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Shear and tensile earthquakes: view of seismology and geomechanics

Shear and tensile earthquakes: view of seismology and geomechanics. Tomáš Fischer 1,2 , Alice Guest 3 , Václav Vavryčuk 1 (1) Institute of Geophysics, Academy of Sciences, Prague (2) Charles University in Prague, Faculty of Science (3) ESG, Calgary, Canada. Outline.

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Shear and tensile earthquakes: view of seismology and geomechanics

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  1. Shear and tensile earthquakes: view of seismology and geomechanics Tomáš Fischer1,2, Alice Guest3, Václav Vavryčuk1 (1) Institute of Geophysics, Academy of Sciences, Prague (2) Charles University in Prague, Faculty of Science (3) ESG, Calgary, Canada AIM meeting Bratislava 2010, Fischer, Guest, Vavryčuk

  2. Outline • Moment tensors of swarms and injection-induced earthquakes • Moment tensor of Tensile earthquake • Stress conditions for shear and tensile failure • Differential stress and depth • Application on data – Soultz-sous-Forets, Cotton Valley AIM meeting Bratislava 2010, Fischer, Guest, Vavryčuk

  3. Source-type plot (Hudson, 1989) MT eigenvalues (abs) M′1<M′2<M′3 AIM meeting Bratislava 2010, Fischer, Guest, Vavryčuk (1, 1, 1) k (1, 1, 3) (0, 0, 3) T (-1, -1, 2) (1, -1, 0)

  4. Moment tensors of injection-induced microearthquakes Soultz-sous-Forets geothermal field(Horálek et al., 2010) Cotton Valley, gas field (Šílený et al, 2008) pure DC various types Source-type plot (Hudson, 1989) AIM meeting Bratislava 2010, Fischer, Guest, Vavryčuk

  5. Hydraulic fracturing in sediments(Baig & Urbancic, 2010) co-injection post-injection opening dipoles closure dipoles AIM meeting Bratislava 2010, Fischer, Guest, Vavryčuk

  6. Injection in Coso geothermal field(Julian et al., 2010) pre-injection co-injection post-injection dipoles dipoles opening dipoles AIM meeting Bratislava 2010, Fischer, Guest, Vavryčuk

  7. Earthquake swarms in West Bohemia(Horálek et al., 2002; Horálek et al, submitted) 1997 swarmdipoles 2000 swarmpure DC AIM meeting Bratislava 2010, Fischer, Guest, Vavryčuk

  8. Motivation • Ambigous non-DC components of MT of high-pressure related earthquakes • Some are dipoles (opening, closure) – Coso geothermal field, gas fields, 1997 swarm in West Bohemia • Some are pure DC – Soultz geothermal field, 2000 swarm in West Bohemia => what is the role of high-pressurized fluid ? AIM meeting Bratislava 2010, Fischer, Guest, Vavryčuk

  9. Moment tensors -> geomechanics • DC component of MT –> shearing (Mode 2 and 3 fractures) • Volumetric component of MT–> explosion or opening crack (Mode 1 fracture) Fracture opening needs extensive normal stress => explore the conditions for tensile stress AIM meeting Bratislava 2010, Fischer, Guest, Vavryčuk

  10. Interpretation of non-DC components • Combined pure tensile and pure shear faulting (Julian et al., 1998) • Shear wing crack (Julian et al, GRL 2010)pure shear+pure tensile φ AIM meeting Bratislava 2010, Fischer, Guest, Vavryčuk

  11. Interpretation of non-DC components • Tensile earthquake (shear+opening – close to reality)(Dufumier & Rivera, 1997; Vavrycuk, 2001) α>0 45° α<0 Slip is deviated from the fault Moment tensor is non-DC (DC+CLVD+ISO) Normal stress is extensive 0° -45° AIM meeting Bratislava 2010, Fischer, Guest, Vavryčuk

  12. MT of Tensile earthquake • Poisson solid, λ=μ gives crack faulting (MT=[1 1 3]) • Dipole faulting (MT=[0 1 1])possible for λ/μ-->0 (fully compressible) • Pure volumetric for only λ/μ-->∞ ν: 0 .17 .25 .37 .49 Crack κ=λ/μ: 0 .5 1 3 100 Dipole α>0 α<0 1.41,Dipole 1.58 1.73, Crack 2.24 vP/vS 10 AIM meeting Bratislava 2010, Fischer, Guest, Vavryčuk

  13. Tensile earthquake • Simultaneous shear and opening • Explains MT of injection-induced earthqaukes • Dipole faulting requires small vP/vS (non-physical?) • What are the stress conditions for shear+opening? AIM meeting Bratislava 2010, Fischer, Guest, Vavryčuk

  14. Stress and fractures in Mohr diagram σn • Stress state τ • Fracture σ3 σ1 • Traction on a fracture 2θ AIM meeting Bratislava 2010, Fischer, Guest, Vavryčuk

  15. Failure envelope • nonlinear for small and negative normal stress • linear for larger positive normal stress sandstone (Zoback, 2007) AIM meeting Bratislava 2010, Fischer, Guest, Vavryčuk

  16. Failure envelope • real failure envelope is non-linear • linearized form τf = S0 + μi(σn – P) S0 AIM meeting Bratislava 2010, Fischer, Guest, Vavryčuk

  17. Rock failure, stress drop σ - p Failure: release of accumulated shear stress => stress drop Δσ (depends on the available τ) => decrease of differential stress σ1-σ3 τ stress drop p AIM meeting Bratislava 2010, Fischer, Guest, Vavryčuk

  18. Failure types 2θ • Tensile failure possible only for • small angle θ • small diferential stress σ1-σ3 AIM meeting Bratislava 2010, Fischer, Guest, Vavryčuk

  19. Failure stresses and fracture orientation • Griffith‘s failure envelope: • gives the following relations between • tractions σ1 and σ3 • mean and differential stress • fracture orientation θ AIM meeting Bratislava 2010, Fischer, Guest, Vavryčuk

  20. Failure stresses and fracture orientation σ1-σ3 (σ1-σ3)MAX ≈ 2.8S0 τf θMAX=22.5° σn • Tensile failure occurs only • If diff. stress is small • On fractures striking less than 22.5° to σ1 AIM meeting Bratislava 2010, Fischer, Guest, Vavryčuk

  21. Differential stress σ1-σ3 (in depth) Tensile earthquakes: • (σ1-σ3)MAX ≈ 2.8S0 • Intact sandstone:S0≈20 MPa => (σ1-σ3)MAX ≈56 MPa • Pre-existing fractures:S0≈0..4 MPa => (σ1-σ3)MAX <10 MPa • σ1-σ3 increases with depth=> depth limit of tensile earthquakes? (Zoback, 2007) NO LIMIT of depth in overpressurized formations !! AIM meeting Bratislava 2010, Fischer, Guest, Vavryčuk

  22. Small differential stress σ1-σ3 in overpressurized formation How it happened? • sealed formations - pressure buildup in geological past • step-wise release of stress on optimally oriented fractures (Zoback, 2007) AIM meeting Bratislava 2010, Fischer, Guest, Vavryčuk

  23. Scenario Fluid injection in • Hydrostatic conditions • tensile failure is limited to small depths only on fractures trending <22.5°off SHmax • other fractures fail in shear mode • Overpressured formation • tensile failure is possible at any depth for fractures trending <22.5°off SHmax • other fractures fail in shear mode AIM meeting Bratislava 2010, Fischer, Guest, Vavryčuk

  24. Example 1: Soultz-sous-Foretsmacroscopic faults • Valley & Evans (2007): • hydrostatic conditions • σ1-σ3 = 54 MPa @ 4.7km depth (too much for tensile failure) • Tischner et al. (2007): map of event density for all stimulations Fractures trending 25° off SHmax & high diff. stress => shear stimulation Agrees with pure DC events of Horálek et al. (2010) AIM meeting Bratislava 2010, Fischer, Guest, Vavryčuk

  25. Example 1: Soultz-sous-Foretsfocal mechanisms • Horálek et al. (2010) • 45 full MT – dip slip/strike slip • Valley & Evans (2007) • 0.90·Sv ≤ SHmax ≤ 1.05·Sv • SHmax 169°; unconstrained plunge • Cuenot et al. (2005) • stable subhorizontal σ3, NE-SW => • choose σ3 of 250°/10° • get θ between σ3 and fault normal θ ≈ 25° σ3 AIM meeting Bratislava 2010, Fischer, Guest, Vavryčuk

  26. Example 1: Soultz-sous-Foretsstress and fault failure Stress @ 4.7 km depth (Valley & Evans, 2007): • Pformation= 47 MPa (hydrostatic) • SHmin = 64 MPa • SV = 118 MPa • SHmax = 0.9 .. 1.05 Sv • Pnet= 15 MPa (Tischner et al, 2007) => σmeaneff = 29 MPa σdiff = 54 MPa (? smaller near fractures?) μ=0.8 (Valley 7 Evans, 2007) • => shear faulting, because • σ3 = 2 MPa (positive) • σdiff too large • θ ≈ 25° too large Shear faulting agrees with pure-DC events of Horálek et al. (GJI 2010) AIM meeting Bratislava 2010, Fischer, Guest, Vavryčuk

  27. Example 2: Cotton Valley injectionfocal mechanisms • Sandstone formation with many natural fractures, shale interbeds • Gas reservoir => probably overpressured • Rutledge et al. (2004): • narrow bands of seismicity along vertical fractures trending close SHmax • fault plane solutions showing both left- and right-lateral strike-slips Alternating opposite slip on sub-parallel fractures (<10°difference in strike) => small shear stress => negative normal stress? AIM meeting Bratislava 2010, Fischer, Guest, Vavryčuk

  28. Example 2: Cotton Valley injectionstress and fault failure • Opposite shears on faults striking within ±10° possible only if σn<0 • Assuming cohesion S0=2 MPa • |τf|<0.7 MPa => small stress drops • -1 MPa < σn < -0.97 MPa => Opposite shears in DC-constrained mechanisms prove that induced events had large non-DC component Proved by Šílený et al. (2008): full MTs of Cotton Valley events show up to 50% of non-DC AIM meeting Bratislava 2010, Fischer, Guest, Vavryčuk

  29. Cotton Valley x Soultz-sous-Foretsoverpressured x hydrostatic Stress drop releases the accumulated shear stress small τ => small Δσ !! AIM meeting Bratislava 2010, Fischer, Guest, Vavryčuk

  30. Conclusions • Tensile earthquake explains crack and dipole MTs and fits the stress components resolved • Non-linear failure envelope gives stress for non-DC events • Any fracture with σn<0 shows shear component, “tensile earthquake “ • Pure tensile event (τ=0) is very unlikely • Tensile eq. is possible only for fracs within small angles and small σ1-σ3 (θ<22.5°off SHmax and σ1-σ3 < 2.8 S0) • Small σ1-σ3 possible in small depths or in naturally overpressurized formations • Tensile earthquakes should have small stress drops • We explained the pure-DC events in Soultz-sous-Forets and non-DC events in Cotton Valley AIM meeting Bratislava 2010, Fischer, Guest, Vavryčuk

  31. Preexisting fractures AIM meeting Bratislava 2010, Fischer, Guest, Vavryčuk

  32. 1997 West-Bohemia swarm:two fracture systems? (Vavryčuk, 2001) AIM meeting Bratislava 2010, Fischer, Guest, Vavryčuk

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