Coupled thermal-hydraulic-mechanical (THM) modeling is applied to investigate the performance of a seasonal high-temperature aquifer thermal energy storage operation based on data and conditions from current site investigations at the Geostorage Forsthaus pilot project in Bern (Switzerland). The model includes subhorizontal sand lenses of various lengths and dips that are embedded in a low permeability clay matrix. Thermal energy storage is simulated by seasonal injection and withdrawal of hot (up to 90 degrees C) water from a main well, with reservoir pressure regulated by two auxiliary wells at a distance of about 70 m from the main well. The results show how targeted injection into deeper permeable storage formations, along with active deep well pressure control, can effectively minimize geomechanical impact and the potential risk of damaging subsurface storage and sealing formations, or even surface facilities. With such pressure control, the subsurface mechanical responses are dominated by thermal strain and stress, which can be monitored with subsurface fiber optics. The study demonstrates how coupled THM modeling can be applied for the design of a safe and efficient thermal energy storage operation, and how subsurface fiber optic monitoring can be applied for performance confirmation, allowing for more confident operational forecasting.

The 2017 Pohang M w 5.5 earthquake is currently the largest seismic event induced by Enhanced Geothermal Systems. The high uncertainty on geological and mechanical conditions of the rupture fault of this earthquake has originated a debate on its triggering mechanisms. Here, we propose a stochastic poromechanical analysis approach that combines Monte Carlo sampling and poromechanical models to address the uncertainty problem. By conducting a large number of coupled poromechanical simulations varying the uncertain geomechanical parameters, we yield an exceedance probability of 7%-15% for the Pohang mainshock. Remarkably, this physics-based stochastic prior forecast is quite comparable to the posterior likelihood inferred from the magnitude-frequency relationship of recorded seismicity. Our results reveal a scaling relationship between the earthquake magnitude and the initial fault stability, which indicates a threshold of the initial Coulomb Failure Stress to differentiate if faults are initially, critically stressed, and thus, the earthquake magnitude. This Pohang threshold is -0.2 to -0.1 MPa, about one order of magnitude larger than that proposed for natural earthquakes. This study highlights that the reactivation of critically stressed faults may trigger damaging earthquakes even for small poromechanical perturbations and opens a promising avenue for assessing the likelihood of induced earthquakes based on physical understanding.

Managing induced seismicity risk is needed to enable the widespread adoption of geothermal technologies, facilitating the transition towards a decarbonized energy sector. In April 2022, real-time monitoring and forecasting of induced seismicity were tested during a three-stage hydraulic stimulation in a deep granite heat reservoir at the Utah FORGE site. Here, we analyze the recorded seismicity through statistical inference, and investigate the possible fracturing mechanisms triggered by the injection operation. Our analysis indicates that seismicity is likely induced by opening of a tensile fracture. Through pseudo-prospective forecasting, we then replay the Stage 3 stimulation and related induced seismicity as if it were happening in real-time. We demonstrate that even if the physical processes are complex and likely difficult to disentangle using seismicity alone, physics-based seismicity rate forecasting models show promise for stable forecastability of seismicity induced during hydraulic stimulation. Our results pave the way for Advanced Traffic Light Protocols (ATLP) to become standard operational technology in the mitigation strategies of deep geothermal projects.

Advancing technologies to harvest deep geothermal energy has seen backlash related to unacceptable levels of induced seismic hazard during hydraulic stimulations. A thorough analysis of induced seismic hazard before these operations has recently become standard practice in the last decade. Additionally, more process understanding of the underlying causes of induced seismicity as well as novel approaches to develop geomechanical reservoirs are being explored in controlled underground laboratory experiments worldwide. Here, we present a probabilistic analysis of the seismic hazard induced by the ongoing hectometre-scale stimulation experiments at the Bedretto Underground Laboratory for Geoenergies and Geosciences (BULGG). Our workflow allows for fast updates of the hazard computation as soon as new site-specific information on the seismogenic response (expressed primarily by the feedback afb value and the Gutenberg–Richter b value) and ground motion models (GMMs) become available. We present a sequence of hazard analyses corresponding to different project stages at the BULGG. These reveal the large uncertainty in a priori hazard estimations that only reduces once site-specific GMMs and information on the seismic response of specific stimulation stages are considered. The sources of uncertainty are (1) the large variability in the seismogenic response recorded across all stimulation case studies and (2) uncertain GMMs on the underground laboratory scale. One implication for large-scale hydraulic stimulations is that hazard computation must be updated at different project stages. Additionally, stimulations have to be closely accompanied by a mitigation scheme, ideally in the form of an adaptive traffic light system (ATLS), which reassesses seismic hazard in near-real time. Our study also shows that the observed seismogenic responses in underground laboratories differ from large-scale stimulations at greater depth in that the seismogenic response is substantially more variable and tends to be weaker. Reasons may be lower stress levels, but also smaller injected volumes accessing a more limited fracture network than large-scale stimulations. Controlled underground laboratory experiments can contribute to improving our understanding of the the physical reasons leading to such variable seismogenic responses. The presented analysis implied that such experiments may be limited in terms of upscaling but are likely to be safe in terms of induced seismic hazard.

The world’s energy supply depends critically on hydraulic fracturing (HF) to access otherwise uneconomical resources. Unfortunately, HF also has the potential to induce larger earthquakes – with some projects being prematurely terminated because of perceived earthquake risks. To de-risk HF, we use a suite of statistical tests to discern if some physical process has restricted the growth of earthquake magnitudes. We show that all stage stimulations at both UK PNR-1z and Helsinki St1 indicate bound fracture growth, implying a more controllable operation. Contrastingly, stimulations at Utah FORGE and UK PNR-2 sequentially transitioned into unbound fault reactivation. The problematic stages (that ultimately led to the termination of PNR-2) are clearly distinguishable. We postulate that our research can discriminate fracture stimulation from fault reactivation, contributing to the de-risking of HF operations worldwide. Our statistical tests provide a framework for model falsification, which can guide physical insights into the bounding processes. A newly developed statistical approach using three distinct tests, termed CAP-tests, can discern cases of bound earthquake magnitude growth during hydraulic fracturing stage stimulations and could be used to de-risk future operations.

Hydraulic stimulation of Enhanced Geothermal Systems (EGS) aims at boosting permeability to facilitate fluid circulation, while keeping a low induced seismicity. However, some stimulations have led to poor permeability enhancement or too high induced earthquakes, which suggests that further understanding is needed on poromechanical processes during stimulation. Here, we model a highly-monitored test performed at the Bedretto Underground Laboratory to investigate the impact of fluid injection on permeability enhancement and induced microseismicity. We examine three models: (1) a homogeneous fracture whose transmissivity is manually calibrated to reproduce the observed pressure evolution at the injection borehole (this model fails to capture the spatial distribution of pressure and the corresponding poromechanical processes); (2) an elastic fracture approach, where transmissivity changes locally as a function of fracture aperture following the cubic law (this model overestimates pressure after the onset of fracture slip); and (3) a viscoplastic fracture approach with strain weakening and dilatancy that yields an additional permeability enhancement after shear reactivation. The viscoplastic model captures the spatio-temporal coupled response of the fractured rock to hydraulic stimulation before and after shearing both in terms of pressure and microseismicity. Subsequently to the onset of shear failure, microseismic events occur in every injection cycle as the reactivation front advances when plastic strain and, thus, permeability surpass the previously achieved maximum value. This viscoplastic model permits estimating the extent of the stimulated fracture, the permeability enhancement and its impact on the local state of stress and pore pressure at surrounding fractures, representing a useful tool for the design of effective hydraulic stimulation.

Fractures control fluid flow, solute transport, and mechanical deformation in crystalline media. They can be modeled numerically either explicitly or implicitly via an equivalent continuum. The implicit framework implies lower computational cost and complexity. However, upscaling heterogeneous fracture properties for its implicit representation as an equivalent fracture layer remains an open question. In this study, we propose an approach, the Equivalent Fracture Layer (EFL), for the implicit representation of fractures surrounded by low-permeability rock matrix to accurately simulate hydromechanical coupled processes. The approach assimilates fractures as equivalent continua with a manageable scale (>> 1 mu m) that facilitates spatial discretization, even for large-scale models including multiple fractures. Simulation results demonstrate that a relatively thick equivalent continuum layer (in the order of cm) can represent a fracture (with aperture in the order of mu m) and accurately reproduce the hydromechanical behavior (i.e., fluid flow and deformation/stress behavior). There is an upper bound restriction due to the Young's modulus because the equivalent fracture layer should have a lower Young's modulus than that of the surrounding matrix. To validate the approach, we model a hydraulic stimulation carried out at the Bedretto Underground Laboratory for Geosciences and Geoenergies in Switzerland by comparing numerical results against measured data. The method further improves the ability and simplicity of continuum methods to represent fractures in fractured media.

A Mw 5.5 earthquake struck Pohang (South Korea) on November 2017, following a sequence of five hydraulic stimulations of an Enhanced Geothermal System (EGS). The processes that led to this earthquake, which nucleated two months after the end of the last stimulation in borehole PX2, are not well understood yet. We propose a hydromechanical model that integrates available data to understand the potential relationship between the earthquake and the EGS. Data scarcity is translated into model uncertainties, which we address with sensitivity analyses. Results show that the Mw 5.5 earthquake is linked to the high-injection overpressures of up to 90 MPa induced during the stimulations in borehole PX2 and highlight the usefulness of hydromechanical modeling to forecast the seismicity of an EGS and, more specifically, the need to integrate the low permeability fault core that hindered fluid pressure dissipation at Pohang, which explains the long delay of the mainshock.

Deep geothermal boreholes, often drilled to the crystalline basement, suffer from borehole breakouts that compromise borehole stability and/or lead to low drilling performance. These issues increase the cost of deep geothermal projects and lead to irregular cross-sectional geometries that may entangle well completion (e.g., packer isolation for zonal stimulation, cementing, etc.). Thus, the proper knowledge of rock strength, state of stress and their interactions at the closest vicinity of the borehole is key to the success of deep geothermal drilling. Typically, the magnitudes of the vertical and minimum horizontal principal stresses, Sv and Shmin, respectively, can be estimated while SHmax is difficult to constrain. This paper presents a systematic methodology to jointly evaluate the heterogeneous distributions of the stress tensor principal components and orientations, and the rock strength properties (e.g. cohesion, friction). Model parameters are estimated from measurements available during or shortly after drilling, i.e., breakout width, breakout extent/depth of penetration, breakout orientation and drilling induced tensile fractures. Additionally, measurements of estimated parameters or transformations of them can be considered in the calibration in a generic manner (e.g., Shmin interpreted from XLOT). For illustration purposes, the methodology is applied to the extensive borehole data set along the crystalline section of the borehole BS-1, in Basel (Switzerland). The methodology allows us (1) to derive plausible sets of stress and strength parameters reproducing the complex distribution of breakouts along BS-1, and (2) to unveil the paradox of having no borehole breakouts at sections with high density of natural fractures.

The increased interest in subsurface development (e.g., unconventional hydrocarbon, engineered geothermal systems (EGSs), waste disposal) and the associated (triggered or induced) seismicity calls for a better understanding of the hydro-seismo-mechanical coupling in fractured rock masses. Being able to bridge the knowledge gap between laboratory and reservoir scales, controllable meso-scale in situ experiments are deemed indispensable. In an effort to access and instrument rock masses of hectometer size, the Bedretto Underground Laboratory for Geosciences and Geoenergies (“BedrettoLab”) was established in 2018 in the existing Bedretto Tunnel (Ticino, Switzerland), with an average overburden of 1000 m. In this paper, we introduce the BedrettoLab, its general setting and current status. Combined geological, geomechanical and geophysical methods were employed in a hectometer-scale rock mass explored by several boreholes to characterize the in situ conditions and internal structures of the rock volume. The rock volume features three distinct units, with the middle fault zone sandwiched by two relatively intact units. The middle fault zone unit appears to be a representative feature of the site, as similar structures repeat every several hundreds of meters along the tunnel. The lithological variations across the characterization boreholes manifest the complexity and heterogeneity of the rock volume and are accompanied by compartmentalized hydrostructures and significant stress rotations. With this complexity, the characterized rock volume is considered characteristic of the heterogeneity that is typically encountered in subsurface exploration and development. The BedrettoLab can adequately serve as a test-bed that allows for in-depth study of the hydro-seismo-mechanical response of fractured crystalline rock masses.

The Bedretto Underground Laboratory for Geosciences and Geoenergies (BULGG) is located in central Switzerland and serves as a test bed for geothermal energy research. Several boreholes were drilled from the laboratory section (ca. 1.1 km overburden) to serve as injection boreholes for stimulation and geophysical monitoring boreholes. During a hydraulic stimulation injection in winter 2020 into injection borehole ST2 interval ranging from 313 to 320 m, we observe a thermal perturbation using distributed fiber optic temperature sensing in a neighboring open borehole (MB1) at a depth of 275 m to 295 m. Prior to injection, there is a thermal anomaly in MB1 at about 289 m due to natural fracture fluid flow. Below this depth the temperature is approximately 1.5 °C higher than above. During injection there is a gradual upward movement of the thermal anomaly to ca. 278 m depth. After injection is stopped, the thermal signal gradually recovers to the original depth. The cause for such a temperature change is potentially due to increased warm water flow reaching the base of MB1 from deeper ST2 or poro-elastic fracture closure of the cold-water conducting fractures at 278 and 289 m depth in MB1 during stimulation.

The history of reservoir stimulation to extract geothermal energy from low permeability rock (i.e. so-called petrothermal or engineered geothermal systems, EGS) highlights the difficulty of creating fluid pathways between boreholes, while keeping induced seismicity at an acceptable level. The worldwide research community sees great value in addressing many of the unresolved problems in down-scaled in-situ hydraulic stimulation experiments. Here, we present the rationale, concepts and initial results of stimulation experiments in two underground laboratories in the crystalline rocks of the Swiss Alps. A first experiment series at the 10 m scale was completed in 2017 at the Grimsel Test Site, GTS. Observations of permeability enhancement and induced seismicity show great variability between stimulation experiments in a small rock mass body. Monitoring data give detailed insights into the complexity of fault stimulation induced by highly heterogeneous pressure propagation, the formation of new fractures and stress redistribution. Future experiments at the Bedretto Underground Laboratory for Geoenergies, BULG, are planned to be at the 100 m scale, closer to conditions of actual EGS projects, and a step closer towards combining fundamental process-oriented research with testing techniques proposed by industry partners. Thus, effective and safe hydraulic stimulation approaches can be developed and tested, which should ultimately lead to an improved acceptance of EGS.

We present key observations and analyses of the first and second stimulations conducted at the Pohang enhanced geothermal system (EGS) site in Korea in 2016. The first hydraulic stimulation was conducted in the PX-2 well of 4.3 km depth, with the maximum wellhead pressure of 89.2 MPa, the maximum injection rate of 46.8 L/s, and a total injected volume of 1970 m(3). The first stimulation showed non-linear and reversible fracture-opening behavior with injection pressure increase. The stimulation mechanism of PX-2 is interpreted as a combination of tensile fracture extension and hydraulic jacking. The second hydraulic stimulation was conducted in the PX-1 well of 4.2 km depth, with the maximum wellhead pressure of 27.7 MPa, the maximum injection rate of 18.0 L/s, and a total injected volume of 3907 m(3). The fracture-opening pressure of PX-1 was evaluated from the clear pressure peaks and the pressure at differential injectivity increase, and was drastically lower than that of PX-2. The transmissivity of PX-1 permanently increased by 6.4 times during the second stimulation. The wellhead injectivity of PX-1 was 3.6 times as high as that of PX-2 at the same injection rate. The stimulation mechanism in PX-1 is interpreted as a combination of hydraulic shearing and hydraulic jacking of unmated or shear-dilated fracture. Both stimulations in the two wells showed consistently greater seismicity rate and magnitude during the shut-in phases than during the preceding injection phases. A close correlation between the injected fluid volume and seismic magnitude was observed in both wells, and the seismic events induced by the two stimulations were in general below the maximum magnitude envelopes expected by the previous studies. Despite the close distance of approximately 600 m in the same rock formation, the two wells showed distinctly different behavior in terms of the overall pressure ranges, pressure peaks, pressure for injectivity increases, transmissivity changes, and the stimulation mechanisms due possibly to the heavy mud and lost circulation material used during the drilling and completion of PX-2. The contrasting hydromechanical responses observed in the same reservoir at the two nearby wells emphasize the importance of proper drilling and completion with close consideration of stimulation strategy.

In this study, we investigate numerically the hydro-mechanical behavior of fractured crystalline rock due to one of the five hydraulic stimulations at the Pohang Enhanced Geothermal site in South Korea. We use the commercial code FracMan (Golder Associates) that enables studying hydro-mechanical coupled processes in fractured media in three dimensions combining the finite element method with a discrete fracture network. The software is used to simulate fluid pressure perturbation at fractures during hydraulic stimulation. Our numerical simulation shows that pressure history matching can be obtained by partitioning the treatment into separate phases. This results in adjusted stress-aperture relationships. The evolution of aperture adjustment implies that the stimulation mechanism could be a combination of hydraulic fracturing and shearing. The simulated extent of the 0.01 MPa overpressure contour at the end of the treatment equals to similar to 180 m around the injection point.

Monitoring micro-seismicity during operations of a geothermal field is critical to the understanding of seismic hazard and changes in the reservoir. In the context of a geothermal project, induced earthquakes are an important tool to enhance the permeability and thus productivity of reservoirs and to image structure and processes. However, felt and/or damaging earthquakes are a major threat to societal acceptance and regulatory license to operate. With the adaptive data-driven tool ATLS (Adaptive Traffic Light System), we aim at managing and mitigating the risk posed by induced earthquakes during stimulation and operations, while at the same time ensuring and optimising the productivity. The demonstration site for the application of ATLS lies in the Hengill volcanic region located in the South-West of Iceland, host to two power plants (Hellisheiði and Nesjavellir) with a total production capacity of 423 MWe and 433MWth. The production of energy and heat is accompanied by reinjection of the spent geothermal water in dedicated areas, both to maintain production and to comply with legal requirements. These reinjection areas have been showing different seismic responses to drilling and injection operations. We investigate these different behaviours by performing numerical modelling for two of the reinjection regions. Two models are compared: TOUGH2-Seed, a full 3-dimensional stochastic simulator and an analytical model based on a cumulative density function linking maximum pressure in the reservoir and reactivation. Those two models fulfil two different aspects of the development of an ATLS, with the full 3D allowing an in-depth dive in the driving mechanisms of induced seismicity; and the analytical solution providing a robust and fast approximation of the forecast for real-time application. We show that both models can reproduce observed seismicity patterns in the Hengill geothermal field.