Analysis of Microseismicity and Reactivated Fault Size to Assess the Potential for Felt Events by CO2 Injection in the Illinois Basin

Abstract

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.

Cubic-meter scale laboratory fault re-activation experiments to improve the understanding of induced seismicity risks

“…A potential explanation for the reactivation of critically stressed fault patches in the basement is pressure communication through basement faults 11 , 12 . This explanation is consistent with a reduction in seismic activity at Decatur, when injection was shifted towards a shallower injection well, positioned above some shale baffles which are acting as barriers to fluid flow towards the basement faults 10 .…”

“…In both Decatur and In Salah, microseismicity is strongly clustererd spatially. A high-resolution monitoring network at Decatur allowed to further reveal clear linear structures within the microseismic event clusters 7 and focal mechanisms 8 , which are correlating with the preferred fracture orientation arising from the known regional stress regime in the basement 9 , 10 . A potential explanation for the reactivation of critically stressed fault patches in the basement is pressure communication through basement faults 11 , 12 .…”

A Small CO2 Leakage May Induce Seismicity on a Sub‐Seismic Fault in a Good‐Porosity Clastic Saline Aquifer

“…The Stage 2C injection shares an important similarity to other CCS projects with documented seismicity: the seismogenic faults were only discovered post-injection in the plume images and/or induced seismicity aligned along the faults' surfaces (Dance et al, 2019;Harvey et al, 2021;Williams-Stroud et al, 2020;Zhang et al, 2015).…”

“…The Stage 2C injection shares an important similarity to other CCS projects with documented seismicity: the seismogenic faults were only discovered post‐injection in the plume images and/or induced seismicity aligned along the faults’ surfaces (Dance et al., 2019; Harvey et al., 2021; Williams‐Stroud et al., 2020; Zhang et al., 2015). It is commonly believed that for most settings and relatively mild injection schedules, such undetectable faults are unlikely to serve as either leakage pathways or cause widely felt seismicity (Rutqvist et al., 2016).…”

Potential Seismicity Along Basement Faults Induced by Geological Carbon Sequestration

“…Previous field demonstration projects revealed that the relatively small number and low magnitude of seismic events have been detected after injection operations (White & Foxall, 2016; Williams‐Stroud et al., 2020), suggesting that geological CO 2 storage is unlikely to pose a seismic hazard. However, the amounts of injected CO 2 have typically been small (less pressurization expected), and the difficulty in accurate subsurface monitoring also limits the accurate detection of seismic activities in spatial extent and/or temporal coverage (Kaven et al., 2015; Nicol et al., 2013).…”