From observations of microseismic source parameters to reservoir geomechanics B. Goertz-Allmann 1, V. Gischig, C. Bachmann, and S. Wiemer AGIS workshop, 27 th November 2012 1 now at
Presentation outline Induced seismicity at Basel Observed source property variations: stress drop and b-value Linking seismological observables to geomechanical reservoir properties Geomechanical forward-modeling of induced seismicity based on Coulomb friction Conclusions
Seismicity cloud Over 11 000 detected events ~3500 events located Data are recorded at 6 borehole stations between 0.5 km and 2.7 km depth Dyer et al. (2008)
Data Processing Source spectrum & stress drop 0 corner frequency f c A( f ) 1 ( f / 0 f c ) 2 Compute stress drop from fc
Data Processing Data processing Iterative least-squares approach to isolate source term e i, near-receiver term s j, and travel-time dependent term t k(i,j). Relative source spectra need to be corrected to absolute spectral shapes Event term e i Station term s j Distance term t k(i,j)
Lateral stress drop variations high stress drop low stress drop Goertz-Allmann et al., GRL May 2011
Lateral stress drop variations Two crosssections of all events within 100 m of a plane through the injection point
Distance dependence of stress drop: lower stress drop near injection point. Linear pore pressure diffusion in homogeneous and isotropic medium Linearized injection source-time function (red) (Dinske et al., 2010)
Stress drop versus pore pressure perturbation Mean show a linear correlation with pore pressure in the log-log domain Low correspond to high pore pressure Goertz-Allmann et al., GRL May 2011
3D b-value mapping log N = a - bm Mc b-value is the slope of the Gutenberg-Richter law Allows extrapolation from observed small events to expected larger events Varying magnitude of completeness Mc
b-value calculation Significant changes during and after injection for Basel
Three dimensional b-value distribution Distance dependence: Higher values close to injection point Lower values further out Bachmann et al., GRL (2012)
From source properties to the mechanics of induced seismicity b-value is inversely proportional to differential stress Both source properties have connection to stress state. Observed source property variations correlate with pore pressure Geomechanical forward-modeling to explain observations and infer reservoir mechanics Schorlemmer et al., (2005) normal Strike-slip thrust Amitrano (2003)
Forward-modeling of induced seismicity Temporal pore pressure evolution from linear diffusion
Forward-modeling of induced seismicity Temporal pore pressure evolution from linear diffusion Increase in P p reduces normal stress to effective stress Events are triggered when reaching the Mohr Coulomb failure criterion
Forward-modeling of induced seismicity Temporal pore pressure evolution from linear diffusion Increase in P p reduces normal stress to effective stress Events are triggered when reaching the Mohr Coulomb failure criterion Stress drop reduces differential stress - Mohr circle becomes stable again
Forward-modeling of induced seismicity Random seeds in 3D volume Initial stress state bound by Basel data (Häring et al., 2008) Pore pressure evolution triggers events
Forward-modeling of induced seismicity Random seeds in 3D volume Initial stress state bound by Basel data (Häring et al., 2008) Pore pressure evolution triggers events
Forward-modeling of induced seismicity Random seeds in 3D volume Initial stress state bound by Basel data (Häring et al., 2008) Pore pressure evolution triggers events
Forward-modeling of induced seismicity Random seeds in 3D volume Initial stress state bound by Basel data (Häring et al., 2008) Pore pressure evolution triggers events
Time-distance plot of simulated seismicity Goertz-Allmann and Wiemer, Geophysics (2013)
Event sizes of modeled seismicity Inverse relationship between σ D and the b-value Magnitude randomly drawn from a frequency-magnitude distribution with the respective b-value Assume stress drop as a percentage of σ D
Spatial variations of modeled source parameters stack stack constant b-value stress drop Higher b-values and lower stress drops at higher pore pressure perturbations Model result is consistent with observation Goertz-Allmann and Wiemer, Geophysics (2013)
Differential stress of induced seismicity Stack Goertz-Allmann and Wiemer, Geophysics (2013)
Time and location of large magnitude events Time dependence Shut-in Increased probability of a large event after the shut-in time Distance dependence ML 3.4 Increased probability of a large event at larger distances to the injection point Probability of an event exceeding a certain magnitude M p 1 e ( a Mb)
Time and location of large magnitude events Time dependence Shut-in Increased probability of a large event after the shut-in time Distance dependence ML 3.4 Increased probability of a large event at larger distances to the injection point Probability of an event exceeding a certain magnitude M p 1 e ( a Mb)
Conclusions Stress drop increases with radial distance B-value decreases with radial distance Correlation of both stress drop and b-value with estimated pore pressure perturbations Both observables are linked to reservoir stress state Geomechanical forward modeling can explain basic observation Our model results in higher probabilities of a large magnitude event after the shut-in time and at further distances to the injection point This opens the possibility for an advanced traffic-light system that can predict probabilities based on prior observation of the evolving seismicity cloud during stimulation.
Acknowledgements I thank: Geopower Basel AG for permission to publish Geothermal Exporers Ltd. for providing the data