Tectonic Seismogenic Index of Geothermal Reservoirs C. Dinske 1, F. Wenzel 2 and S.A. Shapiro 1 1 Freie Universität Berlin 2 KIT Karlsruhe November 27, 2012
Introduction M max reservoir location -2.0 Barnett Shale -1.2 Cotton Valley 2.3 Soultz 2000 3.0 Basel 3.7 Cooper Basin 4.3 Paradox Valley
Introduction M max reservoir location -2.0 Barnett Shale -1.2 Cotton Valley 2.3 Soultz 2000 3.0 Basel 3.7 Cooper Basin 4.3 Paradox Valley What are the physical principles describing the magnitude distribution? What are the controlling factors of magnitudes of fluid-induced seismicity? Can we elaborate a model to evaluate the seismic hazard by fluid injections and to mitigate the occurrence of large (felt) events?
Introduction - Magnitude Distribution of Induced Events physical model: combination of Darcy law and continuity equation (principle of mass conservation) (nonlinear) diffusion of pore pressure perturbation event number N is proportional to the cumulative injected fluid mass m c : N m c V I geothermal reservoirs: Basel (M = 0.7) Soultz (M = 1.2) hydrocarbon reservoirs: Barnett Shale (M = 2.9) Cotton Valley (M = 2.1) magnitude-frequency distribution obeys a Gutenberg-Richter probability: log 10 N M = constant + log 10 V I b M
Introduction - the constant: Seismogenic Index Σ describes the seismotectonic state at an injection site combines four (generally unknown) site-specific parameters: ( ) ζ constant = a + log 10 Σ C max S fracture bulk concentration ζ, poroelastic modulus S, maximum critical pressures C max, a value of Gutenberg-Richter probability location (type) Basel (2006/EGS) 0.4 Cooper (2003/EGS) -0.9 Ogachi (1991/EGS) -2.6 Ogachi (1993/EGS) -3.2 Soultz (1993/EGS) -2.0 Soultz (1995/EGS) -3.8 Soultz (1996/EGS) -3.1 Soultz (2000/EGS) -0.5 KTB (1994/SCI) -1.65 KTB (2004-05/SCI) -4.2 Paradox (1997-2000/WD) -2.6 Unterhaching (HYD) -5.0 Barnett Shale (FRAC) -9.2 Cotton V. A (FRAC) -6.2 Cotton V. B (FRAC) -4.4 Cotton V. C (FRAC) -9.4 Canada, BC, A (FRAC) -2.9 Canada, BC, B (FRAC) -2.3 Canada, BC, C (FRAC) -3.5 Australia (FRAC) -8.0 Σ
Significance of Seismogenic Index expected number of events with a given minimum magnitude: N M = V I 10 Σ b M occurrence probability of an event with a given magnitude: P(0, M, V I ) = exp( V I 10 Σ bm ) implementation in real-time monitoring: Σ & b
Motivation How can we evaluate a potential seismic hazard prior to development / hydraulic stimulation of a reservoir?
Motivation How can we evaluate a potential seismic hazard prior to development / hydraulic stimulation of a reservoir? 1. collect a database based on observations from different types of reservoirs (different geological and tectonical settings), analysis of parameterization and correlations of seismogenic index) application to undeveloped reservoir sites 2. use tectonic seismic activity in order to evaluate the seismotectonic state of an area where fluid injections are planned seismogenic index for tectonic seismicity Σ tec
Seismogenic Index: Correlations 1 - CC 0.65 2 - CC 0.4 3 - CC 0.79 4 - CC 0.9 5 - CC 0.79 1. PGA (tectonic seismicity): GSHAP hazard map [Giardini et al., 2003] 2. depth of stimulated reservoir and induced seismicity 3. b value of the frequency magnitude distribution 4. cumulative seismic moment release 5. seismic energy normalised to hydraulic energy
Seismogenic Index: Continuing Work How can we evaluate a potential seismic hazard prior to development / hydraulic stimulation of a reservoir? 1. collect a database based on observations from different types of reservoirs, in different geological and tectonical settings (including analysis of parameterization and correlations of seismogenic index) application to undeveloped reservoir sites 2. use tectonic seismic activity in order to evaluate the seismotectonic state of an area where fluid injections are planned seismogenic index for tectonic seismicity Σ tec
Fluid-Induced Seismicity Tectonic Seismicity fractures and faults with bulk concentration ζ tec contained in a rock volume fractures are characterised by critical (shear stress) values C necessary for activation of shear slip continuous but slow perturbation of stress resulting from tectonic deformation an earthquake occurs along a fracture if shear stress exceeds local criticality value σ 3 seismogenic volume V tec with fault concentration ζ tec σ 1 σ 1 shear slip event at fracture location if accumulated shear stress τ will be larger than critical value C σ 3
Tectonic Seismogenic Index event rate R is proportional to the shear stress rate τ = G ǫ in a given seismogenic volume V tec : R = Gζ tec C maxtec ǫv tec G: shear modulus ζ tec: bulk concentration of pre-existing fractures 1/C maxtec : probability density function of critical shear stresses event rate R Mtec with magnitude larger than M: R M = Gζ tec C maxtec ǫv tec 10 a bm combine the unknown quantities to the seismogenic index Σ tec : ( ) Gζtec Σ tec a + log 10 = log 10 R M log 10 ( ǫ V tec ) + b M C maxtec
Work-flow: (I) Estimation of Seismicity Parameters 1. Search available earthquake databases 2. Define a seismogenic volume based on the spatial distribution of earthquakes and limiting accordingly the catalogue 3. Process the EQ catalogue: - Decluster in order to identify and remove aftershocks - Define a completeness level 4. Determine the b value from the frequency-magnitude distribution 5. Compute the annual probability of a given magnitude earthquake
Work-flow: (I) Estimation of Seismicity Parameters 1. Search available earthquake databases 2. Define a seismogenic volume based on the spatial distribution of earthquakes and limiting accordingly the catalogue 3. Process the EQ catalogue: - Decluster in order to identify and remove aftershocks - Define a completeness level 4. Determine the b value from the frequency-magnitude distribution 5. Compute the annual probability of a given magnitude earthquake
Work-flow: (I) Estimation of Seismicity Parameters 1. Search available earthquake databases 2. Define a seismogenic volume based on the spatial distribution of earthquakes and limiting accordingly the catalogue 3. Process the EQ catalogue: Decluster in order to identify and remove aftershocks - Define a completeness level 4. Determine the b value from the frequency-magnitude distribution 5. Compute the annual probability of a given magnitude earthquake
Work-flow: (I) Estimation of Seismicity Parameters 1. Search available earthquake databases 2. Define a seismogenic volume based on the spatial distribution of earthquakes and limiting accordingly the catalogue 3. Process the EQ catalogue: Decluster in order to identify and remove aftershocks Define a completeness level 4. Determine the b value from the frequency-magnitude distribution 5. Compute the annual probability of a given magnitude earthquake
Work-flow: (I) Estimation of Seismicity Parameters 1. Search available earthquake databases 2. Define a seismogenic volume based on the spatial distribution of earthquakes and limiting accordingly the catalogue 3. Process the EQ catalogue: Decluster in order to identify and remove aftershocks Define a completeness level 4. Determine the b value from the frequency-magnitude distribution 5. Compute the annual probability of a given magnitude earthquake
Work-flow: (I) Estimation of Seismicity Parameters 1. Search available earthquake databases 2. Define a seismogenic volume based on the spatial distribution of earthquakes and limiting accordingly the catalogue 3. Process the EQ catalogue: Decluster in order to identify and remove aftershocks Define a completeness level 4. Determine the b value from the frequency-magnitude distribution 5. Compute the annual probability of a given magnitude earthquake
Work-flow: (II) Estimation of Strain Rates deformation strain rates ǫ can be calculated from geodetic measurements (i.e. GPS, displacement) use published results: Basel & Soultz - [Tesauro et al., 2005] Ogachi - [Hasegawa et al., 2005] Paradox Valley - [Blume and Sheehan, 2003] and [Chang et al., 2006] seismic strain rates can be calculated from the seismic moment provided by the earthquake magnitudes [Kostrov, 1974; Molnar, 1983] use published results: Basel - [Delacou et al., 2004] application of the approach to reservoir locations in the Upper Rhine Graben seismicity parameters - [Burkhard and Grünthal, 2009] deformation strain rates - [Tesauro et al., 2005]
Tectonic Seismogenic Index Σ tec = log 10 R Mtec log 10 ( ǫ V tec ) + b M Location M Event b Strain Seismogenic Σ tec Σ inj Rate [ 1 yr ] Rate [ 1 yr ] Volume [m3 ] Basel 1 2.3 0.74 0.89 2 10 9 3.26 10 13-2.9 0.4 Basel 3 2.3 0.74 0.89 2.5 10 10 3.26 10 13-1.8 0.4 Soultz 2.3 1.33 0.81 2 10 9 1.63 10 14-3.5-3.8 to -0.5 Ogachi 1 5.5 0.02 0.9 1 10 7 9.3 10 13-4.0-3.2 to -2.7 Ogachi 2 5.5 0.028 0.78 1 10 7 9.3 10 13-4.4-3.2 to -2.7 Ogachi 3 5.0 0.011 0.9 1 10 7 9.3 10 13-4.2-3.2 to -2.7 Ogachi 4 5.0 0.017 0.85 1 10 7 9.3 10 13-4.3-3.2 to -2.7 Paradox V 1 3.5 0.1 0.81 1.3 10 8 3.89 10 14-4.9-2.6 Paradox V 2 3.5 0.1 0.81 5 10 9 3.89 10 14-4.5-2.6 Rhine Graben 1 2.3 0.89 0.86 2 10 9 6.33 10 13-3.3 Rhine Graben 2 2.3 0.89 0.86 1 10 9 6.33 10 13-3.0 up to two units difference
Comparison of Seismogenic Indices seismogenic index for tectonic seismicity is slightly smaller than seismogenic index for fluid-induced seismicity at a given location Σ inj Σ tec = ( ainj a tec ) ( ) ( ) ( ) ζinj Cmaxtec 1 + log 10 + log 10 + log 10 ζ tec C maxinj S G regional (averaged) versus local values of a, ζ and C max S 10 11 Pa 1 and G 3 10 10 Pa difference of 0.5 different stressing rates: stress changes due to fluid injection occur rather instantaneous compared to the slow tectonic loading seismic strain approximately 1/100 of tectonic strain [Kostrov, 1974]
Conclusions number of fluid-induced earthquakes increases proportional to the injected fluid mass site-specific, seismotectonic parameters control their frequency-magnitude statistics seismogenic index quantifies the seismotectonic state of a reservoir this index permits to compute the occurrence probability of a given magnitude event strategies to evaluate potential seismic hazard prior to reservoir development: correlations between seismogenic index and known parameters seismogenic index based on tectonic seismicity the tectonic seismogenic index can be used as a proxy of the seismogenic index for fluid-induced seismicity contributes to site selection and injection design
Acknowledgments Data are kindly provided by: Basel: M. Häring Cooper Basin: H. Kaieda Ogachi: H. Kaieda Soultz: A. Jupe (courtesy of G.E.I.E.) Paradox Valley: K. Mahrer Barnett Shale: S. Maxwell Cotton Valley: J. Rutledge Unterhaching: J. Wassermann and by sponsors of the PHASE research project. EQ catalog data were processed using ZMAP [Wiemer, 2001].
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