Aspects of induced seismic activity and deep-well sequestration of carbon dioxide

Geologic sequestration of carbon dioxide (CO (sub 2) ) is being investigated as an option to reduce greenhouse gas emissions. An environmental concern with this and any other subsurface injection practice is the potential to cause low-level induced seismicity events. Such induced seismic activity may be minimized through careful site characterization and facility operation. Induced seismic activity mostly occurs along previously faulted rocks and may be investigated by analyzing stress conditions at depth, well testing, and geophysical surveying. Cases of induced seismic activity have been documented at waste disposal wells, oil fields, and other sites. Locating an injection facility requires investigation of subsurface geologic structures, review of seismic activity in the area, well testing, seismic monitoring, and modeling of pressure buildup and fluid migration. Special considerations related to the properties of CO (sub 2) should be noted for an injection system. Supercritical CO (sub 2) liquid is less dense than water and may cause density-driven stress conditions at depth. The fluid may also interact with formation water and rocks, altering permeability and resulting in changes in pressure conditions at depth. Injection of large volumes of CO (sub 2) may affect formation pressures in rock units distant from the injection facility. The possibility for seismic activity induced by deep-well injection has been considered when evaluating the disposal of CO (sub 2) in deep saline reservoirs. The potential for seismic activity is greatest in seismically vulnerable locations with preexisting faulting. The current framework involving proper siting, system installation, and monitoring provides mechanisms to mitigate the risks of induced seismic activity.