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Fuxian Song, M. Nafi Toksöz

Full-waveform approach for complete moment tensor inversion using downhole microseismic data during hydraulic fracturing. Fuxian Song, M. Nafi Toksöz. Earth Resources Laboratory, Department of Earth, Atmospheric and Planetary Sciences,

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Fuxian Song, M. Nafi Toksöz

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  1. Full-waveform approach for complete moment tensor inversion using downhole microseismic data during hydraulic fracturing Fuxian Song, M. NafiToksöz Earth Resources Laboratory, Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, 02139 Aug 15, 2011

  2. Outline 1. Objective 2. Introduction • Microseismic monitoring for hydraulic fracturing • Microseismic moment tensor • Downhole microseismic moment tensor inversion: previous work and challenges • Introduction to full waveform based moment tensor inversion and source estimation 3. Test with synthetic data • Condition number of the sensitivity matrix A • Unconstrained inversion of a non-double-couple source: near field • Constrained inversion of a non-double-couple source: far field 4. Field test 5. Conclusion

  3. Objective • Study the feasibility of inverting complete seismic moment tensor and stress regime from one single monitoring well by matching full waveforms • Fracture plane geometry, together with shear and volumetric components derived from complete moment tensors contain important information on fracturing dynamics. A better understanding of fracturing mechanism and growth eventually leads to a better hydraulic fracturing treatment.

  4. Conclusion Understanding the dynamics of fracture growth requires knowledge of complete moment tensors At near field (< 5 S-wavelengths), a complete moment tensor solution can be obtained from one well data without a priori constraints. At far field (> 5 S-wavelengths), a priori constraints are needed for complete moment tensor inversion using one well data. Twowells are sufficient to resolve complete moment tensors, even at far field. Initial field results show a dominant double-couple component inhydrofrac events, while a non-negligiblevolumetric component is also seen in some events.

  5. Microseismic monitoring for hydraulic fracturing Event locations to map fractures 2) Source studies to determine fracture orientation, size, rock failure mechanism and stress state

  6. Seismic moment tensor Complete moment tensor: 6 independent components of this symmetric matrix Ref: Stein & Wysession, 2003

  7. Previous studies and challenges X1(N) X1(N) (0,0,x’3) (0,0,x’3) X2(E) X2(E) (x1,0, x3) (x1,0, x3) Our approach: Full waveform: both near and far field 1D layered velocity model, multiple arrivals Goal: Invert for the complete moment tensor from single well data and estimate source parameters Assumption: Assume far field Assume homogeneous velocity model, use only direct P and S arrivals Limitation: Can not invert for M22 from single well data X3(D) X3(D) Ref: Vavrycuk, 2007; Baig & Urbancic, 2010 Ref: Song et al., 2010, 2011

  8. Full waveform based moment tensor inversion Multi-component microseismic data Preprocessing (noise filter) Pre-calculate Green’s function (for each said event location) Linear inversion to obtain the complete MT (for each said event location and origin time) Grid search over event location and origin time Determine the best MT (with the smallest fitting error) Evaluate the inverted MT (for source parameters) Ref: Song et al., 2011

  9. Methodology for source parameter estimation Inverted complete MT Full waveform inversion Diagonalize MT into Md Determine (strike, dip, rake) Md decomposition: Mdc, Mclvd, Miso Calculate seismic moment M0 and component percentages Analyze S-wave displacement spectrum Determine corner frequency fc and source radius r0 Source parameters (M0 , r0 , cISO, cDC, cCLVD, strike, dip, rake) Ref: Jost & Herrmann, 1989; Vavrycuk, 2001; Song et al., 2010, 2011

  10. Source receiver configuration: single well vs. multiple well Well azimuth: East of North, B1: 00, B2: 450 , etc. Sensitivity matrix A: elementary seismograms derived from Green’s function Condition number of matrix A: 1) Provides an upper bound on errors of the inverted moment tensor due to noise in the data; 2) The least resolvable MT element determined by the eigenvector of the smallest eigenvalue. Observed data: velocity data Complete moment tensor: 6 independent elements

  11. Influence of well coverage and mean source receiver distance • Condition number increases with increased source receiver distance  • Near field: waveforms sensitive to all 6 components;unconstrained inversion • Far field: waveforms not sensitive to M22, additional constraints needed;constrained inversion • Condition number doesn’t improve much when comparing 2 wells with 8 wells  • 2 wells sufficient to recover all 6 components • Condition number only increases slightly when using only horizontal components Ref: Song et al., 2011

  12. Unconstrained inversion of a non-double-couple source: near field Clean synthetic data: mean source-receiver distance: 60 ft (3.5 ) North component in red, East component in blue True moment tensor: [0.43 -0.72 0.78 -0.72 -0.37 0.02 0.78 0.02 0.39] (cDC, cISO, cCLVD): (74%, 11%, 15%) (Strike, Dip, Rake): (1080, 800, 430) 1D velocity model derived from field study Source time function: smooth ramp with f0 = 550 Hz Ref: Song et al., 2011

  13. Unconstrained inversion of a non-double-couple source: near field Contribution from near field Total wave-fields Near field terms only Average peak amplitude ratios (near-field terms/total wave-fields): 9%, 11%, 14%, 18%, 22% and 60% for geophones 1 to 6 Ref: Song et al., 2011

  14. Unconstrained inversion of a non-double-couple source: near field Input: an approximate velocity model (up to 2% random perturbation) and a mislocated source (up to 20 ft in each direction). 10% Gaussian noise. • a) waveform fitting: • North component • b) waveform fitting: • East component Ref: Song et al., 2011

  15. Unconstrained inversion of a non-double-couple source: near field Mean absolute errors: One well: CISO ~ 4%, CCLVD ~ 4%, CDC ~ 6%, M0~ 6%, strike ~ 10, dip ~ 20, rake ~ 10 Two wells: CISO ~ 3%, CCLVD ~ 3%, CDC ~ 4%, M0~ 4%, strike ~ 10, dip ~ 20, rake ~ 10 • At near field (<5 S wavelength), complete MTs are invertible using full waveforms from one well without constraints. Ref: Song et al., 2011

  16. Constrained inversion of a non-double-couple source: far field One well at 00 azimuth, mean source-receiver distance: 345 ft (20 ) • At far field, M22 is the least resolvable element • Invert for the rest 5 MT • elements and use a-priori information as constraints to determine M22 •  Constrained inversion! Bothnear field information and additional refracted/reflected rays from layered structure contributes to the decrease of the condition number Ref: Song et al., 2011

  17. Synthetic test • Constrained inversion of a non-double-couple source: far field Additional constraints: dip, strike uncertainty range +/- 150 around true values  The cyan strip! Maximize DC percentage within that strip  Green vertical line: M22! Ref: Song et al., 2011

  18. Constrained inversion of a non-double-couple source: far field Input: 10% Gaussian noise, up to 2% velocity model errors, up to 20 ft location errors in each direction; Constraints: known strike value Mean absolute errors: One well: CISO ~ 16%, CCLVD~13%, CDC ~ 13%, M0~ 11%, strike ~ 00, dip ~ 40, rake ~ 70 Two wells: CISO ~ 6%, CCLVD~13%, CDC ~ 13%, M0~ 7%, strike ~ 30, dip ~ 40, rake ~ 50 • At far field (> 5 S wavelength), by introducing a-priori constraints, • complete MTs are invertible using full waveforms from one well

  19. Field test: event horizontal view (Bossier gas play) Select high SNR waveforms from the lower 6 geophones (12835 ~12940 ft) : Average noise level ~ 7% 7 test events: Depth range: 13040 ~ 13100 ft Average distance from center receiver: 350 ft Only horizontal components used in inversion: noisy vertical component due to poor clamping Ref: Sharma et al., 2004

  20. Field test: constrained inversion Additional constraints: Dip range: 600 ~ 900 Strike range: +/- 600 around N870E or N-930E  The cyan strip! Maximize DC percentage within that strip  Green verticals: M22! Ref: Song et al., 2011; Warpinski et al. 2010

  21. Constraints (one well data): Strike range: +/- 600 around the average fracture trend Dip range: 600 ~ 900 Field test: full waveform fitting Modeled data in black, observed data in red: a) North component, b) East component • Good fit in both major P and S wave trains • Un-modeled wave packages probably due to un-modeled lateral heterogeneity

  22. Field test: corner frequency determined from S-wave Madariaga source model Ref: Madariaga, 1976

  23. Field test: source parameter estimates from constrained inversion Observations: Strike valuesare generally consistent with average fracture trend (N870E / N-930E) Double-couple component is dominant for most events, but for some events, the isotropic component is non-negligible. Event moment magnitude ranges from -4 to -2. Rupture area of these events are also small, only a few m2.

  24. Conclusion Understanding the dynamics of fracture growth requires knowledge of complete moment tensors At near field (< 5 S-wavelengths), a complete moment tensor solution can be obtained from one well data without a priori constraints. At far field (> 5 S-wavelengths), proper a priori constraints are needed for complete moment tensor inversion using one well data. Twowells are generally sufficientto resolve complete moment tensors, even at far field. Initial field results show a dominant double-couple component inhydrofrac events, while a non-negligiblevolumetric component is also seen in some events. Future work includes more field tests and some geo-mechanical modeling to understand the observed source mechanisms.

  25. Acknowledgement • Dr. Norm Warpinski, Dr. Jing Du, and Dr. QinggangMa from Pinnacle/Halliburton • Dr. Bill Rodi, Dr. Mike Fehler, and Dr. H. SadiKuleli from MIT

  26. Thanks for your attention! Questions or comments?

  27. -----Backup----

  28. Discussion: Open questions about dynamics of hydrofractures • Why a dominant double-couple component? Why hydrofracture propagates as shearing instead of tensile growth?  Griffith’s crack model to calculate stress distribution • What is the influence of pre-existing fractures on hydraulic fracture growth? • In the far field, does the hydraulic fracture propagate along the pre-existing fracture or along the maximum horizontal stress direction?

  29. Griffith’s 2D crack model: shear stress distribution >0, Shearing Overburden pressure: 89.8 MPa (1 psi/ft, 13040 ft), Fluid net pressure: 6.9 Mpa (1000 psi), Shear strength: 7.4 MPa, Tensile strength: 4.58 MPa Ref: Zhao et al., 2009

  30. Griffith’s 2D crack model: shear stress distribution >0, Shearing Overburden pressure: 89.8 MPa (1 psi/ft, 13040 ft), Fluid net pressure: 6.9 Mpa (1000 psi), Shear strength: 7.4 MPa, Tensile strength: 4.58 MPa Ref: Zhao et al., 2009

  31. Griffith’s 2D crack model: normal stress distribution <0, tensile >0, compressive Overburden pressure: 89.8 MPa (1 psi/ft, 13040 ft), Fluid net pressure: 6.9 Mpa (1000 psi), Shear strength: 7.4 MPa, Tensile strength: 4.58 MPa Ref: Zhao et al., 2009

  32. Griffith’s 2D crack model: normal stress distribution <0, tensile >0, compressive Overburden pressure: 89.8 MPa (1 psi/ft, 13040 ft), Fluid net pressure: 6.9 Mpa (1000 psi), Shear strength: 7.4 MPa, Tensile strength: 4.58 MPa Ref: Zhao et al., 2009

  33. Constrained inversion of a non-double-couple source: far field Comparison of mean absolute errors in the inverted source parameters from the one-well case and two-well case

  34. Field test: source parameter estimates from constrained inversion Observations: Strike estimatesare generally consistent with event trends (N870E or N-930E) Double-couple component is dominant for most events, but for some events, the isotropic component is non-negligible. Event moment magnitude ranges from -4 to -2. Rupture area of these events are also small, only a few m2.

  35. Unconstrained inversion of a non-double-couple source: near field Input: an approximate velocity model (up to 2% random perturbation) and a mislocated source (up to 20 ft in each direction). 10% Gaussian noise. Grid search range, space 15*15*11, origin time: 2 dominant periods, space: 5ft; origin time: 0.25 ms (Sampling frequency: 4KHz) • a) waveform fitting: • North component • b) waveform fitting: • East component Ref: Song et al., 2011

  36. Source studies from seismic moment tensor 1. Infer fracture size from event size: 2. Analyze rock failure mechanism: Moment tensor inversion of a single event Multiple event location 3. Determine induced fracture plane orientation: fracture strike, dip, rake 4. Estimate stress state: SHmin , SHmax , Ref: Finck, 2004

  37. Constrained inversion of a non-double-couple source: far field Input: 10% Gaussian noise, up to 2% velocity model errors, up to 20 ft location errors in each direction, Constraints: dip, strike range, +/- 150 around true value Mean absolute errors: One well: CISO ~ 23%, CCLVD~11%, CDC ~ 10%, M0~ 25%, strike ~ 120, dip ~ 90, rake ~ 90 Two wells: CISO ~ 6%, CCLVD~13%, CDC ~ 13%, M0~ 7%, strike ~ 30, dip ~ 40, rake ~ 50 • At far field (~ 20 S wavelength), by introducing a-priori constraints, • complete MTs are invertible using full waveforms from one well

  38. Velocity model and perturbation

  39. Field test: Bonner gas play in East Texas Ref: Griffin et al., 2003; Sharma et al., 2004

  40. Unconstrained inversion of a non-double-couple source: near field True moment tensor: [0.43 -0.72 0.78 -0.72 -0.37 0.02 0.78 0.02 0.39] (cDC, cISO, cCLVD ): (74%, 11%, 15%) (Strike, Dip, Rake): (1080, 800, 430) 1D velocity model derived from field study Source function: smooth ramp with f0 = 550 Hz Ref: Song et al., 2011

  41. Unconstrained inversion of a non-double-couple source: near field • a) After adding 10% Gaussian noise • Reference signal level: • maximum absolute • amplitude averaged • across receivers (max • over components) • b) After [200 900] Hz band-pass filtering Zooming factor: 30 Ref: Song et al., 2011

  42. Summary of estimated source parameters 1) Seismic moment, moment magnitude, isotropic component percentage and strike estimate 2) Source radius according to Madariaga ‘s source model

  43. Field test: full waveform fitting Test event 2: a) North component fitting, b) East component fitting

  44. Field test: full waveform fitting Test event 3: a) North component fitting, b) East component fitting

  45. Why M22 not invertible at far field? X1(N) X2(E) (0,0,x3) (0,x2,x3) Far field P-wave Far field S-wave X3(D) Ref: 1) Nolen-Hoeksema, & Ruff, Tectonophysics, 2001 2) Vavrycuk, Geophysical Prospecting 2007 3) Baig & Urbancic, The Leading Edge, 2010

  46. Statement Hydraulic fracturing has become an important process in the energy industry. Production of oil, natural gas from unconventional sources (tight sands, gas shales) and geothermal energy require hydraulic fracturing at some stage of their development. Even CO2 injection for geologic sequestration produces hydraulic fracturing. Understanding the dynamics of fracture initiation, propagation and growth in the earth is a challenging problem. Mechanisms of microearthquakes generated during fracturing contain important information for fracture dynamics. Analysis of observed events is essential for developing a better understanding of fracturing.

  47. Source studies from seismic moment tensor 1. Infer fracture size from event size: 2. Analyze rock failure mechanism: Ref: Finck, 2004

  48. Source studies from seismic moment tensor 3. Determine induced fracture plane orientation: fracture strike Moment tensor inversion of a single event Multiple event location 4. Estimate stress drop:

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