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Institute of Geophysics, Academy of Sciences Prague, Czech Republic

Shear-tensile/implosion (STI) model - a prospective alternative to moment tensor in seismic detection of fracture mode during hydrofracture treatment of oil/gas wells. Jan Šílený. Institute of Geophysics, Academy of Sciences Prague, Czech Republic. Outline:.

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Institute of Geophysics, Academy of Sciences Prague, Czech Republic

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  1. Shear-tensile/implosion (STI) model - a prospective alternative to moment tensor in seismic detection of fracture mode during hydrofracture treatment of oil/gas wells Jan Šílený Institute of Geophysics, Academy of Sciences Prague,Czech Republic

  2. Outline: Motivation: hydrofracturing of oil/gas wells example of extreme data deficiency Inversion for MT: Cotton Valley MT summary: advantages, pitfalls MT alternative: STI (shear-tensile/implosion) reprocessing of Cotton Valley Conclusion

  3. Hydrofracturing of oil/gas wells monitoring well monitoring well treatment well

  4. Hydrofracturing of oil/gas wells Possibility to recover MT (6 components) far field: Vavryčuk (2007) 3 (or more) wells: OK from P, P+S rarely 2 wells: OK from P+S occasionally 1 well: Not enough ! standard • additional constraint needed Jechumtálová & Eisner(2008) • near-field needed Song& Toksoz(2011) • a simpler source model needed less parameters, e.g. shear +off-plane comp.

  5. Micro-earthquakes induced by hydrofracturing Cotton Valley gas field hydrofractureexperiment Rutledge et al. 2004

  6. Cotton Valley mechanisms Šílený, J., Eisner, L., Hill, D. & Cornet, F., 2009. Non-double-couple mechanisms of microearthquakes induced by hydraulic fracturing. J. Geophys. Res., 114, doi:10.1029/2008JB005987.

  7.  66% CLVD(T)  66% CLVD(P) + 33% ISO(impl) + 33% ISO(expl) Hydrofracturing of oil/gas wells Cotton Valley case study Far field P+S from 2 wells = even-determined inversion for MT No over-determination ! Viewpoint of fracture mechanics: MT is unnecessarily complex Question of particular interest: Was permeability of the reservoir increased? i.e, were tensile cracks created?  discriminate between shear and tensile modes of fracturing shear slip tensile crack crack closure  100% DC

  8. traditional intuitive explanation (opening crack with fluid influx) is incorrect ! tensile crack / cavity closure 66% CLVD(P) 66% CLVD(T) + 33% ISO(expl) + 33% ISO(impl) physical sources of interest are combinations of ISO and CLVD Prospects: MT … general dipole source advantage:linearity an alternative BUTtoo general to the MT  includes unphysical sources Traditional decomposition ISO+DC+CLVD motiv: • split isotropic and deviatoric part ISO DC+CLVD BUTthere is something • single out DC in addition, namely CLVD an unphysical source benefits:  shear – tensile / implosion source model • physical source Dufumier & Rivera 1997 Vavrycuk 2001,2011 • less parameters  advantage in inversion a + / disadvantage: • non-linearity slope (4 angles + magnitude) angle a shear slip + tensile crack / cavity closure

  9. Shear-tensile/implosion source model • DC angles dip, strike, rake Parameters: • slope angle a • magnitude (scalar moment) Retrieval of STI parameters: • via MT (DC,ISO,CLVD): through 6 parameters  vulnerability of ISO,CLVD • directly: 5 parameters  more robust  preferred in scarce configurations Inversion method: 2-step grid search • coarse grid global search • fine grid local refinement advantageous mapping of model space: possibility of other inversion options: at ‘each’ point (in terms of the sampling) information available on the goodness of fit: chi-square • different norm (L1) • adjustment of s possibility to construct a confidence region: subspace in the model space around estimated solution with a priori specified probability content

  10. Tensile/implosion part 0 Histogram of values of the slope angle a of the mechanisms with NRMS < NRMSthreshold Shear-tensile/implosion source model Plots of ‘confidence zones’ (optional) adjustment of s material property of fault zone Shear-slip part Source planes of the mechanism • fault plane Projection of T,P,N axes • plane normal to slip of the mechanisms • info on fault plane (ambiguous, but…) with NRMS < NRMSthreshold • estimate of uncertainty in the orientation of shear-slip part • estimate of uncertainty in the tensile/implosion part a positive  tensile fracturing negative  implosion

  11. slope angle tensile crack shear slip vertical strike-slip source lines 450 dip-slip source lines

  12. Synthetic testing of STI: two-well vs. single well monitoring Cotton Valley gas field hydrofractureexperiment Simulation of a single-well monitoring Source model: • vertical strike-slip • 450 dip-slip

  13. MT: noise, velocity mismodeling, mismodeling+mislocation T Inversion of data from two monitoring wells dip-slip + tensile (slope 50) strike-slip + tensile (slope 50) T model model P T P T T P P P P T T mismodeling mismodeling + mislocation mismodeling mismodeling + mislocation noise noise 10% 30% 10% 30% T T T T P P P P model model mismodeling mismodeling mismodeling + mislocation mismodeling + mislocation

  14. MT: velocity mismodeling+mislocation Inversion of data from two monitoring wells dip-slip + tensile (slope 50) strike-slip + tensile (slope 50) T P T model model P

  15. MT: velocity mismodeling+mislocation T Inversion of data from one monitoring well dip-slip + tensile (slope 50) strike-slip + tensile (slope 50) T P T model model P not existing STI: yes!

  16. T T P model P STI: lite velocity mismodeling dip-slip + tensile (slope 50) strike-slip + tensile (slope 50) model T T T T P P P P

  17. T model P STI: heavy velocity mismodeling + mislocation dip-slip + tensile (slope 50) strike-slip + tensile (slope 50) T P model P P P P T T T T

  18. STI: heavy velocity mismodeling + mislocation P P T T T P P P T T Poisson constant s l/m = 2s/(1-2s) fixed at s=0.25 free

  19. strike-slip + tensile (slope 50) T P STI: noise contamination

  20. dip-slip + tensile (slope 50) T P STI: noise contamination

  21. Re-processing of Cotton Valley: G1 STI two wells STI single well MT (two wells)

  22. Re-processing of Cotton Valley: G4 STI two wells STI single well MT (two wells)

  23. Re-processing of Cotton Valley: R1 STI two wells STI single well MT (two wells)

  24. Conclusions: MT: general description of a dipole source too general in case of a simple fracturing: but hydrofracturing in oil/geothermal industry: opening/closing of tensile cracks additionalconstraint helpful STI model less parametersthan MT (5 vs. 6) non-linear model  exploration  • advantage in estimate of modelspace of uncertainty of the solution • minimization in different norms • estimate of Poisson constant beneficial in deficient configurations in particular,single-well monitoring robust even with: • reasonable noise in data • realistic mislocation • slight velocity mismodeling

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