Faults slip in response to changes in stress or fluid pressure, and these changes can be natural or anthropogenic. Estimating the likelihood of fault slip for a given change in loading is critical for safe geological storage and energy extraction in faulted rocks, as well as effective communication of risks to policy makers and the public. The energy transition and decarbonization are urgent and essential tasks: we will only be successful if we manage to balance public perceptions of risk with the technical challenges inherent to the exploitation of faulted rock. To accomplish both, we need to do a better job of quantifying the uncertainties in our mechanical and geometrical data. 

Measures of fault stability include slip (T_s) and dilation (T_d) tendency, and fracture susceptibility (S_f, the change in fluid pressure to push effective stress to failure). The input values for any of these measures are always uncertain, and they are uncertain to varying degrees. For example, while the vertical stress can be well constrained from wireline density log data, the maximum horizontal stress is generally much harder to quantify from any source.

Probabilistic models of fault stability for the Mississippian (Lower Carboniferous) Limestone underlying much of northern England are presented. This is an undrilled target for geothermal energy. Fault maps are derived from published geological maps and recently reprocessed seismic reflection data. Stress and pressure constraints are derived from legacy onshore hydrocarbon wells and wireline logs. Coefficients of friction and cohesive strength remain poorly constrained, not only in terms of their magnitude, but critically in the shapes of their statistical distributions. In addition, the applicability of simplified indices of fault stability (T_s, T_d, S_f) to complex natural fault zones is questionable, and our predictions could be improved through weighting by information derived from long-term seismological records.  

This contribution raises two key points: 1) Laboratory data rarely include repeat measurements from quasi-identical samples of the same rock, and therefore the statistical distributions of friction or cohesion are poorly known. This is important because skewed distributions – and the direction of that skewness (high or low) – can significantly affect predictions of fault stability; 2) What is the best way to weight the calculated probability of slip in complex natural fault zones to account for geometrical weakening? Specifically, is total fault length more or less important than fault smoothness (roughness)?

How to cite: Healy, D. and Hollis, C.: A probabilistic assessment of induced seismicity and fluid flow in the Carboniferous Limestone of northwest England: Implications for geothermal energy, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2798, https://doi.org/10.5194/egusphere-egu23-2798, 2023.

Hydraulic fracturing and horizontal drilling technology had contributed to the commercial shale gas production. However, hydraulic fracturing requires a water for the drilling fluids. Many countries have shale gas fields and are water stress country such as China. CO2 fracturing have many advantages than water fracturing. One is that CO2 gas is a greenhouse gas. The other is that SRV of the CO2 fracturing is much larger than that of the  water fracturing due to the low viscosity of CO2 gas. Devonian age Besa River shale gas play in the Liard Basin of northeastern BC with net estimated sales gas of 48 TCF over an area of 430,000 acres. We have conducted many case studies about CO2 fracturing simulation and reservoir simulation in the shale gas of Liard basin. In order to study, mechanical earth model was constructed. From this case study, we designed optimum fracturing pattern and development plan.

How to cite: Lee, T. and Ahn, T.: CO2 fracturing simulation and reservoir simulation case study in the Liard Basin of Canada, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4577, https://doi.org/10.5194/egusphere-egu23-4577, 2023.

Riedel shear structures (RSS) are often observed in the embryonic stage of strike-slip fault development. Typical RSS often involve a set of conjugated fault segments that interact with each other prior to the growth of a through-going fault, that is, the principal deformation zone (PDZ), sub-structures R, R’ and P. Ideally, sub-structures R and P should be symmetric to the PDZ while structures R and R′ are conjugated to each other. It is a critical concept linking the geomechanical behavior of individual earthquakes to structural geology at both local and regional scales. The depiction of RSS can help understand the geomechanical behavior of individual earthquakes. In the meantime, the influence of long-term fluid injections on the developing process of RSS, as manifested by the common occurrences of injection-induced earthquakes, has been rarely addressed.

Here we document a clear case in western Canada where the development of a local RSS system is expedited by 25 years of wastewater injection. RSS are manifested by an earthquake sequence consisting of 187 events (M_L ranging 1.3–3.9) between 2018/01/01 and 2021/07/15 in an area without any previous seismic history.

Focal mechanisms of these events exhibit various faulting types with the majority (87%) being compatible with the background stress regime. The orientation of derived nodal planes and six fault segments depicted from the refined earthquake distribution collectively define the overall geometrical characteristics of RSS, which consists of four primary strike-slip structures striking 19º (R’), 79º (R), 94º (PDZ) and 109º (P), respectively. Mohr-Coulomb failure analysis further suggests a cumulative stress perturbation of up to 10.0 MPa, probably bringing the local structures very close to the critically stressed state.

Overall, our observations suggest that long-term fluid injection can expedite the development of local fault systems. Consequently, it is probably necessary to consider the full dimension of the local/regional RSS rather than the size of individual events in the assessment of the overall injection-induced seismic hazard.

How to cite: Yu, H., Kao, H., Wang, B., and Visser, R.: Long-term Fluid Injection Can Expedite Fault Development: Riedel Shear Structures Illuminated by Induced Earthquakes in Alberta, Canada, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4598, https://doi.org/10.5194/egusphere-egu23-4598, 2023.

The Earth’s crust is permeated with faults and fractures due to its long tectonic history. Faults and fractures not only act as zones of weakness but can increase permeability and act as conduits to circulate fluids in the underground, making them ideally suited for geothermal energy production. However, human-induced changes to in-situ pressure fields have a documented history of leading to fault (re-)activation in the form of earthquakes (seismic slip) or aseismic slip that does not generate measurable ground motion.

This study aims to quantify the current background earthquake activity, the spatiotemporal relationship to the seismotectonic setting, and the remote earthquake-earthquake triggering propensity in the Lower Rhine Embayment (LRE) in western North-Rhine Westphalia. The study area is targeted for extensive exploration activities focused on geothermal energy production. While regional mean slip rates do not exceed 0.1 mm/yr, paleo-seismic studies suggest that the normal faulting system has hosted a series of ~14 earthquakes of M_w > 5.0 since the 14th century, including the 1992 M_w 5.3 Roermond earthquake. Therefore, estimating background seismicity rates independent of anthropogenic stress perturbation is one important element in developing strategies to minimize the probability of felt earthquakes caused by industrial activity. 

We evaluate waveform data from a temporary deployment of 48 seismic stations operating between July 2021 and May 2022 in a radius of roughly 10 km around a future exploration well drilling site in Weisweiler. We implement a machine learning-based earthquake detection algorithm, SeisBench, to detect earthquakes and denoise the continuous seismic waveforms (DeepDenoiser), estimate P- and S-phase arrivals (PhaseNet and Generalised Phase Detection; GPD), and associate earthquake phases using a Bayesian Gaussian mixture model (GaMMA). We locate 81 local earthquakes to complement the 14 recorded in the Earthquake Observatory Bensberg (BNS) catalog for a total of 95 seismic events recorded between July 2021 and December 2021 with magnitudes ranging from 0 < M_L < 1.3. In addition, we use the continuous BNS catalog to evaluate the dynamic triggering potential in the LRE starting in 1990. We select 20 teleseismic mainshocks with M > 6 (1990 – 2015) and M > 7 (2016 – present), as well as the 1992 Roermond M_w 5.3 due to the high amplitude shaking it caused within the study area. Preliminary remote dynamic triggering results suggest that the passing surface waves of the July 2021 M 8.2 Chignik, Alaska earthquake may have triggered a seismic sequence of about 16 locatable earthquakes. The migrating aftershock sequence of the Roermond earthquake also suggests the mainshock caused limited dynamic triggering within the study area, which lies outside of the classical aftershock zone of ~2-3 fault lengths.

How to cite: Roth, M. P., Harrington, R. M., Knapmeyer-Endrun, B., Finger, C., and Dietl, M.: Deriving the triggering potential in the Lower Rhine Embayment using background seismicity near Weisweiler, Germany, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7498, https://doi.org/10.5194/egusphere-egu23-7498, 2023.

In 2018, after more than 700 years, the last black coal mine ceased operation in the greater Ruhr region in western Germany. In addition to the repurposing of the unused subsurface infrastructure for heat storage projects, the utilization of deep geothermal resources, located below mining levels in the Devonian Massenkalk formations, is the most promising way of facilitating the green energy transition in this highly populated region. The operation of fluid injection or withdrawal during geothermal production alters subsurface stress and poses questions about the possible reactivation of faults in the greater Ruhr region, with strong implications for seismic hazard potential. Deep geothermal resources depend on permeable pathways (i.e., fault zones and fracture networks) in the subsurface, which are known to have certain intrinsic pre-operational seismic hazard. In this study, we evaluate the probability of reactivation (using the so-called slip tendency and fracture susceptibility indicators) and dilation tendency of major faults accounting for uncertainties of in situ stress conditions, fault geometries, and frictional properties. Furthermore, we investigate the spatio-temporal evolution of the slip tendency of a major fault in the region during geothermal operations utilizing coupled thermo-hydro-mechanical numerical models. This study benefits from a recently published in situ stress database of the greater Ruhr region derived from 429 hydrofracturing tests performed across several coal mines. The assessment of fault reactivation utilizing probabilistic and coupled numerical modelling approaches, as presented in this study, aims at de-risking the exploration of deep geothermal systems in the greater Ruhr region and can hopefully serve as an example for quantification of pre-operational seismic hazards in other regions. Based on preliminary results, we find that the NW-SE striking faults are much more prone to reactivation, in comparison to the NE-SW ones, which are oriented unfavorably under prevailing stress conditions. We also find that thermal stress contributes significantly to fault stability, especially during long-term geothermal production.

How to cite: Kruszewski, M., Verdecchia, A., Heidbach, O., Harrington, R. M., and Healy, D.: Quantification of pre-operational seismic hazard of deep geothermal systems exemplified in the greater Ruhr region (Germany), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8557, https://doi.org/10.5194/egusphere-egu23-8557, 2023.

Pore pressure changes due to fluid injection or withdrawal alter the rock stresses, which may potentially induce seismic events. Activities that have been associated with induced seismicity include geothermal energy production, subsurface gas storage, and natural gas production. Physics-based models are required to gain insight in the processes that lead to induced seismicity. When computing induced stresses with such models, a commonly made simplification is the assumption of uniform pressure changes across the entire reservoir. In reality, pressure gradients arise due to fluid production or injection. Here, we assess the effect of non-uniform pressure fields under steady-state flow conditions on induced stresses. We employ (semi-)analytical techniques to compute the corresponding pressure field and fault stresses. We are particularly interested in reservoirs with displaced faults (i.e., cases with nonzero fault offset), as shear stresses tend to concentrate at the reservoir corners along the faults. The stress profile along the fault becomes asymmetric under steady-state flow. The effect of fluid flow on fault stresses is larger in case of injection than in case of depletion. Injection with up-dip flow results in increased zones of fault slip near the bottom of the reservoir, while injection with down-dip flow results in increased slip near the top of the reservoir. The significance of steady-state flow on induced stresses can be estimated from the ratio of the average pressure change along the fault and the applied pressure gradient. The effect of steady-state flow is most relevant at the start of production or injection and diminishes with time. Thus, the effect of steady-state flow is only expected to be relevant when initially critically stressed faults are present. For non-critically stressed faults, the assumption of uniform depletion or injection is expected to be reasonable. An order-of-magnitude estimate of the effect of steady-state flow across displaced faults in the Groningen natural gas reservoir shows that the effect on fault stresses is probably negligible. A similar estimate of the effect in typical low-enthalpy geothermal doublets indicates that steady-state flow may possibly play a small role, in particular close to the injector. Nevertheless, site-specific assessments are necessary to quantify the effect in greater detail.

How to cite: Cornelissen, P. and Jansen, J. D.: The effect of steady-state flow on induced stresses along displaced faults, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8515, https://doi.org/10.5194/egusphere-egu23-8515, 2023.

Deep geothermal is a clean and renewable source of energy with a high potential for heat and electricity production which can help Switzerland meet its energy and climate objectives. Worldwide, several geothermal projects have been successfully operated for decades. Unfortunately, some projects have also been suspended due to unexpected levels of induced seismicity. Thus, adequate risk management is essential to establish safe and economically viable geothermal projects.

In Switzerland, the subsurface is under the sovereignty of the cantonal authorities. Within the GEOBEST2020+ project, the Swiss Seismological Service (SED) supports the cantons in adequately handling the risk of induced seismicity associated with deep geothermal projects. Funded by the Federal Office of Energy (SFOE) in the scope of its SwissEnergy program, the GEOBEST2020+ program aims to provide operator-independent seismological consulting and baseline seismic monitoring services to the cantonal authorities.

In this framework, we deploy dedicated seismic networks in the vicinity of the monitored projects. These networks must be sensitive enough to follow the evolution of microseismicity and allow the operators to run traffic-light systems and take action before larger events occur. Before the station installation, we perform a careful site survey analysis, considering the background noise conditions and evaluating the signal-to-noise ratio at each site. To evaluate beforehand the detection sensibility of a seismic network, we estimate the Bayesian Magnitude of Completeness (BMC), optimized for Switzerland. We additionally estimate the theoretical location uncertainties inside the network considering the background noise level at the stations and the network geometry.

Here we show the comparison results between our methodology to the ground truths of several years of continuous monitoring data at Lavey-les-Bains, canton of Vaud. We use a combination of three types of detection methods: Machine Learning combined with migration methods (MALMI), coherence (Pyrocko/Lassie) and template matching (QuakeMatch), to produce a high-quality, manually revised seismic catalog. We compare our theoretical methods of network performance to the real data measured at the geothermal site.

How to cite: Antunes, V., Kraft, T., Roth, P., Toledo, T., and Wiemer, S.: GEOBEST2020+ baseline seismic monitoring workflow and network performance evaluation for deep geothermal projects in Switzerland, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16577, https://doi.org/10.5194/egusphere-egu23-16577, 2023.

Hydraulic stimulation has been used as a standard practice for rock mass treatment to fulfill the intended engineering goal of the stimulation procedure in various applications such as oil & gas, mining, and enhanced geothermal systems (EGS). High-pressure fluid injection during the hydraulic stimulation perturbs the local stress field around the injection borehole which has a direct influence on the hydro-mechanical behavior of the rock mass and the measured induced seismicity. Hydraulic stimulation is usually conducted using a predefined injection protocol, a sequence of pressure and/or flow rate injection cycles. Extensive attempts have been designed to develop a proper injection protocol in laboratory and field scales to reach the stimulation goals as well as reduce the risk of induced seismicity.

In the context of the ERC “Fault Activation and Earthquake Rupture (FEAR)” project, injection protocols are developed to further our understanding of earthquake rupture processes such as nucleation and premonitory slip. A series of stress-preconditioning injection strategies were tested in the Bedretto Underground Laboratory for Geoenergies and Geosciences, Switzerland (Ma et al., 2022, Solid Earth), using an existing stimulation and monitoring array (Plenkers et al., under rev. with Sensors). The main objective of these tests was to develop atypical injection strategies that could be implemented in different geological conditions that target aspects of induced seismicity associated with high-pressure fluid injection. We performed two injections and monitored the rock mass response using six monitoring boreholes each equipped with multi-component seismic monitoring systems, pressure, temperature, and distributed strain sensing arrays. Two different injection protocols have been applied in two packed-off injection intervals of a stimulation borehole, allowing us to characterize the hydro-mechanical response of the rock mass around these intervals. Each test was associated with pre- and post-characterization tests including hydraulic and hydro-mechanical tests such as HTPF tests.

The preliminary results of these tests show a direct dependency of the location and magnitude of seismicity with injection protocols. Moreover, the effect of the injection protocols on the characteristics of the rock mass such as transmissivity and normal stress could be quantified. Preconditioning of the fault appears to require low permeability, preferably undrained, fault structures and suffers if any fluids pathway are mitigated by the long range, slow pressurization of this region. Complex fluid pathways made re-stimulation and precondition of the target structures difficult. The outcomes of these tests play a key role in the design of upcoming FEAR stimulation experiments to better understand the earthquake physics on a field scale.

References

Ma, X., Hertrich, M., Amann, F., Bröker, K., Gholizadeh Doonechaly, N., et al., 2022. Multi-disciplinary characterizations of the BedrettoLab–a new underground geoscience research facility. Solid Earth, 13(2), pp.301-322.

Plenkers K., Reinicke A., Obermann A., Gholizadeh Doonechaly N., Krietsch H., et al., under rev. with Sensors, Multi-disciplinary monitoring networks for mesoscale underground experiments: Advances in the Bedretto Reservoir Project.

How to cite: Jalali, M., Selvadurai, P., Dal Zilio, L., Durand, V., Obermann, A., Meier, M.-A., and Amann, F. and the Bedretto Team: An Attempt to Control Induced Seismicity and On-Fault Stress Pre-Conditioning in the Bedretto Underground Laboratory, Switzerland, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11422, https://doi.org/10.5194/egusphere-egu23-11422, 2023.

Most traffic-light systems (TLS) related hydraulic fracturing stimulations still adopt the maximum observed magnitude as the decisive metric to aid decision making by stakeholders. However, waiting for the red-light magnitude to be observed is not a proactive stance, especially given that jumps of up to two magnitude units are evidently common enough between events. Clearly there is a need to actively forecast rather than to passively record the size of the next largest earthquake (NLE). In this study, we demonstrate that we can do just that using an ensemble of 6 existing models from the literature designed with similar purposes in mind (Shapiro et al. 2013; McGarr 2014; Mendecki 2016; van der Elst et al. 2016; Galis et al. 2017; Cao et al. 2020). Following a logic-tree approach, these 6 models are calibrated and dynamically weighted in near real-time using as sole inputs the initial parts of the earthquake catalogs and the reported injection rates. The proposed forecasting tool is tested against 18 past stimulations from 9 different Enhanced Geothermal Systems around the world (Helsinki, Basel, Soultz, Cooper Basin, Basel, Pohang, FORGE, Paralana, Newberry). Overall, the results indicate a consistent (across sites and time) and accurate estimation of the next largest magnitude with a tight uncertainty range (1σ) of less than 0.5 magnitude units. Our proposed framework underestimated the next largest magnitude only in one occasion (out of the 18 stimulations), while reliably maintaining a tight safety margin of less than 1 magnitude unit. We recommend that the forecasted NLE replaces the largest observed magnitude as the default metric adopted by future TLS governing any type of fluid-injection operation.

How to cite: Grigoratos, I., Kwiatek, G., and Wiemer, S.: Dynamic forecasting of the next largest earthquake during hydraulic fracturing stimulations, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15041, https://doi.org/10.5194/egusphere-egu23-15041, 2023.

We performed a similarity based hierarchical cluster analysis and carried out a master event relative relocation of seismicity induced during the hydraulic stimulation of a brittle and highly fractured shear zone. The shear zone was stimulated in an extend of about 10 m and is located in crystalline rock at the Grimsel Test Site, Switzerland. The objective of the experiment was to improve our understanding of stimulation processes associated with high-pressure fluid injections used for reservoir creation in enhanced geothermal systems. In addition to the seismic network exhibiting high spherical coverage at close distances (i.e., eight acoustic emission sensors ~10 – 15 m away from the injection interval), fluid pressures, rock mass deformations and fault dislocations were monitored around the injection location. 1/3 of the located seismic events could be assigned to 44 repeater families exhibiting high waveform similarity (i.e., >95%) when comparing waveform similarities sequentially in time. When comparing similarities to the chosen master event, similarities decay continuously within a repeater family which also reflects in a spatial migration of hypocenters. However, source areas of seismic events within repeater families mostly overlap assuming an appropriate stress drop. But the spatial extent of the observed migration of hypocenters compared to the measured mechanical deformation in the volume is larger and thus suggests that multiple asperities are responsible for seismic events within a repeater family. The repeater families themselves are distributed in clusters which are attributed to the stimulation of distinct fractures in the damage zone of the targeted shear zone. No repeating events were found in the seismicity accompanying a tensile fracture splaying off the stimulated pre-existing fractures. Statistically, events of repeater families are rather Gaussian than power-law distributed. The extended analysis of this multisensor array data presented here reveals a highly complex fracturing process on the 10 m scale during hydraulic stimulation (see also an earlier study of the same experiment focusing on source mechanism Villiger et al. (2021)).

References

Villiger, L., Gischig, V. S., Kwiatek, G., Krietsch, H., Doetsch, J., Jalali, M., Amann, F., Giardini, D., & Wiemer, S. (2021). Meter-scale stress heterogeneities and stress redistribution drive complex fracture slip and fracture growth during a hydraulic stimulation experiment. Geophysical Journal International. https://doi.org/10.1093/gji/ggab057 (Geophysical Journal International)

How to cite: Villiger, L., Kraft, T., Gischig, V., Krietsch, H., and Wiemer, S.: Repeating earthquakes induced during a decameter-scale hydraulic stimulation experiment at the Grimsel Test Site, Switzerland, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13239, https://doi.org/10.5194/egusphere-egu23-13239, 2023.