TY - JOUR A1 - Bentz, Stephan A1 - Kwiatek, Grzegorz A1 - Martinez-Garzon, Patricia A1 - Bohnhoff, Marco A1 - Dresen, Georg T1 - Seismic moment evolution during hydraulic stimulations JF - Geophysical research letters N2 - Analysis of past and present stimulation projects reveals that the temporal evolution and growth of maximum observed moment magnitudes may be linked directly to the injected fluid volume and hydraulic energy. Overall evolution of seismic moment seems independent of the tectonic stress regime and is most likely governed by reservoir specific parameters, such as the preexisting structural inventory. Data suggest that magnitudes can grow either in a stable way, indicating the constant propagation of self-arrested ruptures, or unbound, for which the maximum magnitude is only limited by the size of tectonic faults and fault connectivity. Transition between the two states may occur at any time during injection or not at all. Monitoring and traffic light systems used during stimulations need to account for the possibility of unstable rupture propagation from the very beginning of injection by observing the entire seismicity evolution in near-real time and at high resolution for an immediate reaction in injection strategy. Plain Language Summary Predicting and controlling the size of earthquakes caused by fluid injection is currently the major concern of many projects associated with geothermal energy production. Here, we analyze the magnitude and seismic moment evolution with injection parameters for prominent geothermal and scientific projects to date. Evolution of seismicity seems to be largely independent of the tectonic stress background and seemingly depends on reservoir specific characteristics. We find that the maximum observed magnitudes relate linearly to the injected volume or hydraulic energy. A linear relation suggests stable growth of induced ruptures, as predicted by current models, or rupture growth may no longer depend on the stimulated volume but on tectonics. A system may change between the two states during the course of fluid injection. Close-by and high-resolution monitoring of seismic and hydraulic parameters in near-real time may help identify these fundamental changes in ample time to change injection strategy and manage maximum magnitudes. Y1 - 2020 U6 - https://doi.org/10.1029/2019GL086185 SN - 0094-8276 SN - 1944-8007 VL - 47 IS - 5 PB - American Geophysical Union CY - Washington ER - TY - JOUR A1 - Wang, Lei A1 - Kwiatek, Grzegorz A1 - Rybacki, Erik A1 - Bonnelye, Audrey A1 - Bohnhoff, Marco A1 - Dresen, Georg T1 - Laboratory study on fluid-induced fault slip behavior: the role of fluid pressurization rate JF - Geophysical research letters : GRL N2 - Understanding the physical mechanisms governing fluid-induced fault slip is important for improved mitigation of seismic risks associated with large-scale fluid injection. We conducted fluid-induced fault slip experiments in the laboratory on critically stressed saw-cut sandstone samples with high permeability using different fluid pressurization rates. Our experimental results demonstrate that fault slip behavior is governed by fluid pressurization rate rather than injection pressure. Slow stick-slip episodes (peak slip velocity < 4 mu m/s) are induced by fast fluid injection rate, whereas fault creep with slip velocity < 0.4 mu m/s mainly occurs in response to slow fluid injection rate. Fluid-induced fault slip may remain mechanically stable for loading stiffness larger than fault stiffness. Independent of fault slip mode, we observed dynamic frictional weakening of the artificial fault at elevated pore pressure. Our observations highlight that varying fluid injection rates may assist in reducing potential seismic hazards of field-scale fluid injection projects.
Plain Language Summary Human-induced earthquakes from field-scale fluid injection projects including enhanced geothermal system and deep wastewater injection have been documented worldwide. Although it is clear that fluid pressure plays a crucial role in triggering fault slip, the physical mechanism behind induced seismicity still remains poorly understood. We performed laboratory tests, and here we present two fluid-induced slip experiments conducted on permeable Bentheim sandstone samples crosscut by a fault that is critically stressed. Fault slip is then triggered by pumping the water from the bottom end of the sample at different fluid injection rates. Our results show that fault slip is controlled by fluid pressure increase rate rather than by the absolute magnitude of fluid pressure. In contrast to episodes of relatively rapid but stable sliding events caused by a fast fluid injection rate, fault creep is observed during slow fluid injection. Strong weakening of the dynamic friction coefficient of the experimental fault is observed at elevated pore pressure, independent of fault slip mode. These results may provide a better understanding of the complex behavior of fluid-induced fault slip on the field scale. KW - fault slip KW - fluid injection KW - induced seismicity KW - fluid pressurization KW - rate KW - stick-slip KW - fault creep Y1 - 2020 U6 - https://doi.org/10.1029/2019GL086627 SN - 0094-8276 SN - 1944-8007 VL - 47 IS - 6 PB - Wiley CY - Hoboken, NJ ER - TY - JOUR A1 - Wang, Lei A1 - Kwiatek, Grzegorz A1 - Rybacki, Erik A1 - Bohnhoff, Marco A1 - Dresen, Georg T1 - Injection-induced seismic moment release and laboratory fault slip BT - implications for fluid-induced seismicity JF - Geophysical research letters N2 - Understanding the relation between injection-induced seismic moment release and operational parameters is crucial for early identification of possible seismic hazards associated with fluid-injection projects. We conducted laboratory fluid-injection experiments on permeable sandstone samples containing a critically stressed fault at different fluid pressurization rates. The observed fluid-induced fault deformation is dominantly aseismic. Fluid-induced stick-slip and fault creep reveal that total seismic moment release of acoustic emission (AE) events is related to total injected volume, independent of respective fault slip behavior. Seismic moment release rate of AE scales with measured fault slip velocity. For injection-induced fault slip in a homogeneous pressurized region, released moment shows a linear scaling with injected volume for stable slip (steady slip and fault creep), while we find a cubic relation for dynamic slip. Our results highlight that monitoring evolution of seismic moment release with injected volume in some cases may assist in discriminating between stable slip and unstable runaway ruptures. KW - induced seismicity KW - seismic moment release KW - fluid injection KW - stick slip KW - fault creep KW - acoustic emission Y1 - 2020 U6 - https://doi.org/10.1029/2020GL089576 SN - 0094-8276 SN - 1944-8007 VL - 47 IS - 22 PB - American Geophysical Union CY - Washington ER - TY - JOUR A1 - Kwiatek, Grzegorz A1 - Martinez-Garzon, Patricia A1 - Plenkers, K. A1 - Leonhardt, Maria A1 - Zang, Arno A1 - von Specht, Sebastian A1 - Dresen, Georg A1 - Bohnhoff, Marco T1 - Insights into complex subdecimeter fracturing processes occurring during a water injection experiment at depth in Aspo Hard Rock Laboratory, Sweden JF - Journal of geophysical research : Solid earth N2 - We investigate the source characteristics of picoseismicity (M-w < -2) recorded during a hydraulic fracturing in situ experiment performed in the underground Aspo Hard Rock Laboratory, Sweden. The experiment consisted of six stimulations driven by three different water injection schemes and was performed inside a 28-m-long, horizontal borehole located at 410-m depth. The fracturing processes were monitored with a variety of seismic networks including broadband seismometers, geophones, high-frequency accelerometers, and acoustic emission sensors thereby covering a wide frequency band between 0.01 and 100,000Hz. Here we study the high-frequency signals with dominant frequencies exceeding 1000 Hz. The combined seismic network allowed for detection and detailed analysis of 196 small-scale seismic events with moment magnitudes M-W < -3.5 (source sizes of decimeter scale) that occurred solely during the stimulations and shortly after. The double-difference relocated hypocenter catalog as well as source parameters were used to study the physical characteristics of the induced seismicity and then compared to the stimulation parameters. We observe a spatiotemporal migration of the picoseismic events away and toward the injection intervals in direct correlation with changes in the hydraulic energy (product of fluid injection pressure and injection rate). We find that the total radiated seismic energy is extremely low with respect to the product of injected fluid volume and pressure (hydraulic energy). The radiated seismic energy correlates well with the hydraulic energy rate. The obtained fault plane solutions for particularly well-characterized events signify the reactivation of preexisting rock defects under influence of increased pore fluid pressure on fault plane orientations in good correspondence with the local stress field orientation. KW - induced seismicity KW - fracking KW - picoseismicity KW - seismomechanics KW - source parameters KW - maximum magnitude Y1 - 2018 U6 - https://doi.org/10.1029/2017JB014715 SN - 2169-9313 SN - 2169-9356 VL - 123 IS - 8 SP - 6616 EP - 6635 PB - American Geophysical Union CY - Washington ER - TY - JOUR A1 - Zang, Arno A1 - Stephansson, Ove A1 - Stenberg, Leif A1 - Plenkers, Katrin A1 - von Specht, Sebastian A1 - Milkereit, Claus A1 - Schill, Eva A1 - Kwiatek, Grzegorz A1 - Dresen, Georg A1 - Zimmermann, Günter A1 - Dahm, Torsten A1 - Weber, Michael T1 - Hydraulic fracture monitoring in hard rock at 410 m depth with an advanced fluid-injection protocol and extensive sensor array JF - Geophysical journal international N2 - In this paper, an underground experiment at the Aspo Hard Rock Laboratory (HRL) is described. Main goal is optimizing geothermal heat exchange in crystalline rock mass at depth by multistage hydraulic fracturing with minimal impact on the environment, that is, seismic events. For this, three arrays with acoustic emission, microseismicity and electromagnetic sensors are installed mapping hydraulic fracture initiation and growth. Fractures are driven by three different water injection schemes (continuous, progressive and pulse pressurization). After a brief review of hydraulic fracture operations in crystalline rock mass at mine scale, the site geology and the stress conditions at Aspo HRL are described. Then, the continuous, single-flow rate and alternative, multiple-flow rate fracture breakdown tests in a horizontal borehole at depth level 410 m are described together with the monitoring networks and sensitivity. Monitoring results include the primary catalogue of acoustic emission hypocentres obtained from four hydraulic fractures with the in situ trigger and localizing network. The continuous versus alternative water injection schemes are discussed in terms of the fracture breakdown pressure, the fracture pattern from impression packer result and the monitoring at the arrays. An example of multistage hydraulic fracturing with several phases of opening and closing of fracture walls is evaluated using data from acoustic emissions, seismic broad-band recordings and electromagnetic signal response. Based on our limited amount of in situ tests (six) and evaluation of three tests in Avro granodiorite, in the multiple-flow rate test with progressively increasing target pressure, the acoustic emission activity starts at a later stage in the fracturing process compared to the conventional fracturing case with continuous water injection. In tendency, also the total number and magnitude of acoustic events are found to be smaller in the progressive treatment with frequent phases of depressurization. KW - Geomechanics KW - Fracture and flow KW - Broad-band seismometers Y1 - 2016 SN - 0956-540X SN - 1365-246X VL - 208 SP - 790 EP - 813 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Durand, Virginie A1 - Bentz, Stephan A1 - Kwiatek, Grzegorz A1 - Dresen, Georg A1 - Wollin, Christopher A1 - Heidbach, Oliver A1 - Martinez-Garzon, Patricia A1 - Cotton, Fabrice A1 - Nurlu, Murat A1 - Bohnhoff, Marco T1 - A two-scale preparation phase preceded an M-w 5.8 earthquake in the sea of marmara offshore Istanbul, Turkey JF - Seismological research letters N2 - We analyze the spatiotemporal evolution of seismicity during a sequence of moderate (an M-w 4.7 foreshock and M-w 5.8 mainshock) earthquakes occurring in September 2019 at the transition between a creeping and a locked segment of the North Anatolian fault in the central Sea of Marmara, northwest Turkey. To investigate in detail the seismicity evolution, we apply a matched-filter technique to continuous waveforms, thus reducing the magnitude threshold for detection. Sequences of foreshocks preceding the two largest events are clearly seen, exhibiting two different behaviors: a long-term activation of the seismicity along the entire fault segment and a short-term concentration around the epicenters of the large events. We suggest a two-scale preparation phase, with aseismic slip preparing the mainshock final rupture a few days before, and a cascade mechanism leading to the nucleation of the mainshock. Thus, our study shows a combination of seismic and aseismic slip during the foreshock sequence changing the strength of the fault, bringing it closer to failure. Y1 - 2020 U6 - https://doi.org/10.1785/0220200110 SN - 0895-0695 SN - 1938-2057 VL - 91 IS - 6 SP - 3139 EP - 3147 CY - Boulder ER - TY - JOUR A1 - Ziegler, Moritz O. A1 - Reiter, Karsten A1 - Heidbach, Oliver A1 - Zang, Arno A1 - Kwiatek, Grzegorz A1 - Stromeyer, Dietrich A1 - Dahm, Torsten A1 - Dresen, Georg A1 - Hofmann, Gerhard T1 - Mining-Induced Stress Transfer and Its Relation to a 1.9 Seismic Event in an Ultra-deep South African Gold Mine JF - Pure and applied geophysics N2 - On 27 December 2007, a 1.9 seismic event occurred within a dyke in the deep-level Mponeng Gold Mine, South Africa. From the seismological network of the mine and the one from the Japanese-German Underground Acoustic Emission Research in South Africa (JAGUARS) group, the hypocentral depth (3,509 m), focal mechanism and aftershock location were estimated. Since no mining activity took place in the days before the event, dynamic triggering due to blasting can be ruled out as the cause. To investigate the hypothesis that stress transfer, due to excavation of the gold reef, induced the event, we set up a small-scale high-resolution three-dimensional (3D) geomechanical numerical model. The model consisted of the four different rock units present in the mine: quartzite (footwall), hard lava (hanging wall), conglomerate (gold reef) and diorite (dykes). The numerical solution was computed using a finite-element method with a discretised mesh of approximately elements. The initial stress state of the model is in agreement with in situ data from a neighbouring mine, and the step-wise excavation was simulated by mass removal from the gold reef. The resulting 3D stress tensor and its changes due to mining were analysed based on the Coulomb failure stress changes on the fault plane of the event. The results show that the seismic event was induced regardless of how the Coulomb failure stress changes were calculated and of the uncertainties in the fault plane solution. We also used the model to assess the seismic hazard due to the excavation towards the dyke. The resulting curve of stress changes shows a significant increase in the last in front of the dyke, indicating that small changes in the mining progress towards the dyke have a substantial impact on the stress transfer. KW - Induced seismicity KW - static stress change KW - deep-level mining KW - tabular mining KW - Coulomb failure stress KW - 3D geomechanical numerical model Y1 - 2015 U6 - https://doi.org/10.1007/s00024-015-1033-x SN - 0033-4553 SN - 1420-9136 VL - 172 IS - 10 SP - 2557 EP - 2570 PB - Springer CY - Basel ER -