To understand fluid induced seismicity, we have designed a large-scale laboratory experiment consisting of a one-cubic-meter sandstone with an artificial fault cut and fluid-injection boreholes. The sandstone block is assembled in a true triaxial loading frame and equipped with 38 piezoelectric sensors to locate and characterise acoustic emission events. The differential stress on the artificial fault is increased in stages to bring it towards a critically stressed state. After each stage of differential stress increase, fluids are injected at low pressures through boreholes to test the potential of fault re-activation. In addition, a high-pressure injection was conducted that created a hydraulic fracture from the injection borehole towards the artificial fault. The newly generated fluid pathway resulted in an activation of the complete block through a stick-slip movement. We compare acoustic emission measurements from the laboratory experiment with seismicity observations from the field-scale CO2 injection at Decatur, Illinois, U.S., and conclude that the existence of fluid pathways plays a decisive role for the potential of induced seismicity.

The Illinois Basin–Decatur Project safely injected, over 3 years, nearly 1.1 million tons (1 million tonnes) of supercritical carbon dioxide (scCO 2 ) into the base of a 1,640 ft (500 m) thick saline sandstone reservoir at a depth of 7,025 ft (2.14 km) during CCS1 injection. Baseline data collection started 1.5 years before injection, documenting air, soil, groundwater, and bedrock conditions. Reservoir monitoring included microseismicity, formation pore pressures, surface acquired 2D and 3D seismic data, and petrophysical bedrock properties through cores and downhole geophysical logging. Data guided early hydromechanical models to predict pressure and scCO 2 fronts through time. A 3D velocity model developed for the site was critical for accurate location of microseismicity, which during injection averaged a little over four events per day, and maximum magnitudes as other CO 2 injection sites, while only eight microseismic events were detected during the 1.5 years of preinjection monitoring. Events appear to be related to previously undetected planes of weakness, which developed during diagenetic or compactional processes associated with the Precambrian surface topography and faults in the highly fractured Precambrian basement. The orientation of the planes of weakness in relation to the in situ stress field match the calculated focal mechanism, and modeling shows them to be critically stressed at these low pressure increases and capable of inducing seismicity.

To investigate mechanisms causing microseismicity (M-w < 2) at a CO2 injection site, a large-scale triaxial block experiment was carried out on a faulted (saw-cut) cubic-meter of Castlegate Sandstone. The experiment consisted of injection tests at varying differential stresses, while monitoring and recording pore pressure and acoustic emissions (AEs). During the experiment, similar to 33,000 AEs and-14 mm of horizontal displacement/slip, like a strike-slip fault movement, occurred. To un-derstand the AE responses and ascertain fault characteristics near the located AEs, we modeled the topography of the fault surface, fault aperture, and fault-gouge thickness using pre-and post-experiment laser scans of the fault surface on each half of the block. Additionally, we characterized surface roughness parallel and perpendicular to slip. Models show crushing and flattening of the fault surfaces can be linked to the spatiotemporal distribution of AEs within 50 mm of the fault surface. Approximately 65% of AEs were in areas with small aperture (<= 300 mu m); thicker fault gouge was observed in adjacent areas with wider aperture and shows a two-fold reduction in grain size relative to unaltered Castlegate Sandstone. This work provides a conceptual understanding on fault surface evolution, which can be applied towards modeling of seismic slip.

Accurate estimation of the moment magnitude of microseismic event catalogs is essential for proper estimation of moment release as well as for the extraction of source parameters that can reveal variations in the reservoir geomechanics and hydraulic properties. Using Decatur as an example we present how an improper choice of the analysis bandwidth and lack of instrument correction can lead to an underestimation of larger magnitudes which may affect the functionality of traffic light protocols. Spatial variations of Gutenberg’s b-value within selected seismicity clusters at Decatur are associated with seismicity migrating over time along the cluster orientations, suggesting differences in either the medium or fault properties and changes in pore pressure. The different b-value regions are supported by clear systematic difference in frequency-magnitude distributions.

ABSTRACTThe 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.

ABSTRACT The three-year CO2 injection period at the Illinois Basin - Decatur Project site (Decatur, Illinois, United States) produced a number of microseismic events distributed in very distinct spatiotemporal clusters with different orientations. Further characterization of the microseismicity encompasses the determination of the event source mechanisms. Initially, the microseismic monitoring network consisted solely of borehole sensors, but has been extended with surface sensors, thereby significantly improving the data coverage over the focal sphere. This article focuses on 23 events from the northernmost microseismic cluster (about 2 km from the injection point) and takes advantage of both, surface and downhole, recordings. The resulting strike-slip east–west-oriented focal planes are all consistent with the east–west orientation of the cluster in map view. The injection-related increase of pore pressure is far below the formation fracture pressure; however, small stress-field changes associated with the pore-pressure increase may reach as far as to the investigated cluster location. Monte Carlo modeling of the slip reactivation potential within this cluster showed that the observed maximum stress-field orientation of N068° is the optimum orientation for fault reactivation of the east–west-oriented cluster. Our results suggest that the east–west orientation of the investigated cluster is the main reason for its activation, even though the cluster is about 2 km away from the low-pressure injection point.

The overburden above an active, underground longwall coal mining operation was characterized before and after subsidence using core drilling, geotechnical instrumentation and in situ testing. The analysis of mining-induced changes in the overburden provides a better understanding of the mechanisms which lead to the surface expression of subsidence and hydrologic changes.

PreviousNext No AccessSEG Technical Program Expanded Abstracts 2020Importance of monitoring seismicity induced by CO2 sequestration at Illinois Basin – Decatur ProjectAuthors: Frantisek StanekSherilyn Williams-StroudRobert BauerLeo EisnerFrantisek StanekIRSM CASSearch for more papers by this author, Sherilyn Williams-StroudIllinois State Geological Survey (ISGS)Search for more papers by this author, Robert BauerIllinois State Geological Survey (ISGS)Search for more papers by this author, and Leo EisnerSeismik s.r.o.Search for more papers by this authorhttps://doi.org/10.1190/segam2020-3427548.1 SectionsAboutPDF/ePub ToolsAdd to favoritesDownload CitationsTrack CitationsPermissions ShareFacebookTwitterLinked InRedditEmail AbstractProcessing of combined microseismic data from two independent sparse seismic monitoring networks deployed at the (near-)surface around the Illinois Basin – Decatur Project (IBDP) injection site was done to characterize the detected seismicity by locations and inverted source mechanisms. By employing waveform similarity analyses, consistent processing was achieved for similar events, making it possible to distinguish between different clusters of events. The seismicity response was observed to vary with injection location. Different clusters and trends of locations in space and time support an interpretation of activated (mostly previously unknown) faults. Magnitudes of the observed seismic events range from -2.1 to 1.2, no felt event was recorded. Source mechanisms of the stronger events inverted from body-wave arrival amplitudes show that the activated faults are dominated by strike-slip and oblique-slip type of failure. Faults are interpretable in the recently reprocessed 3D reflection seismic data volume, which helped to confirm the existence of faulting in some of the locations where microseismicity was detected. This study shows that proper continuous microseismic monitoring, and updated processing of 3D seismic volume with a special focus on areas with observed microseismicity helps to better understand the response of the reservoir to CO2 injection.Presentation Date: Wednesday, October 14, 2020Session Start Time: 8:30 AMPresentation Time: 11:25 AMLocation: 362APresentation Type: OralKeywords: sequestration, microseismic, monitoring, risk, induced seismicityPermalink: https://doi.org/10.1190/segam2020-3427548.1FiguresReferencesRelatedDetailsCited byStudy of CO2 injection in a depleted oil reservoir using geomechanically coupled and non-coupled simulation modelsMaterials Today: Proceedings, Vol. 57 SEG Technical Program Expanded Abstracts 2020ISSN (print):1052-3812 ISSN (online):1949-4645Copyright: 2020 Pages: 3887 publication data© 2020 Published in electronic format with permission by the Society of Exploration GeophysicistsPublisher:Society of Exploration Geophysicists HistoryPublished Online: 30 Sep 2020 CITATION INFORMATION Frantisek Stanek, Sherilyn Williams-Stroud, Robert Bauer, and Leo Eisner, (2020), "Importance of monitoring seismicity induced by CO2 sequestration at Illinois Basin – Decatur Project," SEG Technical Program Expanded Abstracts : 2226-2230. https://doi.org/10.1190/segam2020-3427548.1 Plain-Language Summary Keywordssequestrationmicroseismicmonitoringriskinduced seismicityPDF DownloadLoading ...

For holistic seismic monitoring of the progressing CO2 plume within the Mt. Simon sandstone reservoir at the Illinois Basin - Decatur CCS project, we aim to integrate passive microseismic data with complementary surface- and VSP data from active seismic surveys. A detailed microseismic analysis allows for spatial mapping of attenuation Q around the injection area. We identify time periods of possible reservoir changes in between active seismic time-lapse monitors and we can increase the temporal resolution of the 4D signal. In addition, finite-difference FD modeling of microseismic events can help explain the details of the observed waveforms including different phase arrivals for up- and downgoing waves. This, in turn, is essential to improve the depth resolution of microseismic event location and determine whether seismic activity is confined to the injection zone or growing outward. The discrimination of ray paths from FD modelling also allows to select the most relevant waveform phases for 4D time-lapse studies and is an important pre-processing step. In combination with simultaneous tomographic inversion of the velocity- and attenuation structure, it has the potential to maximize the resolution of 4D changes in the medium.

The Illinois Basin – Decatur Project (IBDP) is an integrated carbon capture and geological storage project conducted at the Archer Daniels Midland Company's (ADM) corn processing plant in Decatur, Illinois, USA (Figure 1). Nearly 1.1 million tons (1 million tonnes) of supercritical carbon dioxide (scCO2) was injected into the base of a 500 m thick saline sandstone reservoir at a depth of 2140 m over three years from 2011 to 2014. Prior to injection, an extensive investigation of site conditions included monitoring of air, soil, groundwater, and bedrock. Monitoring activities continued through the three-year injection period, and are ongoing during the post-injection period and scheduled to continue into the first quarter of 2020. As part of this site characterization and monitoring effort, an extensive and varied suite of seismic data has been collected. The seismic data includes three 2D seismic reflection lines across the site, several borehole vertical seismic profile (VSP) acquisitions, two different 3D seismic data volumes acquired four years apart, passive seismic monitoring from geophones installed in two different boreholes (and temporarily in a third), and passive seismic monitoring from an array of earthquake seismometers on the surface above the injection site. Presentation Date: Tuesday, September 17, 2019 Session Start Time: 1:50 PM Presentation Start Time: 2:15 PM Location: 301B Presentation Type: Oral

Safe geologic sequestration of CO2 is important to decrease the concentration of greenhouse gases in the atmosphere. However, its injection could increase the underground pore pressure and potentially induce sliding of critically stressed faults. We here investigated acoustic emission signals from a laboratory test, where fluid injections close to an artificial interface of ~1m length were conducted with different injection pressures and differences in fluid viscosity. During the injection, we could observe three different types of AE signals: 1) AE with P- and S- wave energy, primarily located on the sliding plane and caused by increasing confining stresses, 2) hydraulic fracture type of AE with predominant P-wave energy signals, and 3) long lasting oscillations or tremor-like signals along with a stick-slip motion along the artificial interface. The pore pressure at the injection point reached up to 6.2 MPa during the hydraulic stimulation and after shut-in, it dropped down to almost zero. However, about 10 minutes later, a sudden sliding of the interface (stick-slip motion) was recorded. The analysis of the spatial distribution of the AE energy was applied to monitor first the evolution of the hydraulic fracture and thereafter the dynamics of stick-slip, indicating a nucleation phase of the sliding, then the rupture propagated through the whole interface with an average rupture velocity of a few m/s. The speed and the energy radiated during this event were approximately 6 orders of magnitude larger than observed during quasi-static sliding preceding the stick-slip. This observed stick-slip motion can be considered a laboratory analogue to earthquakes, and its occurrence can be related to the injection of fluids.

Laboratory tests were conducted in a triaxial load frame with acoustic emission and transmission capability to investigate mechanisms that might be initiating the microseismicity experienced in CO2 injection operations. Although often related to reactivation of mapped faults or local fracturing due to reduced injectivity, the case of the Illinois Basin - Decatur Project is used here to illustrate the need for better understanding of what triggers microseismic events in relatively large permeability, good reservoir candidates. There, microseismicity has occurred in the CO2 storage target formation, the Mt. Simon sandstone, as well as in the underlying Precambrian basement. The microseismicity in the Mt. Simon sandstone occurred ahead of CO2 plume arrival and at relatively low injection pressure conditions, well below the fracturing pressure at the injection well. A hypothesis is suggested for the occurrence of such events in the field, whereby critically stressed planes are activated by the passage of the pressure front at injection start; these faults are small and thus not visible in the seismic survey. In order to test this hypothesis, sandstone plugs were prepared by two different methods to incorporate a fracture plane, which we attempted to reactivate by pore pressure pulses. The reactivation was successful at low pressure for a fracture created in the laboratory at reservoir conditions but was unsuccessful except at a much higher pore pressure in a saw-cut artificial fracture. The results suggest that tortuous, rough stress-induced fractures may be easier to reactivate because of the higher probability that sections are already favorably oriented with respect to critical shear stress at a low pore pressure increase. Saw-cut fractures may close completely under isotropic stress loading and may be difficult to activate unless exactly oriented with respect to critical shear stress at a low pore pressure increase. Acoustic emission accompanying fracture reactivation was also recorded and analyzed. This revealed a different event distribution energy between creating and reactivating the fracture.

Summary Safe geologic sequestration of CO2 is important to decrease the concentration of greenhouse gases in the atmosphere. However, the injection could increase the underground pore pressure and potentially induce sliding of critically stressed faults. We report results from a laboratory test where fluid injections close to an artificial interface of ∼1m length were observed to induce sliding. During the injection, the pore pressure at the injection point reached up to 6.2 MPa and after shut-in, it dropped down to almost zero. However, about 10 minutes later, a sudden sliding of the interface (stick-slip motion) was recorded. Two types of acoustic emission (AE) signals were detected: short bursts and long-lasting oscillations (tremors). The analysis of the spatial distribution of the AE energy was applied to monitor the dynamics of stick-slip, indicating a nucleation phase of the sliding, then the rupture propagated through the whole interface with an average rupture velocity of a few m/s. The speed and the energy radiated during this event were approximately 6 orders of magnitude larger than observed during quasi-static sliding preceding the stick-slip. This observed stick-slip motion can be considered a laboratory analogue to earthquakes, and its occurrence can be related to the injection of fluids.

One way to make large-scale geological storage of CO2 a publicly more accepted practice is to develop more convincing methodologies for monitoring verification and accounting (MVA). Passive seismic data has a large potential to enhance this current lack of credibility in MVA. Presented here are three case studies, where passive seismic data were analysed in innovative ways to provide relevant information on the CO2 and its behaviour in the subsurface. We briefly describe the applied analysis methods and the added value in the cases of In Salah (Algeria), Decatur (Illinois, U.S.) and the Longyearbyen CO2Lab (Svalbard). We also describe the differences in deployment of sensors for passive seismic data acquisition, and the resulting difficulties in unified data analysis workflows. Finally, we provide an overview on recent CO2 storage sites and the generally increased acceptance for the importance of passive seismic data acquisition. To avoid further catastrophic global warming, a drastic and immediate reduction of greenhouse gas emissions is necessary. A whole range of international organizations have realized this for a long time, including the Intergovernmental Panel on Climate Change (IPCC), the International Energy Agency (IEA), and the United Nations Framework Convention Climate Change (UNFCCC). On 4 November, 2016 the Paris Agreement finally entered into force with most of the world’s political leaders showing that they are ready to take immediate action. Among many greenhouse gas reduction strategies, Carbon Capture and Storage (CCS) is an important technology, because it can generate net negative CO2 emissions, mostly with regards to bio-energy CCS (BECCS). Advancement of renewable energy sources is vital to reducing reliance on fossil fuels. However, regardless of the speed in the advancement in renewable energy sources, CCS can reduce the CO2 footprint from ‘dirty’, greenhouse gas-producing, industrial and agricultural processes, which cannot be replaced by renewable energies.

We analyse microseismicity induced during the Decatur, Illinois, carbon capture and storage (CCS) demonstration project. More than 10,000 microseismic events were detected during the injection of 1 Mio metric tons of CO2 over the course of 3 years. The seismicity occurs in distinct clusters and shows little to no correlation to the progressing CO2 front. For geomechanical reservoir characterization and seal integrity assessment, we need very high depth resolution for the event locations, such that events can be unambiguously attributed to specific formations. We therefore compare event locations using different sensor distributions (borehole and surface sensors) and/or including additional phase arrival information besides direct phases. Analysis of Brune-type stress drop of induced microseismic events with two different estimation methods exhibits signs of pore pressure diffusion processes within individual clusters, although the overall seismicity across clusters does not correlate with pressure gradient or CO2 front. An observed distance dependence of stress drop from the nucleation point within a cluster suggests a local pressure gradient over the extent of the cluster.

The Illinois Basin – Decatur Project (IBDP) has demonstrated the safety, effectiveness, and efficiency of the process of isolating the carbon dioxide (CO2) stream from biofuels production and storage in a deep saline reservoir at a depth of more than 2,000 meters. Geologic assessment and controls have proven essential to understanding reservoir conditions and predicting CO2 behavior. The injectivity and storage capacity of part of the lower Mt. Simon Sandstone at IBDP have been confirmed. Modeling, microseismic event analysis, and MVA continue to provide significant insights into reservoir response to stored CO2 and the development of commercial-scale project workflows.

Subsidence is the sinking of land surface, commonly resulting from underground mining. In Illinois, property damage has been sufficiently severe that a state law was enacted to provide subsidence insurance for homeowners. This publication has been prepared for homeowners in Illinois: (1) to inform them whether they live in subsidence-prone areas, (2) to aid them in understanding some frequently encountered effects of mine subsidence as well as problems sometimes mistaken for mine subsidence, and (3) to suggest further sources of information. Although the new subsidence insurance program for homeowners in mining areas prompted the writing of this report, we do not attempt to explain the details of the insurance program. Our purpose is to explain the causes and the nature of subsidence and discuss ways to minimize damage caused by subsidence. About 750,000 acres of Illinois land have been undermined for coal, and many homeowners are concerned about the effects underground mining may d at nine areas alalitative methods are presented. Chapter Five presents conclusions and suggestions for future research.