Analysis of microseismic events from a stimulation at Basel, Switzerland (original) (raw)
Related papers
Geothermics, 2013
Induced seismicity with large events occurred during and after a hydraulic stimulation at Basel, Switzerland, in 2006. This paper describes a study of the characteristics of the large events (those of moment magnitude greater than 2.0) to understand their origin. The large events during the stimulation and just after bleeding off had hypocenters within the seismic cloud while the large events that occurred several weeks after shut-in were located outside of the seismic cloud. We found no evidence that either local stress concentration or increased pore pressure caused the increase of event magnitudes as no shear slip with extremely high stress drop, or a significant correlation between pore pressure and large event magnitude were identified. Our integrated analysis of the fault plane solution and rock failure mechanism showed unbalanced seismic activity and seismic energy release in the pre-existing fracture system. From these observations we conclude that the large events did not originate from the rupture of rigid asperities triggered by increased pore pressure. Our observations suggest instead that critical changes of the stress state or coefficient of friction on fracture planes during stimulation triggered the unstable shear slip of large events. We also conclude that the characteristics of the large events are dependent on their occurrence times and hypocentral locations.
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.
SEG Technical Program Expanded Abstracts 2008, 2008
Microseismic multiplets, which are groups of events showing 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. We then interpreted the behavior of each multiplet cluster from the orientation and size of the multiplet seismic structure, fault plane solution, source radius, and hydraulic record. Multiplets which correlate to (a) identical shear slip on a macroscopic single fracture or thin fracture network, (b) repeating slip within a microscopic fracture, and (c) extension of rupture zone on a microscopic fracture, are identified and their response to the stimulation was also interpreted.
Reservoir structure delineation by microseismic multiplet analysis at Basel, Switzerland, 2006
SEG Technical Program Expanded Abstracts 2007, 2007
In order to develop an enhanced geothermal reservoir as part of the Deep Heat Mining project at Basel (Switzerland) a hydraulic stimulation program was conducted in deep geothermal well Basel 1 during December 2006. This EGS project is financed by Geopower Basel AG. The stimulation was operated and monitored by Geothermal Explorers Ltd for microseismic activity; more than 13,000 microseismic events were observed during the stimulation and afterwards. Hypocenters of approximately 2,900 events were located onsite. During subsequent analysis, we analyzed microseismic multiplet events that exhibit similar waveforms to those among the located events. 70% of the located events comprise multiplets which may be assigned to over 100 distinct multiplet clusters. We estimated relative hypocenters for 1,635 of the multiplet events using a double differential hypocenter location technique. The relocated hypocenter distribution clearly delineates the reservoir structure to an accuracy of typically less than 10 m. Each multiplet cluster defines a planar or streak structure which has vertical inclination and strikes predominantly along a NW-SE direction; this is generally consistent with local tectonic stress analyses. Hypocenter distribution is uniform within the stimulated volume, suggesting that the reservoir structure lacks any dominant fluid flow path. The temporal and spatial pattern of multiplet response corresponds clearly to the time line and injection volumes of the pumping operation.
Microseismic multiplets, which are groups of events showing 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. We then interpreted the behavior of each multiplet cluster from the orientation and size of the multiplet seismic structure, fault plane solution, source radius, and hydraulic record. Multiplets which correlate to (a) identical shear slip on a macroscopic single fracture or thin fracture network, (b) repeating slip within a microscopic fracture, and (c) extension of rupture zone on a microscopic fracture, are identified and their response to the stimulation was also interpreted.
Application of microseismic multiplet analysis to the Basel geothermal reservoir stimulation events
Geophysical Prospecting, 2010
The stimulation of a geothermal well in Basel, Switzerland produced a distribution of microseismic event locations with an overall alignment in the direction of the maximum horizontal stress. Fault plane solutions of individual larger events indicated movements on fracture planes at an angle to the maximum horizontal stress that could not be reliably interpreted from the event locations. To obtain higher resolution images of the microseismic event locations, events with similar waveforms have been identified by multiplet analysis. A number of receivers were used in the multiplet processing to ensure each multiplet is represented by a unique group of waveforms. The location accuracy within each multiplet has been significantly improved using cross-correlation to refine the shear-wave traveltime picks. The distribution of events within each multiplet can be interpreted as being due to movements on a single fracture or a number of near parallel fractures. It is shown that whilst the overall distribution of events is around the direction of the maximum horizontal stress, the individual multiplets representing fracture planes have a variety of azimuths and dips.
Complexities in the analysis and interpretation of microseismic data
SEG Technical Program Expanded Abstracts 2007
Passive microseismic monitoring offers the promise of a reliable and valuable tool to aid in the diagnosis and control of a hydraulic fracture stimulation program. Timely processing, analysis, and interpretation of microseismic data allows for the adjustment of pumping parameters as the stimulation proceeds, with the aim of improving return on investment. While methods exist today for automated computation and event localization, care needs to be exercised in the quick interpretation of the traditional display, where each event location estimate, is plotted as a single point. We will show an example of data, and highlight some of the limitations of the point source display.
GEOPHYSICS
The forecasting and risk assessment of induced seismicity associated with fluid injection have considerable importance for subsurface energy development. We have developed a seismic evaluation method called the possible seismic moment (PoSeMo) model to assess the potential seismic moment that could be released in the future based on current seismic activity. The PoSeMo model assumes the existence of a representative parameter that can describe the seismic characteristics of a given field. This parameter is defined as the seismic moment density, which quantifies the seismic moment able to be released per rock volume. The rock volume presumed to be in critical condition because of stimulation is defined as the stimulated rock volume. The current stimulation condition for the PoSeMo model can be estimated from the product of these two parameters. The difference between the output of the PoSeMo model and the observed cumulative seismic moment corresponds to the cumulative seismic moment that could be released in the future. This value can be transformed into the possible maximum magnitude that has clear physical meaning and that can be used as feedback on the stimulation operation for seismic hazard assessment. We have applied this model to a microseismic data set from the Basel engineered geothermal system project. We have successfully estimated reasonable values for seismic moment density and stimulated rock volume. The PoSeMo model performed well, and it provided reasonable estimates of seismic moment. The maximum magnitude estimated by the PoSeMo model was almost identical to the largest event that had occurred previously. Thus, it was concluded that the PoSeMo model satisfactorily demonstrated its feasibility as a real-time seismic evaluation method, based on physical parameters derived from microseismic information.
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.