seismicity, such as that which might cause damage to buildings in the area or a significant public nuisance, triggers a designation of red-light status and a halt to operations.
Several states in the U.S. have adopted a TLS for responding to possible induced seismicity. Jurisdictions using a TLS may also require well operators to provide more detailed information about the geological properties of a given region, and they may offer more limited (e.g., time-restricted) drilling permits.
The traffic-signal approach can take into account an array of factors likely to influence seismicity near injection wells. Regulators may conduct a risk assessment that accounts for natural factors such as earthquake history in the area, operational factors such as knowledge of formation characteristics and operating experience, and exposure factors such as populations, structures and infrastructure.
Images: “Stoplight” by Chris Voll licensed under CC BY SA 2.0