Characteristics of large-magnitude microseismic events recorded during and after stimulation of a geothermal reservoir at Basel, Switzerland (original) (raw)
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The authors analyzed microseismic events with large magnitude collected during and after a hydraulic stimulation of engineered geothermal reservoir at Basel, Switzerland in 2006. Fundamental characteristics of the microseismic events with large magnitude were investigated by hypocentral distribution, source radius, similarity of waveforms to neighboring events, and fault plane solution. Results from these analyses have revealed that most of the large events from the deep part of the stimulated zone originated in ruptures involving multiple asperities. It has also been estimated that the large events in the shallow part of the seismic cloud occurred in fractures that were sub-parallel to the stimulated zone, suggesting that they were the result of different rupture processes to that of the mid-depth and deep large events. We investigated volumetric strain induced by preceding microseismic events to a large event and pore pressure, which are known as possible triggers of shear slip, to interpret physics behind the large events at Basel. Some amount of strain was accumulated around the hypocenters of the large events just before the occurrences, although no clear evidence that strain is the trigger of the large events was obtained. Spatio-temporal distribution of the critical pore pressure for shear slip was estimated using tectonic stress and fault plane solution (FPS). This analysis revealed that critical pore pressure does not directly correlate to the magnitude of microseismic events.
Proceedings of the 10th SEGJ International Symposium, 2011
The authors analyzed seismic events which include large events with moment magnitude larger than 2.0 observed during and after the hydraulic stimulation at Basel, Switzerland in 2006. The characteristics of the large events, including time of occurrence, hypocenter location, waveform similarity/difference to the neighboring events, fault plane solutions (FPSs), and seismic source parameters, have been investigated by the authors. We found that the characteristics of the large events, which occurred in deeper and shallower parts of the seismic cloud are different from each other. In this study, we identified orientation of fracture planes on which seismic activity was observed. Critical pore pressure for shear slip, which is considered as one of the triggers of the shear slip on existing fracture plane has been also evaluated for the large events. Many large events had fault plane solution with trend of N-S azimuth and sub-vertical dip, even the stress state of the fracture plane is not the most preferential to slip. Meanwhile, the rests of the large events including the largest one, occurred on the fracture plane with WNW-ESE azimuth trend strikes, which can be interpreted as the most preferential plane to slip in case of coefficient friction is 0.6. Such unbalanced seismic activity and release of seismic energy on fractures with different azimuths are clearly observed for this data.
Proceedings of the 11th SEGJ International Symposium, Yokohama, Japan, 18-21 November 2013, 2013
Induced seismic events with moment magnitude larger than 2.0 occurred during and after a hydraulic stimulation for creation of reservoir of Engineered Geothermal System (EGS) at Basel, Switzerland in 2006. The large events considerably damaged the constructions and resulted in shutdown of EGS project in Basel after risk analyses for further large events. The authors have analyzed the large events and investigated fundamental characteristics of the large events, including time series of occurrence, epicenter location, waveform similarity to the other events, Fault Plane Solutions (FPSs), and source parameters. In this study, we investigated relation between the occurrence of the large seismic events and some possible trigger mechanisms for shear slip of the large seismic events. One of the possible trigger mechanisms is an increase in pore pressure. We estimated the critical pore pressure from the Coulomb failure criterion using the information of the stress state at Basel and the orientation of the fault planes. We observed that the large events occurred just after the final stage of the stimulation at relatively low critical pore pressure. We also estimated diffusion pressure based on diffusion model and the results showed diffusion pressure disturbed even after two months of the stimulation. The other possible trigger mechanism is the static stress redistribution caused by the preceding events. The static stress changes were calculated from orientation of the fault planes and seismic source parameters as input. The result showed that the stress state on the fault plane became stable for most of large events and that the magnitude of the stress change was negligibly smaller than change in pore pressure.
2009
Microseismic multiplets, which are groups of events with highly similar waveform despite different origin time and magnitude, can be effectively used to determine relative location of the hypocenters with high accuracy. The authors clustered multiplets from microseismic events collected during a hydraulic stimulation in the Deep Heat Mining Project at Basel, Switzerland in 2006. The similarity of waveform was quantitatively evaluated in the frequency domain using the magnitude square coherence function, and this made it possible to cluster the events with different criteria which correlate to different physical phenomena associated with shear slip on fractures. In this study, the similarity of waveform was evaluated in lower frequency (1-60 Hz) and higher frequency (40-97 Hz), respectively. We then interpreted the behavior of each multiplet cluster from the relocated hypocenter distribution, fault plane solution, source radius, and hydraulic record. Multiplets which correlate to (a)...
We show that microseismic events—earthquakes with small magnitudes—can be fruitfully used to gain insight into the properties of the fracture network of large-scale porous media, such as oil, gas, and geothermal reservoirs. As an example, we analyze extensive data for the Geysers geothermal field in northeast California. Injection of cold water into the reservoir to produce steam leads to microseismic events. It is demonstrated that the analysis can also lead to insight into whether the fractures are of tectonic type or induced by injection of cold water. To demonstrate this we estimate, using the catalogue of the microseismic events, the fractal dimension D f of the spatial distribution of hypocenters of the events in three seismic clusters associated with the injection of cold water into the field, as well as the b values in the Gutenberg-Richter frequency-magnitude distribution. The fractal dimensions are all in a narrow range centered around D f 2.57 ± 0.06, comparable to the measured fractal dimension of fracture sets in the greywacke reservoir rock. For most cases the b values are about b 1.3 ± 0.1, consistent with the Aki relation, D f = 2b. Both D f and b are significantly higher than those commonly observed for regional tectonic seismicity or aftershock sequences for which D f ≈ 2 and b ≈ 1 are typical. Our results do not imply that no tectonic triggering exists in the reservoir, but rather that the overpressure allows the activation of less favorably oriented fractures that produce an increase in both b and D f. The estimate D f ≈ 2 for tectonic seismicity has been interpreted as indicating that most tectonic events occur on the subset of near-vertical faults—because they have lower normal stress—or that they occur on the backbone of the fracture and fault network, the multiply connected part of the network that enables finite shear strain. Our results lend support to the latter. The results that the entire fracture network, and not just its backbone, is active at the Geysers indicate that the seismicity is not a result of the triggered release of tectonic stress, but is induced by the release of local stress concentrations, driven by thermal contraction that is not constrained by friction. The possible implication for hydraulic fracturing—so-called fracking—is also briefly discussed.
Large earthquakes during hydraulic stimulations at the geothermal site of Soultz-sous-Fores
Several deep wells were drilled in the Rhine Graben (Soultz-sous-Foreˆts, France) to evaluate the geothermal Hot Dry Rock potential of a deep fractured granite reservoir. Three main boreholes, which reached about 5 km depth, intersected a crystalline basement overlain by 1.4 km of Cenozoic and Mesozoic sediments. Stimulations of these three wells were carried out in 2000 for GPK2, 2003 for GPK3 and 2004 for GPK4. During these stimulations and other hydraulic activities a seismological surface network was installed in order to monitor the seismicity induced by the massive fluid injection.
2010
The technical feasibility of geothermal power production in a low enthalpy environment will be investigated in the geothermal site at Groß Schönebeck, North German Basin, where a borehole doublet was completed in 2007. In order to complete the Enhanced Geothermal System, three massive hydraulic stimulations were performed. To monitor injection-induced seismicity during fluid injection a seismic network was deployed including a single 3-component downhole seismic sensor at only 500 m distance to the injection point. Injection rates reached up to 9 m/min and maximum injection well-head pressure was as high as ~60 MPa. A total of 80 very small (-1.8 <MW< -1.0) induced seismic events were detected only at the deep borehole sensor. The hypocenters were determined for 29 events using P and S wave onset times and polarization analysis. The events show a strong spatial and temporal clustering and a maximum seismicity rate of 22 events per day. Spectral parameters were
We have inverted the peak amplitudes of direct P waves of 45 micro-earthquakes with magnitudes between M = 1.4 and 2.9, which occurred during and after the 2003 massive fluid injection in the GPK3 borehole of the Soultz-sous-Forêts Hot Dry Rock facility. These events were recorded by a surface seismic network of 15 stations operated by the Ecole et Observatoire des Sciences de la Terre, University of Strasbourg. The unconstrained moment tensor (MT) expression of the mechanism was applied, allowing the description of a general system of dipoles, that is, both double-couple (DC) and non-DC sources, as tensile fractures. The mechanisms of all but one event are dominantly DCs with a few per cent additional components at the most. We have checked carefully the reliability of the MT retrieval in bootstrap trials eliminating some data, by simulating the mislocation of the hypocentre and by applying simplified velocity models of the area in constructing Green's functions. In some of the trials non-DC components amounting to several tens of per cent appear, but the F-test classifies them as insignificant. Even the only micro-earthquake with an exceptionally high non-DC mechanism cannot be classified unambiguously-the F-test assigns similar significance to the pure DC solution. The massive dominance of the DC indicates the shear-slip as the mechanism of the micro-earthquakes investigated. The mechanisms display large variability and are of normal dip-slip, oblique normal to strike-slip types. The T-axes are fairly stable, being concentrated subhorizontally roughly in the E-W direction. On the contrary, the P-axes are ill constrained varying in the N-S direction from nearly vertical to nearly horizontal, which points to heterogeneous stress in the Soultz injected volume. This is in agreement with the stress pattern from in situ measurements: the minimum stress axis is well constrained to E-W, whereas the maximum and intermediate stress values are close to one another, enabling the ambiguity of the P-axis direction. We found no significant dependence of source mechanisms either on magnitudes or depths. The time-space distribution of the events analysed suggests that the injection activated two segments of the natural faults existing in the area (I and II in our notation) showing different source mechanism patterns. The dip-slip regime is characteristic of fault segment I where the seismicity occurred during and also after injection, whereas the strike-slip regime prevails in segment II where the seismicity was triggered only after the injection shut in. This indicates that the tensile fractures, which are assumed to be created during injection, may have occurred on a smaller scale than the pure shear micro-earthquakes investigated.
Rupture Characteristics of —M3 to M1 Hydraulically Induced Seismicity
SEG Technical Program Expanded Abstracts 2015, 2015
Besides the typical negative magnitude fractures induced in the treated formations by hydraulic fracture stimulation programs, small positive magnitude events associated to small faults located underneath or cross-cutting the reservoir also frequently occur. In this study we investigate discriminant source and rupture characteristics to distinguish between the two event types and also between reservoir fracture types. To achieve this goal we estimate source and failure properties of -M3 to M1 seismic events recorded during a hydraulic fracturing stimulation of a shale reservoir in Horn River Basin, Canada, for which the >M0 events are associated with slip on a pre-existing fault underneath the reservoir. Comparison between static and dynamic source parameters suggests distinct signatures of the two event types associated to two distinct failure processes. Positive magnitude events occurring beneath the reservoir have slightly higher static and dynamic stress drops, higher apparent stress and energy release, and rupture faster than shallower reservoir events. These differences reflect fracturing of harder rocks at higher confining stresses for the deep events, but also a possible release of a larger quantity of strain energy stored within the fault zone. The lower stress and energy release and slower rupturing fractures observed in the reservoir fractures, as well as overshoot type failure (slip weakening failure) indicates fluid lubrication by pore pressure increase and frictional resistance reduction. Some trends are also observed when looking only at reservoir fractures, where variations in average rupture velocities correlate with variations in formation depth and fault azimuth reflecting a dominance of the local stress field over other factors. Average rupture velocities also correlates with elapsed time showing an observable imprint of the changed local conditions during treatment over the regional conditions. Identification of more spatial and temporal families of events with similar rupture behaviors and source characteristics can be used as a proxy for specific fracture network development and hydrocarbon production and included in geo-mechanical models and fracture treatment designs. Reservoir and fault related induced events release less stress and radiates less energy than natural occurring tectonic earthquakes of comparable size at similar depths indicating a potential fluid influence in these failures. Considering the ongoing debate regarding the cause-effect relationship between fluid injection programs and nearby deeper earthquakes this study suggests that source parameters can be used as a discriminant factor between the two types of earthquakes.
Analytical investigation of hydraulic fracture-induced seismicity and fault activation
Environmental Earth Sciences, 2018
More recent public discourse has taken place regarding the potential correlation between seismic activity and hydraulic fracturing in shale gas reservoirs. Public fears about the risk of seismicity stem mainly from past earthquakes induced by conventional deep injections because the two types of projects share similar mechanisms of rock failure and fault activation. Although previous earthquake risks associated with fluid injection were not serious, the situation would be far more problematic if hydraulic fracturing in a shale gas reservoir triggered a similar-sized earthquake due to potential environmental issues. In fact, almost all documented injection-induced earthquakes have been associated with long-duration and high-volume injection rather than short-term (hours) pressurization (e.g., hydraulic fracturing). In general, hydraulic fracturing operations mostly induce microseismic events through rock failure and activation of small fractures. Although shale reservoirs in tectonically active zones pose a high risk of inducing large-magnitude seismic activities, the internal geological conditions and external stimulation conditions are impossible to be satisfied simultaneously to trigger activation of an entire fault and to result in a destructive earthquake during hydraulic fracturing operations.