ERE5.1

In August 2019 an hydraulic fracturing operation was carried out at the PNR-2 well in Preston New Road, UK. Hydraulic fracturing caused abundant seismic activity that culminated with a M_L 2.9 event. This event prompted the operator (Cuadrilla Resources Ltd.) to halt any further stimulation of the well. The seismic activity was recorded by a downhole array of 12 sensors located in a monitoring well (PNR-1z). The operator released a seismic catalog created in real time during the fracturing operation. The catalog consists of 55555 events detected and located with a coalescence microseismic mapping method. The catalog also reports moment magnitudes, but no precise information on the method and on the parameters used to estimate them is available. In our study, we attempt to improve the number of detections and the location accuracy of the events by applying template matching and a double-difference relocation method, respectively. We also recalculate moment magnitudes using spectral fitting to look for any inconsistencies in the real-time catalog. Finally, we use the new information to better understand the spatio-temporal evolution of the seismicity and the dynamics that led to the M_L 2.9 event.

How to cite: Minetto, R., Helmstetter, A., and Edwards, B.: Analysis of the spatio-temporal evolution of the seismicity induced by hydraulic fracturing operations in Preston New Road, UK, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8688, https://doi.org/10.5194/egusphere-egu22-8688, 2022.

Occurred in the Delaware Basin, western Texas, near the town of Mentone, the Mw 5.0 Mentone earthquake is one of the largest induced earthquakes in the central US. Within 25 km of the epicenter, there are a few deep injection wells to the northwest injecting in the high permeable limestone layer at about 22 km averagely, as well as a lot of shallow wells injecting in the upper high permeable sandstone layer at around 18 km averagely. Between the shallow sandstone and deep limestone layers is a thick shale layer with low permeability, which excludes the possibility of downward percolation of the injected fluid in the shallow injection layer. However, the cumulative injection volume of shallow injection wells is about five times as much as that of deep injection wells. Motivated by this, we investigate whether the shallow injection wells may play a role in triggering the Mw 5.0 Mentone earthquake through the injection-induced coupled poroelastic stress perturbations. We first perform focal mechanism inversion and earthquake relocation with the Cut and Paste (CAP) and hypoDD methods, respectively, to constrain the fault plane on which the Mw 5.0 event occurred. A south-facing fault plane with strike/dip of 81^o/52^o is successfully fitted. We then calculate the change of the Coulomb failure stress (ΔCFS) caused by the shallow injection wells at the mainshock location based on the linear fully coupled poroelastic stress model. The calculated ΔCFS of shallow injection wells is approximately 20 kPa and it is mostly contributed by the change in coupled poroelastic stress. Based on findings from other studies, this value of ΔCFS is sufficient in reactivating faults that are well aligned with the local stress field. Since we only account for about half of the total injection volume from the shallow wells in the calculation, we also hypothesize that the actual perturbations caused by shallow injection wells via the coupled poroelastic stress change would be more prominent. Our result reveals the vital role of injection-induced coupled poroelastic stress in triggering seismicity, especially in low permeable geologic settings.

How to cite: Tan, X. and Lui, S. K. Y.: The non-negligible contribution of shallow injection wells on the triggering of the Mw 5.0 Mentone earthquake via coupled poroelastic stress perturbations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3271, https://doi.org/10.5194/egusphere-egu22-3271, 2022.

The Western Canada Sedimentary Basin (WCSB) has experienced an increase in hydraulic fracturing (HF) operations in the last decade, accompanied by an increase in the number of felt earthquakes, including a M_w 4.6 on 17 August 2015 near Fort St. John and a M_L 4.5 (M_w 4.2) on 30 November 2018 near Dawson Creek. While only a small percentage of HF operations induce seismicity, the majority of moderate-sized earthquakes occur in close spatial proximity to HF wells and temporal proximity to individual HF injection stages within the tight shale play of the Montney Formation. Whereas statistical analysis of an enhanced seismicity catalog suggests that the majority of seismicity occurs following HF operations in the relatively older and deeper compartments of the Montney Formation (Lower Montney; LM) and a low number of events are associated with the relatively younger and shallower layers (Upper Montney; UM), the detailed association and triggering mechanism(s) remains unclear.

In this study, we investigate induced earthquake source parameter variations resulting from spatial and/or temporal alteration of injection parameters, including injection time, depth, and volume, at three well pads operating between 2018 and 2020 in the Kiskatinaw area. We use dense local station coverage to create an enhanced seismicity catalog with double-difference relative hypocenter relocations to highlight potential fault orientations, confirmed by focal mechanism solutions. We estimate static stress drop values at the individual well pads and their variation over time as well as variation with the choice of empirical Greens function. We also investigate the temporal changes of the V_P/V_S-ratio in localized areas following HF operations as a proxy for increased fracture density and/or compliance.

The case study at three specific sites targeting both the UM and LM layers investigates the relative influence of a number of factors on the spatial and temporal distribution of source properties. Factors include the scale of HF injection parameters, the target formation layer, and site-specific factors, such as localized fluid accumulation. Preliminary results show that injection in the UM generally leads to significantly fewer earthquakes than injection in the LM, and that lateral variations in compartment properties may significantly influence the seismic response. Moreover, we investigate if repetitive injection at the same wellhead may repeatedly (re)activate sets of faults/fractures and lead to increased hydraulic connectivity between the target sedimentary layers and deeper, pre-existing basement faults. An increase in connectivity would imply an increased potential for triggering large mainshocks.

How to cite: Roth, M. P., Kemna, K. B., Verdecchia, A., Wache, R. M., Pena Castro, A. F., Harrington, R. M., and Liu, Y.: Variation in induced seismicity productivity by alteration of injection parameters: a comparative case study at three hydraulic fracturing wells in the Kiskatinaw area, British Columbia, Canada, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9373, https://doi.org/10.5194/egusphere-egu22-9373, 2022.

Over the last few decades, it has become apparent that different human activities in the subsurface, such as water waste injection, hydraulic fracturing, and geothermal energy production can lead to induced seismicity. Understanding the effects of fluid injection-related parameters on seismic response or evolution of it is essential for finding a method to manage and minimize the induced seismicity risk. Experimental and numerical studies indicate that varying injection patterns and rates can be used to effect and/or mitigate seismicity. However, most of the studies are for intact rock medium, and the mechanism of injection-induced seismicity of faulted rock medium is not clear yet. In this study, we performed fault reactivation experiments on faulted (saw-cut) Red Pfaelzer sandstones to provide new insight into the effect of stress/pressure cycling and rate on fault slip behavior and seismicity evolution. The saw-cut samples were subjected to two different reactivation mechanisms: 1) stress-driven and 2) injection-driven fault reactivation. Three different reactivation scenarios were performed during the stress-driven fault reactivation experiments: continuous sliding, cyclic sliding, and under-threshold cycling sliding. Ten AE transducers were used to detect microseismicity during the fault reactivation experiments, and consequently, different microseismic parameters, such as frequency-magnitude distribution (b-value), AE energy, and AE rate were estimated. Stress-driven fault reactivation experiments showed that (i) a below-threshold cycling scenario prevents seismicity and pure shear slip; however if the shear stress exceeds the previous maximum shear stress, seismicity risk increases drastically in terms of b-value, maximum AE energy, and magnitude. (ii) Compared to continuous sliding, cyclic sliding triggers less seismicity in terms of total b-value and large AE events due to the uniform reduction in roughness and asperity on the fault plane. (iii) By increasing the number of cycles, in general, the number of generated events and AE energy per cycle is reduced. Nevertheless, there is a risk of generating large AE events during the first cycles. In addition, results from the injection-driven fault reactivation experiments demonstrated that high injection rate results in higher peak slip velocity. Compared to the stepwise injection pattern, the cyclic recursive injection scenario showed higher peak slip velocity, due to the high hydraulic energy budget and fault compaction. A proper injection strategy needs to consider various factors, such as fault drainage, critical shear stress, injection rate, and injection pattern (frequency and amplitude). Our results demonstrate that selecting proper stress/pressure amplitude, and pressurization rate for the injection design strategy can help to reduce seismicity risk.  

How to cite: Naderloo, M., Veltmeijer, A., and Barnhoorn, A.: Fault reactivation process in the laboratory: The role of stress cycling and pressurization rate, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4177, https://doi.org/10.5194/egusphere-egu22-4177, 2022.

With human activities in the subsurface increasing, so does the risk of induced seismicity. For mitigation of the seismic hazard and limiting the risk, monitoring and forecasting are essential. A laboratory study was performed to find precursors to fault failure. In this study, Red Pfaelzer sandstones samples were used, which are analog to the Groningen gas reservoir sandstones. A saw-cut fault was cut at 35 degrees, and the samples were saturated. Fault slip was induced by loading the sample at a constant strain rate, and simultaneously active acoustic transmission measurements were performed. Every 3 seconds 512 S-waves were sent, recorded, stacked to reduce the signal-to-noise ratio, and analyzed. The direct seismic wave velocity, coda wave velocity, and transmissivity were monitored before and during the reactivation of the faulted samples. Different loading patterns and confining pressures were investigated in combination with active acoustic monitoring. Velocity and amplitude variations were observed before the induced fault slip and can be used as precursory signals. Two methods to determine changing velocities were used. Direct S-wave velocities are compared to velocity change obtained by coda wave interferometry. Both analyses gave similar precursory signals, showing a clear change in slope, from increase to decreasing velocities and amplitudes prior to fault reactivation. Fault reactivation is preceded by fault creep and the destroying of some of the asperities on the fault plane, causing the seismic wave amplitude and velocity to decrease. Combining all precursors, the onset of fault slip can be determined and therefore upcoming slip can be forecasted in a laboratory setting. Our results show precursory changes in seismic properties under different loading situations and show a clear variation to the onset of fault reactivation. These results show the potential of continuous acoustic monitoring for detection and forecasting seismicity and help the mitigation of earthquakes.

How to cite: Veltmeijer, A., Naderloo, M., and Barnhoorn, A.: Acoustic Precursors to Laboratory Induced Fault Slip and Failure, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4117, https://doi.org/10.5194/egusphere-egu22-4117, 2022.

Fluid injections are known to induce earthquakes in the upper crust. Recent studies have highlighted that fluid injections can contribute to the nucleation of instabilities close to or far from the injection site due to stress transfer induced by poroelastic processes. In addition, recent studies have suggested the maximum magnitude earthquake is expected to be a function of the volume injected. However, the development of the slip front related to the fluid pressure front, as well as its implications on the induced seismic sequence in time and space, remain poorly constrained in the laboratory and in natural fault systems.

Here, we investigated the influence of the initial normal stress (i.e., the permeability of the fault plane) and of the injection rate on the development of both the fluid pressure front and associated slip front during the nucleation stage of laboratory fluid-induced earthquakes. Experiments were conducted on saw cut samples of andesite, presenting a negligible bulk permeability compared to the fault plane one. Strain gauges were glued all around the fault surface to track, (i) the strain transfer associated with slip front propagation during injection and the rupture velocity during dynamic rupture propagation. The dynamics of the fluid pressure front was inverted from pore pressure measurements located at both edges of the fault. The evolution of the slip distribution due to the change in fluid pressure around the injection site was inverted from strain gauge measurements, assuming a 3D modelling of the sample specimen using the Finite Element Method. Our preliminary results show that the initial stress acting on the fault controls the development of the slip front during the nucleation of the instability. In addition, the larger the injection rate, and the faster the propagation of the slip front compared to the fluid pressure front. Finally, the scaling between the volume of fluid injected and the associated nucleation moment differs from the one relating the volume injected to the seismic moment.

How to cite: Passelegue, F. and Dublanchet, P.: Development of slow slip front during the nucleation of laboratory fluid-induced earthquakes, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11535, https://doi.org/10.5194/egusphere-egu22-11535, 2022.

In this study, we conducted injection-driven shear tests on a sawcut fault in granite samples using a triaxial deformation apparatus. The granite samples were drilled from Odenwald basement rocks in Germany. The sawcut fault, inclined 30° to the sample axis, was ground using sandpaper with a particle size of 201 µm. Two boreholes (nominal diameter 1.8 mm) were drilled near the short edge of each sample half to allow direct fluid access to the fault surface. Eight strain gauges, and eight pairs of acoustic emission (AE) sensors attached on the sample surface were used to monitor the deformation, local strain and AE events. 

During the experiments, we first measured the peak shear strength of the faulted sample by advancing the axial piston at a constant rate of 1 µm/s under 36 MPa confining pressure and 1 MPa pore pressure. We then adjusted the shear stress to be 90% of the peak shear strength. Subsequently, the piston was fixed, and the first injection-driven shear test was initiated by injecting distilled water from the bottom borehole at a rate of 0.2 mL/min. We observed three full cycles of fast slip events until the injection pressure was increased up to approximately 18 MPa. We then reduced the pore pressure to the initial 1 MPa and the axial force was removed, followed by the second injection-driven shear test conducted at a higher injection rate of 0.8 mL/min using the same procedure as in the first test. We also observed three episodes of fast slip events until the injection pressure was increased to about 20 MPa. Fluid pressures were monitored continuously at the top and bottom boreholes. We employed a COMSOL model to obtain the time-dependent fluid pressure distribution along the sawcut fault during fluid injection.

For slow fluid injection, we find that the fault surface near the center experiences slight normal dilation and gradual shear stress release prior to the fast slip event. In contrast, for high-rate fluid injection, the same fault patch exhibits normal compaction and shear stress increase preceding fast slip. In both cases, significant normal dilation and abrupt shear stress drops were observed near the fault center during fast slip events. The distinct evolution of local fault deformation and stress are likely attributed to the distribution of slow slipping patches, as signified by the fluid pressure distribution and Mohr-Coulomb failure envelope. At slow injection rate, slow precursory slip may have occurred on the entire fault, initiating a fast slip event. In contrast, at higher rates, slow slip may have been localized around the injection port, resulting in local stress concentration beyond the slow slipping patch. Our results demonstrate that the evolution of local fault deformation and stress can be diverse in different fault patches, depending on the relative location to the fluid pressurized zone and the resulting slow slipping patch. This suggests that the strongly heterogeneous fault deformation should be considered when analyzing the precursors to injection-induced fault reactivation.

How to cite: Ji, Y., Wang, L., Hofmann, H., Kwiatek, G., and Dresen, G.: Injection-rate control on deformation and stress of an experimental fault in granite, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4204, https://doi.org/10.5194/egusphere-egu22-4204, 2022.

Felt induced seismicity compromises the public perception on the deployment of geothermal power-plants in urban areas. Large induced earthquakes have led to the shutdown of Enhanced Geothermal Systems (EGS), such as Basel (Switzerland) and Pohang (Republic of Korea). In the majority of induced seismicity cases in EGS, the largest events occur after shut-in. Different mechanisms can trigger induced seismicity. Pore pressure diffusion is established as the most common triggering mechanism. It reduces the effective normal stress acting across pre-existing fault surfaces, weakening the shear resistance and allowing slip of faults. However, this is not the only triggering mechanisms and it cannot explain the large magnitude of post-injection induces seismicity. Additional influencing processes are poromechanical elastic stressing, shear stress transfer and local tectonic settings. Considering theses mechanisms simultaneously can provide a better understanding of the causes of post-injection seismicity and could allow to develop strategies to mitigate the occurrence of earthquakes with high magnitude. To explain these processes, we investigate the induced seismicity that led to the closure of the Basel EGS project. We set-up a hydro-mechanical finite element numerical model which contains faults corresponding with the clusters of induced events at Basel. We study the reactivation of these pre-existing fractures using a viscoplastic model. We are able to identify the process combinations bringing faults to failure. During injection, faults fail due to pore pressure diffusion in the vicinity of the well, and due to poroelastic stressing further in the reservoir. After the injection shut in, poroelastic stressing and shear stress transfer trigger seismicity, being the most relevant triggering mechanisms of post-injection induced seismicity.

How to cite: Boyet, A., De Simone, S., and Vilarrasa, V.: Identification of The Processes Triggering Induced Seismicity at the Enhanced Geothermal System of Basel (Switzerland), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2160, https://doi.org/10.5194/egusphere-egu22-2160, 2022.

An experimental ~6 km deep enhanced geothermal system in Otaniemi, in the Helsinki capital region, southern Finland, was stimulated in 2018 and 2020. During the two stimulations that lasted seven and three weeks, respectively, signals of the induced earthquakes with a maximum local magnitude of 1.8 were recorded with dense and diverse seismic networks. The intraplate southern Finland setting of the experiment yields an intriguing opportunity to study earthquake and rock failure processes in the precambrian Fennoscandian Shield where the level of natural seismicity is comparatively low. The high confining pressure of 180 MPa at 6 km depth defines the key characteristics of the stress field, together with the previously estimated North-110-degrees-East direction of the maximum horizontal stress. The competent crystalline bedrock has very low attenuation, and yields high signal-to-noise ratio seismograms even at relatively high frequencies. We study the source mechanisms of ~250 induced earthquakes with Mw > 0.5. We perform probabilistic full moment tensor analysis with the Grond package of the software suite Pyrocko. We use data sets from the 2018 and 2020 stimulation experiments. Both experiments were monitored with more than 100 three-component surface stations operated by the Institute of Seismology, University of Helsinki, and 12 three-component borehole stations maintained by the St1 developer company installed at around 300 m depth. The diverse network elements help to evaluate the consistency of the results. We first present results of a detailed analysis of a small event subset characterized by the best data quality and solutions to assess the robustness of the different tensor components to different processing choices. This includes a comparison of surface and borehole sensor data. This allows us to conclude that the majority of the analysed earthquakes have a dominant reverse faulting mechanism and a small subset of events has strike slip mechanisms, which is compatible with solutions reported by the developer group. The predominant fault plane orientations are in agreement with the ambient stress conditions that also seem to control the thrust mechanism. Based on the best quality solutions we discuss the significance of the obtained non-double couple moment tensor components to assess if significant opening or closing elements in the induced earthquake source reflect genuine physical processes or spurious effects associated with imperfect resolution.

How to cite: Rintamäki, A., Heimann, S., Dahm, T., and Hillers, G.: Source mechanisms of earthquakes induced by the 2018 and 2020 geothermal stimulations in Espoo/Helsinki, southern Finland, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7969, https://doi.org/10.5194/egusphere-egu22-7969, 2022.

In 2018 and 2020, two weeks-long geothermal reservoir stimulations were performed some 6 km below the Helsinki capital area, Finland. The seismic activity was recorded by a set of surface broadband sensors and 100 geophones installed by the Institute of Seismology, University of Helsinki, as well as Finnish National Seismic Network stations. The local magnitudes (M_L) of the recorded earthquakes are estimated using a Finnish local magnitude scale and the local magnitude of the largest induced event was 1.8. We apply three different approaches for estimation of moment magnitudes (M_W) to a data base of ~400 induced seismic events from the 2018 stimulation to explore the variability and sensitivity of the magnitude estimates. This is important for real-time monitoring and decision making when the induced event magnitudes approach the pre-defined magnitude limit, and to assess which trends can be robustly associated to earthquake source physics. (1) We employ a time-domain calculation of source parameters based on the application of Parseval's theorem to the integrals of the squared spectral displacement and velocity for the horizontal S-wave trains. The time window between the S-wave arrival time and twice the length of the S-wave travel time is considered for the S-wave train isolation. (2) We obtain moment magnitude estimates from an inversion of 50 s long three-component envelopes based on radiative transfer. (3) We apply a moment tensor inversion to 0.71 s long P and 0.81 s long S-wave signals. We fit a linear M_L-M_W conversion model to the values obtained from the different approaches. Considering the available local magnitude range between –0.5 and 1.8, a comparison of the linear conversion models shows that the moment magnitudes form the envelope inversion are systematically larger by ~0.2 units compared to those obtained from the moment tensor inversion. While the moment magnitudes determined by the time-domain calculation consistently exceed those of the envelope inversion for small local magnitudes (by ~0.2 units), they tend to yield similar estimates towards the larger local magnitudes. Other source parameter systematics include that the smallest seismic moment is obtained with the moment tensor inversion, and the largest with the time-domain equivalent of the spectral integrals. An initial extension of the analysis to 2020 data yields M_L-M_W as well as corner frequency-M_W scaling relations that are, interestingly, different compared to the 2018 results; we will present updated results that inform about the reliability of these trends.

How to cite: Sadeghi-Bagherabadi, A., Eulenfeld, T., Vuorinen, T. A. T., Rintamäki, A. E., and Hillers, G.: Magnitude estimates of earthquakes induced by the geothermal stimulations in Espoo/Helsinki, southern Finland: a comparison of different approaches, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7486, https://doi.org/10.5194/egusphere-egu22-7486, 2022.

The Swiss Energy Strategy 2050 anticipates that by 2050 up to ~7% of the future energy production will come from deep geothermal energy. Likewise, many other countries worldwide are investigating the potential of harnessing deep geothermal energy as a renewable solution. However, seismic risk reduction and reservoir efficiency is the current major coupled problem faced by Enhanced Geothermal System (EGS) reservoirs. Balancing risk and economic output is a key requirement in all EGS projects. The DEEP (Innovation for De-risking Enhanced geothermal Energy Projects) project is an international collaboration whose research-goal is to establish a full-scale protocol for real-time monitoring and risk analysis of potential seismicity triggered by EGS operations. To this end, the project will employ innovative seismic sensors, improved event-cataloguing techniques, fully probabilistic data-driven seismicity forecasts, and loss assessment strategies. In DEEP we plan to apply the so-called Adaptive Traffic Light System (ATLS) where forecasts are continuously updated with real-time data-feeds, providing an integrated and dynamic assessment of the seismic risk to the operators. Field test sites include the Frontier Observatory for Research in Geothermal Energy (FORGE) in Utah (USA), as well as at EGS sites in Germany and France. Parallel to the technology development, the project aims also at defining the next-generation good-practice guidelines and risk assessment procedures in order to reduce commercial costs and enhance the safety of future projects. Here, we will present an overview of the DEEP project to provide a framework for other DEEP presentations. We will also showcase a selection of results from new event detection and location algorithms based on machine learning and using Distributed Acoustic Sensing (DAS), as well as the results from a pilot test of the ATLS workflow for seismicity forecast models for the upcoming FORGE stimulation strategy.

How to cite: Lanza, F. and Wiemer, S. and the DEEP team: The DEEP Project: Innovation for De-Risking Enhanced Geothermal Energy Projects, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4703, https://doi.org/10.5194/egusphere-egu22-4703, 2022.

Carbon Capture and Storage (CCS) sites require microseismic monitoring before, during and after operations to ensure safety of operational personnel and the wider public.

The high dynamic range and low self-noise of broadband seismometers allows for the detection of low magnitude microseismic events which fall below the threshold of less sensitive geophones. Higher long-period sensitivity also allows the full source spectra of earthquakes to be accurately measured, resulting in more accurate magnitude estimations which improve the integrity of any microseismic monitoring system.

Borehole instruments such as the Güralp Radian are a natural fit for detecting low magnitude microseismic events. Optional high gain at the higher frequencies makes the Radian extremely suitable for monitoring low-magnitude induced events while retaining long-period sensitivity for larger ruptures. The slim form factor and omni-angle operation allows the instrument to easily be lowered into decommissioned wells with little information about the orientation at depth.

The Radian is currently being utilised by the British Geological Survey as part of the UK GeoEnergy Test Bed (GTB) to monitor and improve understanding of fluid flow through natural subsurface pathways. A string of 6 interconnected Radians provides vertical profiling around the injection site with a maximum of 8 units able to join in a single string. The Radian will detect and monitor small changes in the subsurface at the GTB as part of the suite of monitoring technologies deployed onsite. 

In addition to onshore networks, offshore depleted gas fields are becoming increasingly scrutinised for potential to store CO2. The advent of Güralp omnidirectional sensor technology combined with acoustic near-real-time data transmission means the Aquarius OBS provides a cost-effective solution for monitoring offshore CCS sites, with infrequent and rapid battery recharging and acoustic data extraction while the unit is still on the seafloor.

How to cite: Reis, W., Cilia, M., Watkiss, N., Mohr, S., Barbara, R., and Hill, P.: Broadband seismic instrumentation for monitoring CCS sites, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8224, https://doi.org/10.5194/egusphere-egu22-8224, 2022.

Injection of fluid in fractured reservoirs triggers seismicity that migrates away from injection point. The enlarging cloud of (micro-)seismicity can be driven by pore-fluid diffusion within fractured rock mass, thus propagating in space proportional to square root of time for an effective isotropic and homogenous medium, or by elastic-stress interactions between over-stressed pre-existing fractures.

In this contribution we adopt an hybrid approach to model seismicity evolution driven by pore-fluid propagation into a Discrete Fracture Network and apply it to a large-scale injection experiment at FORGE Test Site in Utah (USA). We couple a statistical seed model for seismicity with a physic-based solver for non-linear pore-fluid diffusion into a three-dimensional DFN (using TOUGH3). Local inelastic permeability changes mimick irreversible deformations and affect pore-fluid evolution and hence seismicity cloud.

Several synthetic catalogs are generated and compared with one generated with a pure physic-based numerical solver.

How to cite: Ciardo, F. and Rinaldi, A. P.: Modeling of injection-induced seismicity in fractured rock masses with TOUGH3-seed hybrid solver, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5536, https://doi.org/10.5194/egusphere-egu22-5536, 2022.

Induced seismicity is a major challenge for fluid injection operations performed by geo-energy industry to exploit the underground resources. Despite recent developments in the understanding of induced earthquakes, many high-pressure fluid injection operations can still trigger unexpectedly large-magnitude events. A physical understanding of geological parameters controlling the induced seismicity is of central importance for improving our ability to forecast and mitigate the risk of inducing large earthquakes. Current physics-based numerical models are typically based on some simplifications that disregard the multiphysical interactions among fractures and faults. Therefore, the physical linkage between geometrical attributes of the fracture system and the statistics of induced seismicity is poorly understood. The final objective of this research is to determine the impact of fracture network properties on the spatiotemporal evolution of injection-induced seismicity and the emergence of large earthquake events.  

Here, we numerically capture the occurrence of seismic and aseismic slips in fracture systems, represented as discrete fracture networks (DFNs), spanning over two orders of magnitude over the length scale (1-100 m). Then, a 2D finite element model is used to simulate the coupled hydraulic and mechanical processes during fluid injection and analyze the occurrence of earthquakes. We present some preliminary results of our numerical simulations based on synthetic fracture network realizations. We particularly focus on power-law exponent of fracture length distribution and analyze the potential controls on the magnitude frequency of induced seismic events. The results of the analysis could have significant implications injection-related activities such as enhanced geothermal systems.

How to cite: Afshari Moein, M. J. and lei, Q.: Impact of fracture length distribution on the injection-induced seismicity in fractured rocks, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12474, https://doi.org/10.5194/egusphere-egu22-12474, 2022.

Geothermal energy is one of the most promising techniques to exploit renewable energy resources from the Earth and to limit emissions of greenhouse gas. Deep geothermal exploitations are associated with long term fluid circulation and pressure perturbations at great depth, in fractured and faulted zones and are likely associated with a risk of triggering earthquakes. Such earthquakes are usually interpreted as the reactivation of rapid (m/s) shear slip on critically stressed faults caused by fluid flow and poroelastic stress changes. In some cases however, slow aseismic slip (m/d) can take place on faults in response to fluid flow. How fluid pressure perturbations reactivate aseimic or rapid slip still remains poorly understood. A better understanding of the hydromechanical processes controlling fault slip is therefore crucial to mitigate seismic hazards associated with geothermal exploitation.

In this framework, our study aims at constraining the influence of stress state, fluid injection rate, diffusivity and frictional failure criterion on the reactivation of slip on pre-existing faults through mechanical modelling of a set of laboratory experiments. The experiments consist of a fluid injection into a saw-cut rock sample loaded in a triaxial cell. Fault reactivation is triggered by injecting fluids through a borehole directly connected to the fault. This experimental setup is modelled by a 3D Finite Element Method (FEM) coupled with a solver of the fluid diffusion. The sample fault is modelled as a contact surface obeying slip-weakening Mohr-Coulomb friction law. This approach allows to compute slip and stress evolutions, as observed during the laboratory experiment. The FEM model is calibrated and is able to reproduce the experimental results. We show that fluid injection triggers a shear crack that propagates varying from 1 to 300 m/d along the fault. This approach can be used to investigate the relationship between fluid front and slip front during reactivation, which is an important issue to control the effects of fluid injections at depth.

How to cite: Jiang, J., Dublanchet, P., Passelègue, F., Bruel, D., and Pellet, F.: Numerical modelling of fluid-induced fault slip reactivation,application to Geo-Energy systems, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9880, https://doi.org/10.5194/egusphere-egu22-9880, 2022.

The Hellisheiði Geothermal Field is situated in Southwest Iceland and composes the Southern part of the Hengill Volcanic System. This area is characterized by a complex triple junction between three tectonic features: the Reykanes Peninsula rifting, South Iceland Volcanic Zone and West Volcanic  Zone. Reinjection of spent geothermal fluids is distributed mostly in two areas (Gráuhnúkar and Húsmúli), comprising respectively 6 and 5 active injection wells. The Húsmúli reinjection area, commissioned in September 2011 and has seen significant seismicity associated with drilling and injection operations.
In the framework of the Geothermica project COSEISMIQ (http://www.coseismiq.ethz.ch/en/home/), a dense temporary network was installed to monitor the seismicity in the Hengill region between December 2018 and August 2021. With this enhanced network, novel analysis and relocation techniques, a high resolution relocated catalogue was curated and comprises over 3600 events in the Húsmúli area.
We use numerical models, some purely statistical (ETAS and Seismogenic index) and a hybrid model (TOUGH2-Seed) to reproduce observed seismicity in the Húsmúli reinjection area during the COSEISMIQ project. We employ a pseudo-forecasting approach and compare models performances
and fit to the recorded data.

How to cite: Rinaldi, A. P., Ritz, V., Nandan, S., Castilla, R., Karvounis, D., and Wiemer, S.: Modelling injection induced seismicity in the Hengill geothermal field, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10392, https://doi.org/10.5194/egusphere-egu22-10392, 2022.

Enabling a widespread exploitation of Enhanced Geothermal Systems (EGS) around the world by tapping into the heat trapped by the radioactive granites demands a better understanding of the fluid-induced seismicity associated with their stimulation. Induced seismicity occurs not only during hydraulic stimulation, but also after shut-in. The induced earthquakes of Mw > 3 at Basel and Mw = 5.5 at Pohang are two well-known examples that have caused a negative public perception on EGS. Here, we numerically compare the effect of bleed-off on the mitigation of post-injection seismicity for three stimulation schemes: constant rate, step rate and cyclic injection. We find that applying bleed-off in the post-injection phase significantly reduces the post-injection induced seismicity when compared to not applying bleed-off in all the injection schemes.

How to cite: Tangirala, S. K., Parisio, F., and Vilarrasa, V.: Numerical investigation of hydraulic stimulation strategies to mitigate post-injection seismicity in Enhanced Geothermal Systems, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8716, https://doi.org/10.5194/egusphere-egu22-8716, 2022.

Earthquakes caused by human activities receive scrutiny due to the risks and hazards they pose.  Seismicity that occurs after the causative anthropogenic operation stops has been particularly problematic – both because of high-profile cases of damage caused by this trailing seismicity and due to the loss of control for risk management.  With this motivation, we undertake a statistical examination of how induced seismicity stops.  We borrow the concept of Båth’s law from tectonic aftershock sequences.  Båth’s law anticipates the difference between magnitudes in two subsets of seismicity as dependent on their population count ratio.  We test this concept for its applicability to induced seismicity, including ~80 cases of earthquakes caused by hydraulic fracturing, enhanced geothermal systems, and other fluid-injections with clear operational end points.  We find that induced seismicity obeys Båth’s law: both in terms of the magnitude-count-ratio relationship and the power law distribution of residuals.  Furthermore, the distribution of count ratios is skewed and heavy-tailed, with most earthquakes occur during stimulation/injection.  We discuss potential models to improve the characterization of these count ratios and propose a Seismogenic Fault Injection Test to measure their parameters in situ.  We conclude that Båth’s law quantifies the occurrence of earthquake magnitudes trailing anthropogenic operations.

How to cite: Schultz, R., Ellsworth, W., and Beroza, G.: Statistical bounds on how induced seismicity stops, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-16, https://doi.org/10.5194/egusphere-egu22-16, 2022.

A growing number of direct and indirect measurements and observation indicates that aseismic slip transients are often induced during fluid injection operation alongside with swarm-like seismicity. The detection of fluid-induced aseismic slip has made a paradigm shift on our understanding of the spatio-temporal evolution of earthquake activity during injection operation, classically interpreted as triggered by a diffusive front of high pore pressure. Instead, unclamping of the fault by pressurization induced by fluid injection creates the condition to nucleate synchronous aseismic and seismic slip transients.  In this scenario, the spatio-temporal evolution of the induced seismicity is driven by the stressing rate imparted at the leading edges of the aseismic rupture front. However, the relationship between the magnitude of aseismic slip and the hydraulic energy input in the system remains still elusive. A similar mechanism has been proposed for natural earthquake swarms triggered by shallow (5-10 km depth) slow slip events (SSEs), for which a robust power-law scaling has been demonstrated between seismic and aseismic slip. Notably, the power-law moment scaling of shallow SEEs and associated earthquake swarms has been interpreted with a mechanism of fault pressurization enhanced by intense fracturing in a seismogenic volume with abundance of crustal fluids. Similar fault conditions are at play for fluid-induced seismicity. Here, we collected several case studies of recorded induced aseismic deformation during injection experiments together with the accompanying seismic activity. We investigated the spatial distribution and temporal evolution of the seismicity with respect to the ongoing transient aseismic slip. We focused in particular on the seismic and aseismic slip budget of induced seismicity and compared it with previous scaling of SSEs. The aseismic and seismic moments of induced events are compatible with the power-law scaling of shallow natural earthquakes swarms triggered by SSEs, although a data gap exits for SSEs in 0-4 magnitude range, where no SSEs have never been recorded due to the low resolution of surface geodetic instrumentation. We performed also numerical simulation using a 3D hydro-mechanical model using realistic fault and hydraulic parameters in order to fill in the data gap. Our results serve as a basis to build up empirical models that incorporate aseismic slip together with injected volume and pressure to forecast seismicity during fluid-injection experiments.

How to cite: Passarelli, L., De Barros, L., Rinaldi, A. P., and Wiemer, S.: Seismic to aseismic slip scaling during fluid injection experiments, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5699, https://doi.org/10.5194/egusphere-egu22-5699, 2022.

Sufficient shreds of evidence have proved the existence of the remote triggering effect of large earthquakes. To understand its mechanism, it is necessary to conduct detailed investigations on the influence of the far-field dynamic stress changes on the stress state of faults. As an important tool of ultra-broadband crustal stress monitoring, a four-component borehole strainmeter can directly record the dynamic changes of horizontal strain and stress caused by seismic waves. These data are of great importance to study the dynamic Coulomb stress changes and related triggering effects, but have not been paid sufficient attention to so far. This paper analyzes the data of the four-component borehole strainmeter at Gaotai and Tonghua stations, which recorded the far-field strain changes of four major earthquake events in the Pacific region in 2018. We successfully identify the seismic phases of P, S, and surface waves, and analyze the characteristics of different phases through the stress petal method. The dynamic stress changes are calculated, demonstrating the feasibility of using borehole strainmeter data to quantitatively study the triggering effect of teleseismic waves of earthquakes with different magnitudes at different epicentral distances. We find that the direction of the principal stress axis of the dynamic stress changes is generally consistent with the azimuth of the earthquake epicenter. We further discuss the Coulomb stress changes on the major faults near the stations. According to the results, the peak values of the dynamic Coulomb stress changes produced by four earthquakes on the fault planes near Gaotai and Tonghua stations are at the magnitude of hundreds of Pa, which are lower than the threshold value of dynamic triggering. This is also consistent with the observation that no dynamically triggered earthquakes are found on the faults. However, the idea and method of this paper provide useful insight into the detection of possible dynamic triggering of large earthquakes.

How to cite: Li, F., Ren, T., Chi, S., Zhang, H., and Shi, Y.: The dynamic Coulomb stress changes caused by remoteearthquakes based on the borehole strainmeter data, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10043, https://doi.org/10.5194/egusphere-egu22-10043, 2022.