(432e) Leveraging 3D Seismic to Understand Induced Seismicity Risk Associated with CO2 Injection: An Example from the One Earth Energy Sequestration Project in Gibson City, Illinois

One of the key risks in a CO2 sequestration project is induced seismicity caused by the reactivation of pre-existing faults in the subsurface. During the injection of CO2 into saline aquifers, the existing reservoir pressure will increase in response to the injection. This increase in pressure may cause faults to be reactivated and slip along the fault plane, potentially causing seismic events. Induced seismic events that are large enough to be felt by landowners above or surrounding the injection site, particularly if there is property damage, could result in a pause or an entire shutdown of the CO2 sequestration project. Understanding the risk and the potential for felt seismicity is a key component of the overall risk profile of a CO2 sequestration project. Three dimensional (3D) seismic surveys can be a key tool to help characterize the potential induced seismicity risk.

The One Earth Energy site in Gibson City, Illinois, is one of the two sites that make up the Illinois Storage Corridor project, a U. S. Department of Energy supported Carbon Storage Assurance Facility Enterprise (CarbonSAFE) project. The subsurface characterization of this site consisted of a 2164 meter characterization well drilled in 2021, four lines of 2D seismic totaling 43 linear miles acquired during 2019 and 2021, and a 7 square mile 3D seismic survey acquired in 2022. The 3D survey covers the southern portion of the predicted CO2 plume that results from simulating the total planned injection of 4.5 million tonnes of CO2 per year via three injection wells into the lower Mt. Simon Sandstone storage reservoir.

The 3D seismic survey revealed the presence of at least 11 small faults, varying in length from 150 to 1,200 meters, that emanate from the Precambrian basement and die out in the overlying Argenta sandstone or Lower Mt. Simon Sandstone. Both reverse and normal faults were identified with the 3D seismic, with varying orientations. No faults were identified on the 3D survey that completely transect the Mt. Simon Sandstone storage reservoir, and hence no faults penetrate the overlying Eau Claire Formation confining zone.

The previously acquired 2D seismic data suggested that within the area of the 3D survey, between 1 and 3 faults would be identified. However, after the 3D survey was acquired and interpreted, at least 11 faults were identified. This higher density of faulting increases the risk for potential induced seismicity at the One Earth Energy site. However, the relatively small size of the faults, if reactivated, may not lead to felt seismicity. For slip to occur, the fault strike must have an orientation favorable for slip with respect to the maximum horizontal stress direction of 68° in this area of the Illinois Basin. At least three faults have an orientation that is favorable for slip. The magnitude of an earthquake is limited by the area of the slipped patch on the fault, which in turn is generally smaller than the area of the fault surface. Slip along a patch larger than 1 km in diameter could lead to felt seismicity, but only one of the faults, which is also not favorably oriented for slip, is interpreted to have a map length of more than 1 km.

From previous CCS work in the Illinois Basin the risk for induced seismicity was observed to be greatly mitigated by the presence of the Argenta sandstone between the basement and the injection interval in the lower Mt. Simon Sandstone. At locations where the Argenta is thin or missing, more induced seismicity occurred. In addition, thin, very low permeability baffles within the Mt. Simon inhibit vertical pressure migration and leads to primarily horizontal migration, further reducing the likelihood of activating faults in the basement. Because the Argenta appears to be uniformly present in the One Earth Energy injection area with a thickness averaging 125 meters, basement fault reactivation risk could be reduced despite the presence of multiple faults.