The results of monitoring of carbon dioxide (CO2) injection at the Illinois Basin—Decatur Project (IBDP) and the companion Illinois Industrial Carbon Capture and Sequestration Sources (IL-ICCS) project—have shown that reservoir response to fluid pressure changes can vary significantly at different injection locations within the same reservoir. Predrill reservoir characterization is important to identify potentially seismogenic faults. However, interpretations of newly reprocessed 3D seismic reflection data illustrate the challenges related to their identification in a region dominated by faulting with small vertical offsets. Faults interpreted in the 3D seismic volume range from ∼300 to 1200 m wide and are in the same size range as faults that could have been the source of historical events up to Mw 2.7 in central Illinois. The array of monitoring sensors that was installed for the IBDP continues to collect data, as injection operates in IL-ICCS, the second injection well. CO2 injection rates for the IL-ICCS well are on average 1.7 times the rates injected in the IBDP well, but a significantly reduced rate of induced seismicity is observed. This article presents results of passive seismic monitoring for the duration of the project to date, integrating active and passive seismic data to develop a new interpretation of the subsurface structure at the Decatur site that explicitly identifies pathways for fluid flow into the basement leading to induced seismicity, and provides a geological explanation for the sharp reduction of induced seismicity during injection at higher rates into the second well. The use of seismic moment to estimate the length of seismogenic slip planes in the local subsurface suggests that faults large enough to produce felt seismicity are unlikely to be present at or near the Decatur site.
The Illinois Basin -Decatur Project (IBDP) is a large-scale carbon capture and storage (CCS) demonstration project managed by the Midwest Geologic Sequestration Consortium (MGSC). The IBDP is injecting 1 million tonnes of carbon dioxide in the Upper Cambrian Mt Simon Sandstone over three years at a rate of 1000 tonnes per day. The Mt Simon Sandstone can be subdivided into three major units with different geologic and diagenetic histories that have a profound effect on reservoir quality. At the IBDP site, the top of the Mt Simon Sandstone is overlain by 100 m (300 ft) of tight silt and shale in the Eau Claire Formation that forms the primary seal that prevents possible migration of CO 2 into the overlying strata. Below the Mt Simon Sandstone is a Pre-Mt Simon interval that is characterized by its poor reservoir quality. The Pre-Cambrian basement at this site is composed of rhyolite. Three-dimensional seismic refl ection data from the IBDP suggests that as much as 61 m (200 ft) of local Pre-Cambrian topographic relief is present in the study area. The Mt Simon Sandstone appears to thin over topographic high areas and thicken in the valleys. At the IBDP, the best reservoir quality rocks are in the lowermost Mt Simon Sandstone where the average porosity is 22% and permeability is 200 mD, respectively. Regional mapping suggests that these Lower Mt Simon reservoirs are widespread within a basin that is centered just slightly north of the Illinois Basin depocenter.
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