Microseismic assessment and fault characterization at the Sulcis (South-Western Sardinia) field laboratory

https://doi.org/10.1016/j.ijggc.2020.102974Get rights and content

Highlights

  • Background Seismic baseline assessment in SW Sardinia and Sulcis basin area.

  • Background Seismic baseline assessment in the Fault Lab area, where the methods and instruments for CCUS technologies will be developed and tested.

  • Ambient noise tomography (ANT) technique is used to detect and delineate a shallow fault underneath the Fault Lab area.

Abstract

The general acceptance of the CO2 geological storage by stakeholders passes through the assessment and mitigation of risks, potentially induced or increased by the disposal activity. Injection of moderate to large quantities of CO2 in the sub-surface may unbalance local stress and trigger earthquakes if faults are critically stressed, condition that is not easily verifiable. Pilot sites are therefore the best way to proceed further in order to address such challenging issues. In such cases, the reconnaissance of faults and seismicity in the sub-surface, before the onset of activity, is mandatory. In this paper, we present studies carried out in the site where the Sotacarbo Fault Lab is going to be installed. This facility will be located in a very low seismic hazard region of central Mediterranean, where reports on historical large earthquakes are poor. We show results from a series of experiments aimed to monitor the background seismicity around the pilot site. As expected, seismicity is almost absent down to small magnitude close to the future injection-test well. Further seismic imaging of the sub-surface layers obtained by ambient noise tomography offers the ability to resolve the presence of a seismicity-free fault located in the first 200 m below the surface, of which the last episode of activity is difficult to assess. Our results encourage the use of this site to follow the response of the system to injection of small quantity of CO2.

Introduction

The crucial need to reduce the CO2 emission requires a multiplicity of efforts, among which carbon capture, utilization and storage (CCUS) is a viable opportunity (Dipietro et al., 2011; Boot-Handford et al., 2014; Liu et al., 2017). Geological storage of CO2 might produce environmental and safety hazards, (e.g., accidental leakage during the injection at critical pressure, slow leakage after injection in the reservoir following diffusion and upwelling processes in presence of cracking/faulting of the cap rocks) that need to be reliably assessed (Nicol et al., 2011; Vilarrasa et al., 2019). Geo-energy activities that involve fluid injection/extraction may change the state of stress in the subsurface causing deformation, microcracking and fault reactivation (Grigoli et al., 2017). Induced seismicity has received increasing attention by stakeholders (Ellsworth, 2013) and is potentially one of the main obstacles to the volumetric upscale of geo-energy projects.

This is particularly true for areas where the natural seismic hazard is low like in the Sulcis area (Gruppo di Lavoro, 2004; Woessner et al., 2015) and therefore the existing infrastructure may not be adequate to withstand the impact of potentially large anthropogenic seismicity (Takagishi et al., 2014; van Thienen-Visser and Breunese, 2015).

The capacity of storing large quantities of CO2 scales with the volume that will be affected by changes in pressure and stress, increasing the risk of intercepting active faults and inducing large events (Keranen et al., 2014). Different views have emerged about up-scaling storage projects and their applicability or convenience. In particular, some opinions emerged proposing that large-scale CCUS is risky and possibly unsuccessful (Zoback and Gorelick, 2012), for the high probability that earthquakes could violate the seal integrity. Conversely, other opinions assessed that CO2 storage can be performed safely, after proper site characterization and with adequate pressure management (Vilarrasa and Carrera, 2015; Vilarrasa et al., 2019). At present, pilot and demonstration projects focus on verifying the storing capacity and the development of monitoring best practices.

Considering the acceptance of a reasonable level of seismic risk, the general goal is to chart a course and defines clear and plain protocols for safe and efficient monitoring that afford reliable and accurate hazard forecasts in a real-time management of the risk (Bommer et al., 2015). One of the main challenges is to define the existence of faults capable of producing damaging earthquakes. Since activities are generally shallow, even small to moderate magnitude earthquakes can produce significant ground shaking at the surface. In addition, faults that could fail with small to moderate earthquakes are more difficult to be detected in the subsurface. Earthquakes induced by fluid injection often develop on faults barely visible in seismic imaging of the subsurface (Buttinelli et al., 2016). The reliable identification and mapping of aseismic, but possibly critically stressed faults is therefore a first key step in hazard assessment, as became clear in unconventional geothermal systems, like the Enhanced Geothermal Systems (EGS, see Zang et al., 2014). Moreover, faults can act as enhancer or obstacle to fluid flow and their reconnaissance and characterization are important to plan efficient CO2 sequestration.

In this paper, we investigate such aspects for the Sotacarbo Fault Lab (South-Western Sardinia, Italy), a facility specifically designed as a test bed infrastructure in order to develop and test in-field conditions instruments and methods to be applied in the commercial-scale CO2 geological storage site elsewhere. The facility will be located in South-Western Sardinia, a region with low natural seismic hazard (Stucchi et al., 2011; Rovida et al., 2016). According to that, the national permanent seismic monitoring system has an intrinsic high magnitude threshold for detection of local earthquakes in Sardinia, where the Magnitude of completeness (Mc) is higher than 2.9 (Schorlemmer et al., 2010). To improve earthquake detection, various dense seismic arrays have been deployed over the past 4 years. The Sulcis temporary seismic network was installed for a period of about 15 months since July 2014 in order to monitor the South-Western Sardinia (Fig. 1a). After that, in July 2016 we installed the Sotacarbo Fault Lab permanent seismic network of 5 stations around the fault lab, complemented with a temporary network consisting of additional 11 stations operative from the first days of June 2017 to end of August 2017 (Fig. 1b). We present results of the multiscale survey and quality assessment of the overall monitoring efficiency.

In order to enhance the imaging of the fault in the lab site, we performed a focused experiment for ambient noise tomography, a very low cost and efficient tool for subsurface imaging especially in case of absence of local seismicity (Zheng et al., 2008). During the first days of October 2017, we installed a temporary linear seismic array for about 4 days (see line in Fig. 1b), while the Sotacarbo Fault Lab permanent seismic network was operating. Data from both the passive seismic linear array and the Sotacarbo permanent and temporary seismic networks enable the tomographic reconstruction of the fault.

Section snippets

Baseline monitoring of the Sulcis site

The Sulcis is a mining area of South-Western Sardinia, where a former coalfield within the Matzaccara Quaternary basin has been selected as the site of the Sotacarbo Fault Lab (Fig. 1a and b). Aim of this facility is to study CO2 leakage processes through faults and develop advanced monitoring tools to be applied in commercial-scale CO2 geological storage applications worldwide. The characterization of the site in terms of adequateness with respect to natural hazards is therefore a

Results of seismic monitoring and earthquakes location

From inspection of one year of continuous recordings from the first-large temporary network (red and blues triangles in Fig. 1a), we obtained a dataset of 82 earthquakes, which were located using the 1-D velocity model of the INGV seismic monitoring system (Table 1) and Hypoellipse code (Lahr, 1999). Only 5 hypocenters have a high-quality location, r.m.s. less than 0.19 s., with horizontal and vertical errors < 1 and 2 km, respectively, number of stations ≥ 5 and azimuthal gap less than 180°.

Seismic imaging method and results

For imaging the subsurface structure around the Sotacarbo Fault Lab site, and define the position and geometry of the fault, we performed a passive experiment with ambient noise tomography. The dispersion curve of Rayleigh waves for different inter-station paths, from the correlation properties of ambient noise, are inverted to derive 2D and 3D phase velocity maps at distinct frequencies (Shapiro and Campillo, 2004; Saygin and Kennett, 2012). Throughoutthis section, with ‘noise’ or ‘ambient

Discussion and conclusions

The development and acceptance of CCUS requires careful analyses of the hazards associated with the injection and storage activity, passing through the implementation of appropriate mitigation strategies and best practices of the risk management (White and Foxall, 2016).

The caprock integrity and the modification of the stress field due to the confinement of large gas volumes are primary aspects for hazard assessment (Verdon et al., 2011; Verdon, 2014; Cantucci et al., 2016). Experiments at

CRediT authorship contribution statement

M. Anselmi: Conceptualization, Methodology, Data curation, Writing - original draft. G. Saccorotti: Data curation, Methodology, Writing - original draft. D. Piccinini: Methodology, Data curation. C. Giunchi: Methodology, Data curation. M. Paratore: Data curation, Software. P. De Gori: Software, Methodology, Data curation. M. Buttinelli: Conceptualization, Validation. E. Maggio: Resources, Funding acquisition. A. Plaisant: Resources, Supervision. C. Chiarabba: Conceptualization, Writing -

Declaration of Competing Interest

None.

Acknowledgements

This study has been carried out within the “Research on Electric System” project funded by the Italian Ministry of Economic Development. The Sotacarbo Fault Lab is under construction within the “Centre of Excellence on Clean Energy” project (CUP: D83C17000370002) funded by the Regional Government of Sardinia (FSC 2014–2020). The data from the station SU26, belonging to the Sotacarbo Fault Lab seismic network, is currently archived in EIDA waveform repository (eida.rm.ingv.it). We are grateful

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  • Cited by (0)

    1

    Saccorotti, Piccinini, Giunchi, Paratore belong to the branch of Pisa Istituto Nazionale di Geofisica e Vulcanologia, Via Cesare Battisti, 43-56125 Pisa-Italy.

    2

    E. Maggio and A. Plaisant belong to the organization Sotacarbo S.p.A - Grande Miniera di Serbariu, 09013 Carbonia (SU) - Italy.

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