•Model for permeability enhancement by hydroshearing.•Model calibration with a fracture stimulation from Fenton Hill Project.•Combination of pure elastic opening and hydroshearing well explains ...observations.•Sensitivity analysis on critical parameter shows that permeability can be increased with small events.
In this study, we analyze a deep hydraulic stimulation of a fracture zone that was conducted as part of the classical Fenton Hill Hot Dry Rock Program in the 1970s. At the time, it was suggested that a pre-existing fracture or multiple fractures within the fracture zone were jacked open by injection-induced increase in pressure. In this study, we analyze the same stimulation experiment, but investigate the possibility of an alternative mechanism of shear reactivation of pre-existing fractures. We conduct modeling that accounts for both jacking (or elastic-fracture opening) and shear-slip dilation and demonstrate that injection-induced shear reactivation (or hydroshearing) could have occurred simultaneously with seismic events of magnitudes lower than what can be felt by humans. In fact, simulations considering shear reactivation seem to better match observed fluid recovery after multiple injection cycles. Shear reactivation and shear dilation results in locked-open fractures, especially near the injection well that provides permeability of higher flow recovery. We then investigate the sensitivity of the proposed model by varying some of the critical parameters such as maximum aperture, dilation angle, as well as fracture density. Interestingly, none of the simulated cases resulted in a large event that could have been felt by humans, but did result in a cumulative seismic magnitude of less than 1 for each given stimulation step. These results suggest that a permanent irreversible permeability increase of several orders of magnitude can be obtained by hydroshearing in a “seismically” safe manner.
We have conducted numerical simulation studies to assess the potential for injection-induced fault reactivation and notable seismic events associated with shale-gas hydraulic fracturing operations. ...The modeling is generally tuned toward conditions usually encountered in the Marcellus shale play in the Northeastern US at an approximate depth of 1500 m ( similar to 4500 ft). Our modeling simulations indicate that when faults are present, micro-seismic events are possible, the magnitude of which is somewhat larger than the one associated with micro-seismic events originating from regular hydraulic fracturing because of the larger surface area that is available for rupture. The results of our simulations indicated fault rupture lengths of about 10-20 m, which, in rare cases, can extend to over 100 m, depending on the fault permeability, the in situ stress field, and the fault strength properties. In addition to a single event rupture length of 10-20 m, repeated events and aseismic slip amounted to a total rupture length of 50 m, along with a shear offset displacement of less than 0.01 m. This indicates that the possibility of hydraulically induced fractures at great depth (thousands of meters) causing activation of faults and creation of a new flow path that can reach shallow groundwater resources (or even the surface) is remote. The expected low permeability of faults in producible shale is clearly a limiting factor for the possible rupture length and seismic magnitude. In fact, for a fault that is initially nearly-impermeable, the only possibility of a larger fault slip event would be opening by hydraulic fracturing; this would allow pressure to penetrate the matrix along the fault and to reduce the frictional strength over a sufficiently large fault surface patch. However, our simulation results show that if the fault is initially impermeable, hydraulic fracturing along the fault results in numerous small micro-seismic events along with the propagation, effectively preventing larger events from occurring. Nevertheless, care should be taken with continuous monitoring of induced seismicity during the entire injection process to detect any runaway fracturing along faults.
► Modeling of transient ground displacement caused by deep fracture zone opening. ► Comparison of simulation results with all available field data at KB-502 injection well, In Salah, Algeria. ► ...Sensitivity analysis to evaluate the effects of changing the extent of the fracture zone.
The Krechba gas field at In Salah (Algeria), the site of the first industrial scale on-shore CO2 storage demonstration project, is also known for satellite-based ground-deformation monitoring data of remarkable quality. In this work, we focus on the In Salah injection well KB-502, where a double-lobe uplift pattern has been observed in the ground-deformation data. On the basis of previous numerical results, semi-analytical inverse deformation solutions, and seismic analyses, we explain this pattern of uplift as resulting from injection-induced deformation in a deep vertical fracture zone. In this study, we simulate a fracture zone characterized by high permeability and low mechanical stiffness, which activates after a few months of injection, causing irreversible changes in permeability. We study the transient evolution of uplift using the observed injection rate and compare it to the field Interferometric Synthetic Aperture Radar (InSAR) data using the displacement in the satellite line-of-sight. We also carry out a sensitivity study, analyzing the extent of the fracture zone, particularly its height from the reservoir depth. Our analysis supports the notion that the fracture zone is confined within the caprock and does not penetrate into the overlying aquifer.
•Effects on CO2 leakage and induced seismicity and their relation in two different scenarios.•Permeability enhancement in the first years of injection can have a significant impact on leakage ...rate.•Induced seismic events do not always enhance CO2 leakage.
The importance of geomechanics—including the potential for faults to reactivate during large-scale geologic carbon sequestration operations—has recently become more widely recognized. However, notwithstanding the potential for triggering notable (felt) seismic events, the potential for buoyancy-driven CO2 to reach potable groundwater and the ground surface is actually more important from public safety and storage-efficiency perspectives. In this context, this work extends the previous studies on the geomechanical modeling of fault responses during underground carbon dioxide injection, focusing on the short-term integrity of the sealing caprock, and hence on the potential for leakage of either brine or CO2 to reach the shallow groundwater aquifers during active injection. We consider stress/strain-dependent permeability and study the leakage through the fault zone as its permeability changes during a reactivation, also causing seismicity. We analyze several scenarios related to the volume of CO2 injected (and hence as a function of the overpressure), involving both minor and major faults, and analyze the profile risks of leakage for different stress/strain-permeability coupling functions. We conclude that whereas it is very difficult to predict how much fault permeability could change upon reactivation, this process can have a significant impact on the leakage rate. Moreover, our analysis shows that induced seismicity associated with fault reactivation may not necessarily open up a new flow path for leakage. Results show a poor correlation between magnitude and amount of fluid leakage, meaning that a single event is generally not enough to substantially change the permeability along the entire fault length. Consequently, even if some changes in permeability occur, this does not mean that the CO2 will migrate up along the entire fault, breaking through the caprock to enter the overlying aquifer.
We explore the role of earthquake interactions during an injection‐induced seismic sequence. We propose a model, which considers both a transient pressure and static stress redistribution due to ...event interactions as triggering mechanisms. By calibrating the model against observations at the Enhanced Geothermal System of Basel, Switzerland, we are able to reproduce the time behavior of the seismicity rate. We observe that considering earthquake interactions in the modeling leads to a larger number of expected seismic events (24% more) if compared to a pressure‐induced seismicity only. The increase of the number of events is particularly evident after the end of the injection. We conclude that implementing a model for estimating the static stress changes due to mutual event interactions increases significantly the understanding of the process and the behavior of induced seismicity.
Key Points
We model synthetic catalogues for induced seismicity accounting for earthquake interactions in terms of static stress transfer
Static stress interactions affect the number of seismic events, which increase (24% more) with respect to a pressure‐induced only case
Static stress transfer has a significant role in understanding the rate and total number of events, as well as their spatial distribution
•Modeling suggests that tunnel excavation in clay can create stress to reactivate a fault zone.•Results of sensitivity analysis highlight mostly minor slip events.•Critical stress or frictional ...conditions may result in a runaway rupture.•Modeling for nuclear waste repository shows no seismicity, unless unlikely conditions.
Seismic events with magnitude 3 and above have been associated with the removal of rock mass in mining environment since long-time. On the contrary, little is known about the possible seismic events induced by tunneling, although it presents similarities with mining. One great example is the case of the 57 km long Gotthard Base Tunnel excavation, which has been associated more than hundred seismic events, with the largest one having magnitude of ML 2.4, damaging the tunnel infrastructures (e.g. gallery floor or portal area).
Different underground structures will be built probably up to 1000 m below ground for the construction of future deep geological for the storage of nuclear waste. While seismic risk will probably not constitute a liability for the storage site construction, it is important to understand the potential for reactivation of seismogenic features located nearby the future location of emplacement tunnels.
Here we present numerical simulations aimed at understanding the potential for fault reactivation during tunnel construction in clay material, a potential host rock for nuclear waste repository. We evaluate the evolution of the stress changes during the simulation of the excavation with FLAC3D numerical solver. A strain-softening friction model is used to simulate the occurrence of a sudden slip on a fault zone when critical conditions for reactivation are satisfied. This constitutes a worst-case scenario, given the low seismogenic potential of clay rocks. We also present a sensitivity analysis on several critical parameters including fault frictional properties, stress conditions, as well as different tunnel sizes at varying distance from a nearby failure plane, with the final purpose of evaluating safety of a potential nuclear repository site on the short- and long-term.
► The paper deals with induced seismicity from undetected faults. ► Leakage issues related to fault permeability. ► Seismic potentials have been evaluated through analytical methods and ...geo-mechanical simulations.
With developing countries strongly relying on fossil fuels for energy generation, geological carbon sequestration (GCS) is seen as a candidate for large reductions in CO2 emissions during the next several decades. GCS does, however, raise some safety concerns. Specifically, it has been associated with induced seismicity, as a result of pressure buildup arising from prolonged CO2 injection in GCS projects. This seismicity is a delicate issue for two main reasons. First, over a short time scale, deformation of rock could release seismic energy, potentially affecting surface structures or simply alarming the population, with negative consequences for the social acceptance of this kind of projects. Second, over a longer time scale, activated faults may provide preferential paths for CO2 leakage out of reservoirs. While known major faults intersecting target aquifers can be identified and avoided during site screening, the same might not be true for faults that are not resolvable by geophysical surveys. In this study, we use geological observations and seismological theories to estimate the maximum magnitude of a seismic event that could be generated by a fault of limited dimensions. We then compare our estimate with results of geomechanical simulations that consider faults with different hydrodynamic and geomechanical characteristics. The coupled simulations confirm the notion that the tendency of faults to be reactivated by the pressure buildup is linked with the in situ stress field and its orientation relative to the fault. Small, active (critically stressed) faults are capable of generating sufficiently large events that could be felt on the surface, although they may not be the source of large earthquakes. Active, relatively permeable faults may be detrimental concerning the effectiveness of a storage project, meaning that they could be preferential pathway for upward CO2 leakage, although minor faults may not intersect both CO2 reservoirs and shallower potable aquifers.
In 2013, fluid injection during the St. Gallen deep geothermal project, Switzerland, induced hundreds of seismic events, including a
ML 3.5 earthquake on a fault hundreds of meters away from the ...well. Recent studies have suggested the direct pressure effect through permeable hydraulic connections and poroelastic effects as possible mechanisms for inducing seismicity on distant faults. In St. Gallen, operational, seismic, and earthquake data are available to investigate the underlying physical mechanisms using a numerical model. The results show that Coulomb stress changes at the fault can be 3 orders of magnitude greater when a hydraulic connection is present. Combining this with several field observations, we conclude that the direct pressure effect was more likely the predominant mechanism behind the seismicity induced in St. Gallen. The detection of hydraulic connections may be important for future projects as pressure can be driven far from the well and reactivate remote faults.
Plain Language Summary
In 2013, a deep geothermal project carried out in St. Gallen, Switzerland, aimed to generate heat and electricity for that city. But fluid pumped into the 4 km deep borehole caused hundreds of small earthquakes and one larger one that was distinctly felt by the local population. Although it is known that injecting fluid can change the conditions underground and trigger earthquakes, in St. Gallen the tremors occurred several hundred meters away from the well, necessitating a more detailed analysis. Consequently, we performed computer simulations designed to reveal otherwise hidden processes in the subsurface. The results indicate that the injected fluids rapidly flowed through permeable cracks toward the site where the earthquakes occurred, leading to significant changes in subsurface conditions. Knowledge of such highly permeable underground channels may help to improve the control over man‐made earthquakes in future industrial projects.
Key Points
We model stress changes on a fault several hundred meters away from the St. Gallen injection well
A hydraulic connection yields stress changes 3 orders of magnitude greater than poroelastic effects
The direct pressure effect is more likely to be the dominant mechanism inducing the seismicity
Geologic storage of carbon dioxide can efficiently contribute to reduce greenhouse gas emissions to the atmosphere. Two major hazards of this technology are leakage towards the ground surface and ...fluid-induced seismicity. While major faults may be detected and avoided during site characterization, undetected subseismic faults could be encountered once injection has started. This paper investigates leakage and reactivation of undetected faults through coupled thermal–hydraulic–mechanical modeling. The simulations are performed using a recently developed sequential simulator, TOUGH-Pylith, that allows to accurately account for the thermodynamics of brine-CO2 mixtures, and to model faults as surfaces of discontinuity using state-of-the-art fault friction laws. The simulator is benchmarked against well-known analytical solutions and subsequently applied to investigate two cases of CO2 injection close to undetected faults under normal and strike-slip faulting regimes. Although the scenarios are generic and represent unfavorable conditions, they suggest that leakage could occur and that undetected faults could trigger minor seismic events. Therefore, careful site characterization and continuous monitoring during operations should be always performed.
•Modeling of CO2 leakage and reactivation of subseismic faults.•Thermal–hydraulic–mechanical simulations using the sequential simulator TOUGH-Pylith.•Minor seismic events possible, hazard reduces with characterization and monitoring.
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