High-resolution analysis of microseismicity related to hydraulic stimulations in the Berlín Geothermal Field, El Salvador

Transcription

1 High-resolution analysis of microseismicity related to hydraulic stimulations in the Berlín Geothermal Field, El Salvador Grzegorz Kwiatek 1, Fatih Bulut 1 Marco Bohnhoff 1, Georg Dresen 1 1) GFZ Potsdam, Section 3.2: Geomechanics and Rheology

2 Motivation Evaluation of systematic relations between the seismic activity and fluid injection Characterization of fluid-induced seismicity and larger magnitude events - occurrence in time and space, source processes.

3 Berlín Geothermal field Located on the flancs of dormant volcano Cerro Tecapa 8 production wells + 10 injection wells (closed fluid circulation) 1km Rivas et al., 2000; Oates et al. 2004

4 Stimulation and passive seismic monitoring 3 stimulations campaings (each lasting ~3 weeks) performed at two depth intervals in TR8A well to enhance reservoir to the north. 12 Borehole sensors, Natural frequencies: 1, 4.5, 5.5 and 30Hz, 24 sampling frequencies (3000Hz-24Hz), triggered mode, seismic events located assuming isotropic velocity model S N SH max Injection well 1k m Rivas et al., 2000; Oates et al. 2004

5 Stimulation campaigns Not clear correlation between injection and seismic activity (hardly visible relations between injections and seismicity)??

6 Results of monitoring Whole dataset During injection into TR8A 581 events recorded in 16 months ( ) including 3 stimulations of TR8A Weak clustering, low location quality. Source parameters not displaying any particular pattern S N 1km Data from Oates et al., internal Shell report

7 Summary of the industrial monitoring Weak clustering possibly low location quality Hardly visible relations between injection rate, location, and seismicity Source parameters not displaying any particular pattern in response to injection Original seismic data need to be refined!!!

8 Double-difference relocation technique Groups of very similar waveforms observed - coming from the same spot? Double-difference relocation applied (Waldhauser & Ellsworth, 2000) with 1D velocity model: 200,000 cross correlations performed, 393 events relocated.

9 Original dataset before double-difference 1.Weak clustering observed 1km Wells Whole dataset During injection into TR8A

10 Relocated dataset 1.Shifts of locations into the geothermal field 2.Clustering around injection wells 1km Wells Whole dataset During injection into TR8A

11 Relocated dataset 1.Shifts of locations into the geothermal field 2.Clustering around injection wells Wells Whole dataset During injection into TR8A

12 Cluster analysis The measure of similarity between two events depends on interevent distance and average cross-correlation coefficient between waveforms available. Events in families/clusters display similar rupture process TR8A Cluster

13 Cluster analysis 9 families selected Events in families display similar rupture process We focus on family 1 related to injection into TR8A 1km

14 Source parameters using spectral fitting Inversion for moment, source radius and attenuation Ground velocity spectrum Ground velocity spectrum

15 Source parameters using spectral fitting Trade-off between source radius and attenuation resulted in unstable inversion

16 Spectral ratio refinement We applied modified spectral ratio technique (e.g. Imanishi and Ellsworth, 2006) Applied to clusters of data (similar location and travel paths from EQ to receiver) Propagation effects effectively supressed by forming spectral ratios ECGS Workshop 2012, Luxembourg, October 3-5, 2012

17 Spectral ratio refinement Strong improvements in the quality of source parameters (M 0, r 0, Ds) Initial spectral fitting method After spectral ratio refinement

18 Spatial and temporal behavior for stimulation 1 Migration of seismicity outside of injection wells over the fault plane F1 Migration stops immediately after the shut-in Shut-in resulted in an outburst of the larger magnitude events Well head pressure Injection rate SH max 250m

19 Spatial and temporal behavior for stimulation 2 Kaiser effect observed Weak migration outside of injection zone Leaping type of activity due to even higher injection rates and activation of another fault plane F2 (?) Well head pressure Injection rate 250m

20 Spatial and temporal behavior for stimulation 3 No migration Clustering around of injection well (or another fault plane?) Triggered activity on previous faults Another fault plane activated (F4?) Well head pressure Injection rate 250m

21 Static stress drop Static stress drop increases with the distance for events located on the fault plane for I1/I2 Similar feature observed in Basel (Goertz-Allmann et al., 2011) attibuted to the pore pressure perturbations

22 Results We refined locations of induced earthquakes using Double-Difference method. Clear spatial and temporal patterns detected in response to injection Seismicity accumulates in highly permeable zone over pre-existing fault planes We refined source parameters using spectral ratio method Huge improvement in quality of source parameters due to supressed propagation effects Seismicity seem to follow constant stress drop scaling Larger events after shut-its, decrease in magnitudes with time of injection, stronger events on edges of the enhanced reservoir Increase of static stress drop with distance

23 Conclusions Analysis could be possible after the refinement of the dataset Migration of seismicity along existing fault planes due to increasing pore pressure Clustering of seismicity after shut-in or when the injection rate is not high enough Larger magnitude events observed after shut-in possibly triggered by shear stress redistribution over the fault plane Seismicity occurrs in a certain area only if there is an increase in the load (well head pressure, injection rate) - Kaiser effect Static stress drop seem to reflect the pore pressure perturbations (?) Low strength of rocks and high permeability responsible for relatively low seismicity level in comparison to other geothermal sites

24 Thank you for your attention! We would like to acknowledge Julian Bommer, Steve Oates and Gunter Siddiqi for many valuable comments regarding data processing and injection operations performed in Berlin HFR site. We would like to thank Sabrina Andrae and Oliver Germer for improving the quality of the original catalog. Pedro Antonio Santos is acknowledged for providing additional data and comments that helped us to finalize the manuscript. We thank NORSAR (Volker Oye, Julie Albaric) for providing the dataset and comments regarding data preprocessing. We would like to thank Patricia Martinez-Garzon for providing translations of a few papers and the geological profile.