Crosswell waveform tomography in fine-layered sediments: Meeting the challenges of anisotropy R. G. Pratt1, L. Sirgue2, B. Hornby2, J. Wolfe3 1Queen’s University, Canada 2BP Americas, Inc 3BP North America Gas Presented at the 70th EAGE Conference & Exhibition, Rome, Italy (June 2008)

Outline Project description Traveltime tomography Waveform tomography Conclusions Introduction: Geology / Well logs, Survey parameters Traveltime tomography: Data quality and traveltime picks, Anisotropic velocity images Waveform tomography: Pre-processing and results, Verification and interpretation Conclusions:

Outline Project description Traveltime tomography Waveform tomography Conclusions Introduction: Geology / Well logs, Survey parameters Traveltime tomography: Data quality and traveltime picks, Anisotropic velocity images Waveform tomography: Pre-processing and results, Verification and interpretation Conclusions:

Project description Crosswell survey in Western Canada to image hydrocarbon reservoir Objective was the characterization of reservoir architecture Imaging of sandstone channels and potential shale barriers Data are challenging High noise levels, waveform complexity Significant velocity anisotropy

Geological Setting Western Canada, Rocky Mountains Upper Jurassic / Early Cretaceous Finely layered, horizontal beds Sandstones, siltstones, shales Sandstone channels Potential shale barriers Major unconformity within the section

Geological Setting Reservoir sand Receiver Well 160 m Source Well Relative Depths Reservoir sand Formation tops Unconformity Sonic Vp Gamma Ray Receiver Well 160 m Source Well

Survey geometry and survey specs Relative Depths 50 m Sonic Vp Smoothed velocities 150 m Nominal Well Separation 160 m Nominal Aperture +/- 150 m Sweep Frequencies 100 – 2000 Hz Wavelengths ≈ 40 m – 2 m Source/receiver spacing 1.5 m 250 m 350 m Receiver Well 160 m Source Well

Shot gather from 93 m (relative depth) 10 15 20 25 30 35 40 45 50 Time (ms) Relative depth 100 150 200 150 m 250 m 350 m

Shot gather from 286.5 m (relative depth) 10 15 20 25 30 35 40 45 50 Time (ms) 150 200 250 300 350 150 m Upgoing reflection 250 m 350 m

Common level gather (zero vertical offset) 150 200 250 300 350 100 50 10 15 20 25 30 35 40 45 Time (ms) 50 m Average log spectrum of gather 150 m Upgoing reflection 250 m 350 m Relative depth

Outline Project description Traveltime tomography Waveform tomography Conclusions Introduction: Geology / Well logs, Survey parameters Traveltime tomography: Data quality and traveltime picks, Anisotropic velocity images Waveform tomography: Pre-processing and results, Verification and interpretation Conclusions:

Traveltime repick map Receiver depth Traveltime (ms) Source depth 50 m 100 150 200 250 300 350 Source depth 26 50 m 150 m 250 m 350 m 50 28 100 30 150 200 32 250 34 300 36 350 38

Receiver Well 160 m Source Well Traveltime Tomography: Velocity model (from Anisotropic Tomography, Pratt and Chapman 1992) Relative Depths 50 m 150 m 250 m 350 m Sonic Vp Vertical Vp Horizontal Vp Vertical Vp Horizontal Vp Receiver Well 160 m Source Well

Receiver Well 160 m Source Well Traveltime Tomography: Anisotropy model (from Anisotropic Tomography, Pratt and Chapman 1992) Relative Depths 50 m 150 m 250 m 350 m Gamma Ray Thomsen Thomsen Receiver Well 160 m Source Well

Traveltime Tomography: Traveltime residuals Receiver depth Residual traveltime (ms) 50 100 150 200 250 300 350 Source depth 50 m 150 m 250 m 350 m 50 100 150 200 250 300 350

Outline Project description Traveltime tomography Waveform tomography Conclusions Introduction: Geology / Well logs, Survey parameters Traveltime tomography: Data quality and traveltime picks, Anisotropic velocity images Waveform tomography: Pre-processing and results, Verification and interpretation Conclusions:

Waveform tomography Anisotropic starting model Waveform preprocessing Waveform tomography results Introduction: Geology / Well logs, Survey parameters Traveltime tomography: Data quality and traveltime picks, Anisotropic velocity images Waveform tomography: Pre-processing and results, Verification and interpretation Conclusions:

Waveform tomography Anisotropic starting model Waveform preprocessing Waveform tomography results Introduction: Geology / Well logs, Survey parameters Traveltime tomography: Data quality and traveltime picks, Anisotropic velocity images Waveform tomography: Pre-processing and results, Verification and interpretation Conclusions:

Traveltime Tomography: Anisotropy Relative Depths 50 m 150 m 250 m 350 m Gamma Ray Extract smoothed, average 1D vertical profiles for each parameter from these images … Thomsen Thomsen Receiver Well 160 m Source Well

Anisotropy model: Averaged, smoothed and model Relative Depths Relative Depths Thomsen Thomsen 50 m 150 m 250 m 350 m 50 m 150 m 250 m 350 m Gamma Ray Extract “best” elliptical parameter from each depth level …

Traveltime Tomography: Thomsen parameters ( in this example < ) Angle from vertical True anistropy is an-elliptical Current Waveform tomography . is limited to elliptical VTI = is a poor choice Find best fitting value of . over given angle range Some residual error remains Normalized group velocity Velocity vs angle This process is repeated to obtain an appropriate anisotropy for each depth level

WaveformTomography: Anisotropic starting model Relative Depths Horizontal velocity from tomography, vertical velocity from best fit parameter 50 m 150 m 250 m 350 m Sonic Vp Horizontal Vp from traveltimes Vertical Vp Waveform tomography uses the fixed elliptical VTI anisotropy from this model Receiver Well 160 m Source Well

Data waveforms: Synthetic shot gathers in starting model (Source wavelet extracted from data) 50 100 150 200 50 m 150 m 150 250 m 200 250 300 350 m 350 10 15 20 25 30 35 40 45 50 Time (ms)

Data waveforms: Raw shot gathers 50 100 150 200 50 m 150 m 150 250 m 200 Upgoing reflection 250 300 350 m 350 10 15 20 25 30 35 40 45 50 Time (ms)

Waveform tomography Anisotropic starting model Waveform preprocessing Waveform tomography results Introduction: Geology / Well logs, Survey parameters Traveltime tomography: Data quality and traveltime picks, Anisotropic velocity images Waveform tomography: Pre-processing and results, Verification and interpretation Conclusions:

Data waveforms: Raw shot gathers 50 100 150 200 50 m 150 m 150 250 m 200 250 300 350 m 350 10 15 20 25 30 35 40 45 50 Time (ms)

Data waveforms: Shot gathers following tube wave removal 50 100 150 50 m 150 m 150 250 m 200 250 300 350 m 350 10 15 20 25 30 35 40 45 50 Time (ms)

Data waveforms: Shot gathers following edit and 10 ms time window 50 100 150 50 m 150 m 150 250 m 200 250 300 350 m 350 10 15 20 25 30 35 40 45 50 Time (ms)

Frequency domain input files: Format is similar to traveltime map Receiver depth 50 100 150 200 250 300 350 Source depth Traveltime map 50 m 150 m 250 m 350 m 50 100 150 200 250 300 350

Frequency domain input files Receiver depth 50 100 150 200 250 300 350 Source depth Traveltime residual map 500 Hz data for inversion 50 m 150 m 250 m 350 m 50 100 150 200 250 300 350

Frequency domain input files 250 Hz 500 Hz 750 Hz 1000 Hz Similar files are created for all frequencies from 250 Hz to 1600 Hz at 25 Hz intervals

Waveform tomography Anisotropic starting model Waveform preprocessing Waveform tomography results Introduction: Geology / Well logs, Survey parameters Traveltime tomography: Data quality and traveltime picks, Anisotropic velocity images Waveform tomography: Pre-processing and results, Verification and interpretation Conclusions:

WaveformTomography: Anisotropic starting model Relative Depths Waveform tomography uses the fixed elliptical VTI anisotropy 50 m 150 m 250 m 350 m Sonic Vp Horizontal Vp from traveltimes Vertical Vp Receiver Well 160 m Source Well

WaveformTomography: 800 Hz Relative Depths Waveform tomography uses the fixed elliptical VTI anisotropy 50 m 150 m 250 m 350 m Sonic Vp Horizontal Vp from traveltimes Horizontal Vp from waveforms Vertical Vp from waveforms Receiver Well 160 m Source Well

WaveformTomography: 1600 Hz Relative Depths 50 m 150 m 250 m 350 m Sonic Vp Horizontal Vp from traveltimes Horizontal Vp from waveforms Vertical Vp from waveforms Receiver Well 160 m Source Well

WaveformTomography: 1600 Hz Following f-k removal of artifacts Relative Depths 50 m 150 m 250 m 350 m Sonic Vp Horizontal Vp from traveltimes Horizontal Vp from waveforms Vertical Vp from waveforms Receiver Well 160 m Source Well

Waveform tomography QC Do the results match the borehole sonic logs? Do the results predict the true waveforms?

Waveform inversion: Data fit in the frequency domain 250 Hz 500 Hz 750 Hz 1000 Hz

Waveform inversion: Data fit in the time domain 50 100 150 50 m 150 m Shot gathers following tube wave removal 150 250 m 200 250 300 350 m 350 10 15 20 25 30 35 40 45 50 Time (ms)

Waveform inversion: Data fit in the time domain 50 100 150 50 m 150 m Predicted shot gathers 150 250 m 200 250 300 350 m 350 10 15 20 25 30 35 40 45 50 Time (ms)

Waveform tomography QC Do the results match the borehole sonic logs? Do the results predict the true waveforms?

WaveformTomography: Final results Relative Depths 50 m 150 m 250 m 350 m Reservoir sand Sonic Vp Horizontal Vp from traveltimes Unconformity Horizontal Vp from waveforms Vertical Vp from waveforms Receiver Well 160 m Source Well

Conclusions Objective was the characterization of reservoir architecture Imaging of sandstone channels and potential shale barriers Data are challenging High noise levels, waveform complexity Velocity anisotropy can be imaged by traveltime tomography Waveform tomography provides the key to structure Final images require post-processing to remove artifacts Subtle lateral velocity variations cannot be confirmed Points to a need for rigorous anisotropic waveform tomography

Acknowledgement The authors thank BP for permission to use these data and publish this paper