The geomechanics of induced seismicity associated with large-volume fluid injection—implications for risk mitigation

The injection of produced water back into producing permeable formations is regarded to be of low risk of inducing earthquakes because injection into producing conventional reservoirs generally does not lead to a net increase in reservoir pressure. The rise of production from tight unconventional reservoirs, on the other hand, required injection into non-producing aquifers. While unsurprising in hindsight, the concomitant increase in induced seismicity was unexpected based on the assumption, later shown to be false, that faults in stable cratonic sedimentary basins such as those in Texas and Oklahoma are not critically stressed. Complicating matters more, seismicity preferentially occurred in crystalline basement well below the injection target. Geomechanical models demonstrate that this response can be attributed to poroelastic stresses that are active over a larger distance and greater depth than the direct pore pressure disturbance. Our fully coupled poroelastic finite element simulations also demonstrated that in basins of large-volume injection, stress changes cannot be attributed to a single well or injection operation but reflect the cumulative effect of multiple disposal and production wells on a regional scale, making mitigation significantly more challenging. The difficulty of hindcasting observed seismic events on known and well-instrumented faults also demonstrated that effective forecasting of a seismic response would be difficult. This presentation will discuss viable approaches to mitigating the induced seismicity risk, concluding that active pressure management and avoiding injection in close vicinity to known large faults or close to infrastructure are perhaps the most effective approaches for mitigating earthquake risk associated with large-volume injection of wastewater and CO₂ into aquifers.