Wave Imaging Technology Inc.

Transcription

1 Applications of wave imaging technologies to improve onshore US prospecting Morgan Brown Pacific Coast Section SEG Luncheon September 22, 2010 Wave Imaging Technology Inc.

2 Talk Summary (45 min) WIT: Wave Equation Depth Imaging Why Depth Migration? Why Wave Equation? Case studies highlight three Wave Imaging technologies with impact: High-effort depth migration velocity estimation Reverse-time Migration (RTM) Attributes from WEM angle gathers

3 Imaging Technology Hierarchy NMO + stack Reverse-time Migration (RTM) 1970 s 2010 s V(z) Only, Flat Reflectors Complex V(x,y,z) + Overturned Salt Flank 1980 s 1990 s 2000 s Time migration (PSTM) Kirchhoff PSDM Wave equation PSDM (WEM) V(z) Only, Dipping Reflectors Nice V(x,y,z) Fault Shadow problem

4 Why PSDM? Why Wave Equation? Kirchhoff PSDM handles simple refraction. WEM also handles complex focusing. RTM also images overturned beds. Air Simple refraction Water Complex focusing

5 Why PSDM? Practically Speaking Better faults Even shallow Sharper fault truncations Fault plane reflections (especially with RTM) Better steep dips Improved focusing Improved positioning RTM can image very steep

6

7 7 Everything Depends on V(x,y,z)! PSDM got a bum rap (until recently): (Theory) PSDM should always beat PSTM (Practice) PSTM often won Salvation: compute power, volume-based update Depth velocity analysis is iterative Constrained volume-based vs. model-driven solutions WIT: two-phase velocity update WEM Focusing Analysis (MVFA) Robust WEM Angle Gather Update Accurate

8 Checkshot vs. Seismic Velocity X Z(ft) Wilcox WEM Focusing Analysis + Angle Gather Update Velocity (ft/sec) LA Gulf Coast Well 10 miles away

9 Velocity Model has Interpretive Value X Y Z(ft) Top Wilcox Top Wilcox WEM Focusing Analysis + Angle Gather Update Data courtesy ECHO Geophysical

10 Shot Record Migration with Correct Velocity Source wavefield Receiver wavefield x x x Imaging Condition

11 Shot Record Migration with Too-fast Velocity x x x Focusing Analysis If we knew Dt, we could estimate velocity error

12 Z (m) WEM Focusing Analysis t (sec) Time-shift gathers Phase 1 of 2 Relate bestfocusing t to Dv Every shot point Slow Down Good Speed Up Slow Down Good Speed Up Robust to: Large velocity errors Low fold Good for land data Correct Migration Velocity Too-fast Migration Velocity

13 Angle Decomposition for WEM y Compute propagation direction vectors for source and receiver wavefields x Incidence angle, dip angle, and azimuth angle from two vectors z Define angle bins, put image energy at (x,y,z) into correct bin Source wavefield Receiver wavefield

14 14 WEM Angle Gather Velocity Update Incidence angle gathers q x Dv x Phase 2 of 2 z (ft) z (ft) Velocity estimation: Curving up: velocity too slow Curving down: too fast Automatic picking of large angle gather volumes Update velocity at every image point Data courtesy ECHO Geophysical Angle gather target line Residual Velocity panel

15 South Texas Not a typical fault shadow problem lots of little fault shadows Look below velocity anomalies for: Improved event geometry (remove time sags ) Improved event focusing Improved fault resolution PSTM works well here We need PSDM to be as good/better at all locations

16 16 South Texas PSTM PSDM with velocity overlay x x t z Data courtesy ECHO Geophysical

17 17 South Texas PSTM PSTM WIT PSDM converted to time, PSDM overlain with interval velocity Improved event and fault focusing under velocity anomaly Line XLine t (ms) Data courtesy ECHO Geophysical

18 Paradox Basin Thick salt layer = low velocity anomaly Tectonics warps salt, creates velocity lensing WEM + accurate velocity analysis: Better fault imaging Better steep dip imaging

19 Paradox Basin Depth migration velocity overlaying final WEM image Improved steep dip/fault imaging X Y Z(ft) Mostly V(z) salt V(x,y,z) salt Data courtesy Whiting Petroleum

20 Paradox Basin X Y PSDM X Y PSTM Z T Data courtesy Whiting Petroleum

21 Paradox Basin X Y PSDM X Y PSTM Z T Data courtesy Whiting Petroleum

22 Wyoming Monoclinal, hard-rock beds = lateral velocity variation enough to break PSTM WEM + accurate velocity analysis: Better fault imaging Better steep dip imaging

23 Wyoming Migration velocity overlaying final PSDM image Lateral velocity variation is subtle, but sufficient to harm time imaging X Y Z Data courtesy Nadel & Gussman, Rockies

24 Wyoming X PSDM Inline PSTM Inline Z Data courtesy Nadel & Gussman, Rockies

25 Wyoming Y PSDM Xline PSTM Xline Z Data courtesy Nadel & Gussman, Rockies

26

27 What is RTM? RTM = Reverse-time migration, or two-way wave equation depth migration WEM Kirchhoff PSDM RTM Naturally handles complex velocity focusing Yes No Yes Can image steep (>70 o ) dips No Yes Yes Accurate amplitude out of the box Yes No Yes RTM: the best of Kirchhoff and WEM Downsides: More expensive, a bit noisy

28 RTM Tutorial This animation shows a wave propagating from the surface, overturning, and reflecting from an inverted salt flank. The time taken to propagate from source to target is t s ; from target to receiver is t g. t=0 t g t s t s t s +t g t max

29 RTM Tutorial Next, we flip the trace in time (hence the name reverse time migration ) and use the flipped trace as a source function for modeling. The recorded event is injected into the earth at time t max t s - t g. It reaches the salt interface at time t g and propagates for a further time t s before reaching the maximum time. t max t g t s t max - t s - t g One way WEM propagators can t propagate past here (~ 70 o )

30 RTM Tutorial Next we propagate a synthetic source function into the earth. We also back propagate the receiver wavefield in time. At each time step, we multiply the source and receiver wavefields to form an image. Here is the key to RTM: both the source and receiver wavefields are t s seconds from the salt face. We automatically form an image! t s t s X at each time to form image

31 Florida RTM trace RTM WEM Z (m) Data courtesy Spectrum Geo

32 32 Wyoming RTM RTM WEM x y x y z z Data courtesy Nadel & Gussman, Rockies

33

34 AVO/Fracture attributes in complex geology offset Simple earth Complex earth Surface azimuth f f q q Reflection azimuth AVO: In a complex earth, surface offset is no longer a good proxy for incidence angle q at the reflector Azimuthal Fracture analysis: surface azimuth f is not a good proxy for reflection azimuth f in the presence of lateral velocity variation or 3D dip.

35 Improved AVO/Fracture attributes WEM Angle Gathers Measure incidence angle or azimuth angle at the reflector, not at the surface More accurate AVA, more accurate fracture characterization Highly efficient algorithm

36 Attributes from Angle Gathers 5 BCFE well, best in survey No obvious amplitude anomaly at well location Pseudo-Poisson s Ratio Reflectivity Attribute Derived from WEM angle gathers Conforms to Faults, 100x standout over background x y z Data courtesy ECHO Geophysical

37 Attributes from Angle Gathers Here we compare a depth slice through the fluid factor volume with a map of a productive fault block. Note a positive correlation of anomalously high fluid factor (indicating gas) and production. Unfortunately, there is no seismic coverage over a cluster of production. Missing Data Dots indicate wells, numbers indicate cumulative gas/condensate production (BCFE) XLine Missing Data Line Data courtesy ECHO Geophysical

38 Depth Azimuthal fracture anisotropy Azimuth Angle x 0 o 90 o 180 o y 90 o is fast direction, strong azimuthal effect 90 o is slow direction, weak azimuthal effect

39 39 Wyoming: Azimuth Angle WEM Azimuth angle (deg) x slow y fast z Consistent with vertical fractures opening in the strike direction due to flexure of the anticline slow fast Data courtesy Nadel & Gussman, Rockies

40 40 Wyoming: Azimuth Angle WEM Azimuth angle (deg) x fast y slow z slow Away from the anticline, fractures have the opposite orientation. This may indicate the regional stress field fast Data courtesy Nadel & Gussman, Rockies

41 41 Conclusions Intensive depth velocity estimation: the key to aligning PSDM theory and practice Reduced exploration risk from PSDM: More accurate reflector position/attitude Improved fault resolution Improved event focusing Drill in depth, see in depth RTM: Best of Kirchhoff and WEM WEM angle gathers: more accurate attributes

42 42 Acknowledgements Whiting Petroleum (Larry Rasmussen, Pat Winkler, Scott Haberman) Nadel & Gussman, Rockies (Rick Morris, Greg Chapel, Lee Robinson) ECHO Geophysical Spectrum Geo The WIT team: Joe Higginbotham, Cosmin Macesanu, Jo Ottaviano, Oscar Ramirez Bob Clapp (Stanford) Doug Robinson