SUMMARY The injection of CO2 at the Decatur carbon capture and storage site has generated significant microseismic activity, which occurs in distinct spatial clusters up to approximately 2.2 km from the primary injection well. Accurate and precise event locations are vital for the characterization of the microseismicity to help understand the reservoir response to the CO2 injection, whilst enabling the identification of minor faults and fractures below the resolution of conventional active seismic imaging. However, microseismic monitoring of fluid injection sites, such as Decatur, is often performed using a network of borehole sensors often from a single well. While these downhole sensors have excellent detection capabilities, their poor azimuthal coverage limits the ability to precisely determine event locations. We have developed a modified double-difference relocation algorithm suitable for both 1-D and 3-D velocity models, and which incorporates differential back azimuth observations to allow the benefits of the original double-difference algorithm to be applicable to a downhole microseismic monitoring setting. Applying the modified double-difference algorithm to the microseismicity at Decatur, we have successfully relocated 4293 events. The relocation included over 59 million observations for 757 285 event pairs, split across seven geographic regions. Despite the majority of observations being recorded in only two boreholes, with an almost identical azimuthal coverage, the results have shown to be reliable with significantly reduced residuals and low uncertainties associated with the final locations. We have analysed the residuals in terms of their association with each geographic region, data type, station and individual events, to fully appreciate their influence in the inversion and the fit of the data to the final set of event locations. For each region, the relocated seismicity has become less diffuse with improved clustering, and with newly visible linear features often orientated in a NE–SW direction. These results show the potential improvements that can be made to microseismic event locations recorded by a borehole network with a limited and variable azimuthal distribution.

At the Decatur carbon capture and storage site (IL, USA) CO2 has been injected from 2011–2014 and from 2017 to present near the base of the Lower Mt. Simon Sandstone saline reservoir, resulting in microseismicity. Microseismicity is mainly located in the basement and distributed in distinct spatial clusters. The lack of significant impedance contrasts within the basement makes the interpretation of active-source seismic reflection data challenging, however, recent reprocessing allowed to resolve faults above and at the top of the basement. These faults generally do not coincide with the location of microseismic events and their continuation to the general depth of the seismic events cannot be assumed. This paper shows how the interpretation of the microseismicity can complement structural interpretations of active-source seismic reflection data. In particular, we analyze clusters and bursts (abrupt increases) of microseismicity, identify unresolved, smaller-scale weaknesses and extract statistical parameters. These parameters allow comparisons with the interpreted faults, and with fracture sets intercepted by boreholes. During injection at the Decatur site, the injection pressure was kept far below fracture pressure, nevertheless, seismic events were induced and spread far beyond the expected extent of the CO2 plume. We argue that local stress transfers related to the CO2 injection reactivated pre-existing fractures within the critically stressed basement. Finally, we conducted a slip tendency analysis for faults interpreted from active seismic, selected cluster, bursts and nodal planes from focal mechanisms to determine if the interpreted structures are optimally oriented with respect to the stress regime. Our results suggest that the orientation of fractures close to the injection well, generally shows slight deviations from the optimal orientation for slip. This might indicate either slight local deviations of the maximum horizontal stress azimuth from the average direction used in the analysis, or the lack of optimally oriented fractures at this location.

Determining accurate microseismic event locations at the Groningen gas field in the Netherlands has important implications for understanding the ongoing induced seismicity and its associated seismic hazard. To improve the depth constraint of the microseismicity, downhole monitoring arrays have been deployed in the central region of the Groningen field. The observed seismicity at these receivers is characterized by significant complexity in the waveforms, due to the high velocity contrasts that exist and the acquisition geometry. Reliably identifying and picking phases for use in earthquake location algorithms is therefore particularly challenging. Using a well constrained and highly detailed 3-D velocity model, we show how full waveform modelling can be used to understand the causes of the observed phase complexity. By identifying the different modelled phase arrivals in detail, we look to identify any systematic changes in waveform complexity with source location, to aid phase identification of the recorded downhole data. Theoretical traveltimes are often the foundation of earthquake location algorithms. We highlight the associated problems of their computation for the downhole monitoring setup for the Groningen model by complementing the full waveform simulations with traveltime computations and their associated ray paths from both an eikonal solver and using the wavefront construction method. We highlight large inconsistencies in the traveltimes, demonstrate the limitations and sensitivity of ray tracing in a layered velocity model, and show how the theoretical traveltimes do not equate with the dominant phase arrivals observed within the modelled waveforms. Finally, we propose an approach based on computing P-and S-wave traveltimes directly from the full waveform modelling, such that phase picking is based on an amplitude threshold rather than individual phase identification, which can also be adjusted for any given moment tensor.

Monitoring microseismicity during the injection of CO2 has the potential to provide vital data and feedback on a range of timescales: from real-time to longer term analysis. Minimising the risk of seismic hazard and CO2 leakage, while ensuring the injected CO2 is conforming to the injection plan are all possible with an effective microseismic monitoring programme. However, the most reliable interpretation of microseismic data comes from accurate and precisely located events, which is not always achievable with a typical monitoring network. In this study we provide an overview of some different analytical techniques used at the Illinois Basin Decatur project (IBDP) Carbon Capture and Storage (CCS) site, to help constrain microseismic locations with the goal of improving the understanding of the subsurface response to CO2 injection. Presentation Date: Tuesday, September 17, 2019 Session Start Time: 1:50 PM Presentation Start Time: 3:30 PM Location: 301B Presentation Type: Oral

ABSTRACT The sequestration of CO 2 in subsurface reservoirs constitutes an immediate counter‐measure to reduce anthropogenic emissions of CO 2 , now recognized by international scientific panels to be the single most critical factor driving the observed global climatic warming. To ensure and verify the safe geological containment of CO 2 underground, monitoring of the CO 2 site is critical. In the high Arctic, environmental considerations are paramount and human impact through, for instance, active seismic surveys, has to be minimized. Efficient seismic modelling is a powerful tool to test the detectability and imaging capability prior to acquisition and thus improve the characterization of CO 2 storage sites, taking both geological setting and seismic acquisition set‐up into account. The unique method presented here avoids the costly generation of large synthetic data sets by employing point spread functions to directly generate pre‐stack depth‐migrated seismic images. We test both a local‐target approach using an analytical filter assuming an average velocity and a full‐field approach accounting for the spatial variability of point spread functions. We assume a hypothetical CO 2 plume emplaced in a sloping aquifer inspired by the conditions found at the University of Svalbard CO 2 lab close to Longyearbyen, Svalbard, Norway, constituting an unconventional reservoir–cap rock system. Using the local‐target approach, we find that even the low‐to‐moderate values of porosity (5%–18%) measured in the reservoir should be sufficient to induce significant change in seismic response when CO 2 is injected. The sensitivity of the seismic response to changes in CO 2 saturation, however, is limited once a relatively low saturation threshold of 5% is exceeded. Depending on the illumination angle provided by the seismic survey, the quality of the images of five hypothetical CO 2 plumes of varying volume differs depending on the steepness of their flanks. When comparing the resolution of two orthogonal 2D surveys to a 3D survey, we discover that the images of the 2D surveys contain significant artefacts, the CO 2 ‐brine contact is misplaced and an additional reflector is introduced due to the projection of the point spread function of the unresolvable plane onto the imaging plane. All of these could easily lead to a misinterpretation of the behaviour of the injected CO 2 . Our workflow allows for testing the influence of geological heterogeneities in the target aquifer (igneous intrusions, faults, pervasive fracture networks) by utilizing increasingly complex and more realistic geological models as input as more information on the subsurface becomes available.

This paper focuses on modelling the seismic response to a theoretical CO2 injection into the upper reservoir at the Longyearbyen CO2 Lab site in Svalbard, Norway. The modelling aims to assess the monitoring potential of a carbon capture and storage site. We demonstrate the effect of reservoir porosity and CO2 concentration on seismic reflection amplitudes using a prestack depth migration simulator. In addition, we look at five different CO2 plumes of increasing volume and model the seismic response as a proxy to monitoring the evolution of a CO2 plume. We show that even low porosity reservoirs (2%) will generate a significant seismic response from the injection of CO2, and that low CO2 concentrations will also be detected. However, further increases in the CO2 concentration will not be as simple to monitor, with the CO2 contact providing the best seismic imaging potential. Furthermore, we demonstrate how illumination angle and hence seismic acquisition design is critical to avoid imaging artefacts.

In 2013 a seismic caprock monitoring system with 172 nodes was installed on the Oseberg oil and gas field in the Norwegian North Sea. It was specifically designed for active and passive seismic monitoring of a disposal injection well. The aim of the monitoring is for a safer operation with avoidance of leakages to the sea, and increased injection. Despite installation and yearly maintenance costs but owing to benefits in the operation of the injector, the system provides yearly savings of around $1.2 million. Currently, the system does most of the passive seismic analysis – microseismic and interferometry – in real time, and we present data examples from the last two years. Typical noise in the data (seismic shooting, platform generated noise and marine traffic) are removed and the signal-to-noise ratio is significantly improved, resulting in better detections and event location, especially for small microseismic events. Seismic interferometry is used to observe/detect changes in the water column and the shallow subsurface down to several hundred metres. Several offshore hydrocarbon-bearing fields have experienced fractures in the overburden from injection into disposal wells, including leakage to the surface. Although injection procedures to avoid this are effective, they significantly limit the rates and pressures of the injection. To overcome this, a permanent offshore seismic monitoring system using both active and passive seismic methods has been developed. It was installed at the seabed of the North Sea Oseberg field in 2013 with the main aim to control caprock integrity and ensure safe injection, which makes the system unique.

Subducted slab roll-back, lithospheric instability and asthenospheric extrusion have all been proposed as mechanisms that explain the evolution of the extensional Pannonian Basin, within the convergent arc of the Alpine-Carpathian mountain system in central Europe. We determine the P-and S-wave velocity structure of the mantle to depths of 850 km beneath this region using tomographic inversion of relative arrival-time residuals from 225 (P waves) and 124 (S waves) teleseismic earthquakes recorded by 56 stations of the Carpathian Basins Project (CBP) temporary seismic network (16-month duration) and 44 permanent seismic stations. The observed median P-wave relative arrival-time residuals vary between -1.13 s (early) in the Alps and 1.12 s (late) at the western end of the Carpathians; S-wave relative arrival-time residuals are about twice as large (-2.13 s and 3.39 s). We tested the effect of deterministic corrections on our relative arrival-time residuals using crustal velocity models from controlled source experiments, but show that the use of station terms in the inversion provides a robust method of correcting for near-surface crustal variation. Our tomographic models reduce the P-wave rms residual by 71 per cent to 0.130 s and our S-wave rms residual by 59 per cent to 0.624 s. At shallow sublithospheric depths we image several localized lower velocity regions, correlated with higher heat flow and interpreted as upwelling asthenosphere. We image a high velocity structure down to depths of about 350 km beneath the Eastern Alps. Further east, beneath the Pannonian Basin, a deeper continuation of the Eastern Alps fast anomaly is imaged trending E-W from similar to 300 km depth and extending into the mantle transition zone (MTZ). In the MTZ we image a fast anomaly extending outwards as far as the Carpathians, the Dinarides and the Eastern Alps. This higher velocity mantle material is interpreted as being produced by a mantle downwelling, whose detachment from the lithosphere above may have triggered the extension of the Pannonian Basin.