1. Time-Lapse Monitoring of CO2 Injection with Vertical Seismic Profiles (VSP) at the Frio Project T.M. Daley, L.R. Myer*, G.M. Hoversten and E.L. Majer all at Lawrence Berkeley National Lab. *Presenting Conference on Carbon Sequestration May 3, 2005

2. Outline VSP Background Survey Design and Data Acquisition
Data Analysis
Reflection processing
Time-lapse reflection analysis
Plume extent analysis
Conclusions

3. VSP Design and Acquisition Parameters
VSP designed for spatial monitoring of CO2 plume (100 m offsets) and site geologic structural information (500 – 1500 m offsets – not presented in this talk)
Explosive source for highest frequency (resolution) and off-road access.
Source: 3 lb. dynamite in 60 ft shothole, 4 to 10 shots per shot point site
Number of shot points recorded: 8
Maximum Well to Source Offset recorded about 1500 m
Receiver array depth: 0 ft – 5,500 ft, variable spacing 5 – 25 ft
Geophones: 3-component, clamped to casing, unique 80-level string provided with partial support from Paulsson Geophysics

4. VSP and Crosswell Site

5. VSP Explosive Source: Shot Hole Drill

6. VSP Shot Points: Analysis Focused on Site 1,2,4 – 100 m offset up dip
Wells
500 m 9 5 6 8 3 2 1 4 N

7. VSP allows separation of downgoing (direct) wave and upgoing (reflected) wave
Raw Data Site 1
Initial Ray Modeling
Depth
Direct P-Wave
Reflection
Denser spacing in
reservoir interval

8. Reflection Section Site 1
Frio Analysis Window
Enhanced Reflection
Entire VSP
Reflection Section
Frio

9. Processing for Time-Lapse Change
For time-lapse change: normalize reflection amplitude using a shallower reflector above the frio. This removes changes not associated with injection. For example: changes in shallow ground water level and changes in explosive shot energy.
Plot pre and post injection normalized reflection sections and calculate change in reflection amplitude.
Will show sites 1 (N), 2(NE) and 4(NW)

10. Site 1 (North, Up Dip) Reflection Section
Pre Injection
Post Injection
Control
Reflection
Two-way travel time
Frio
Reflection

11. Site 2 (NE) Reflection Section
Pre Injection
Post Injection
Control
Reflection
Two-way travel time
Frio
Reflection

12. Site 4 (NW) Reflection Section
Pre Injection
Post Injection
Control
Reflection
Two-way travel time
Frio
Reflection

13. Estimate Plume Edge
Calculate change in reflection strength for each recording depth.
Estimate reflection point offset for each recording depth. This estimate is straighforward for horizontal layers. However, the Frio site has strong dip and fault blocks.
Intial estimate uses constant 15 degree dip ray tracing results.
Future work will include improved reflection point imaging leading to more accurate estimate of plume edge.

14. Site 1 Reflection Change (Post – Pre)
Sensor Depth (m)
Two-way travel time
Frio
Reflection
Major change in Frio due to CO2 injection.
Smaller change below Frio probably due to transmission through Frio.

15. Time-Lapse Analysis
To estimate extent of CO2 plume, need to map sensor depth of recording to reflection point.
We use ray path modeling for initial estimate.

16. Mapping of sensor depth to reflection point using raypaths: Source sites 1,2,4 (100 m offset)
Injection
Well
Monitor
Well
Distance (m)
Sensor Depth (m)
Ray arrives at
sensor depth
1480 m
= 4860 ft.
Reflection point – 30 m from injection well

17. Site 1 (North): Estimated Plume Size
Range of CO2 induced
reflection amplitude change.
About 70% increase
in peak reflection
amplitude. This is
a strong response.
What is cut-off point
For plume edge?
Choose 60%
Estimated Plume Edge
= 85 m
Amplitude Change (Post - Pre)/Pre
Estimated Reflection Offset (m) not linear

18. Site 2 (NE): Estimated Plume Size
Range of CO2 induced
reflection amplitude change.
About 110% increase
in peak reflection
amplitude. This is
a strong response.
What is cut-off point
For plume edge?
Choose 60%
Estimated Plume Edge
= 45 m
Amplitude Change (Post - Pre)/Pre
Estimated Offset (m) not linear

19. Site 4 (NW): Estimated Plume Size
Range of CO2 induced
reflection amplitude change.
About 80% increase in
peak reflection amplitude.
What is cut-off point
For plume edge?
Choose 60%
Estimated Plume Edge
= 85 m
Amplitude Change (Post - Pre)/Pre
Estimated Offset (m) not linear

20. Current Estimate of Plume Extent survey of Nov 30, 2004
Approximate Plume Extent
from injection well
(85 m N and NW, 45 m NE) 1 4 2
500 m
Wells 3

21. Conclusions
The VSP method is able to ‘see’ the relatively small amount (~1600 tons) of CO2 injected in a thin reservoir at depths of 1500 m.
Time-lapse analysis allows estimate of the spatial extent of the CO2 plume; if the sources are placed correctly and the reflections can be correctly mapped.
At Frio, a strong increase in reflection amplitude is observed. Is this result to be expected for all saline aquifers?
The updip component of the plume is estimated to extend about 85 m, as of Nov 30, 2004.

22. Time-Lapse Monitoring of CO2 Injection using Cross Well Seismic Data at the Frio Project

23. Seismic Crosswell: Orbital Vibrator and P/GSI 80 level array 6-Component O.V. Receiver Gather at 4992' In-Line (left) -> P and SV; Cross-Line (right) -> SH
In-Line
Cross-Line
P-wave
S-Wave

24. Seismic Crosswell Difference Tomography (P and S)
Depth (ft)
Injection
Well
Monitor
Depth (ft)
Injection
Well
Monitor

25. Seismic P-wave and Pulsed Neutron Logs
Distance (ft)
CO2 Plume
G.L. Depth (ft)
Top of ‘C’ Sand
‘B’ Sand
Injection
Well
Monitor

26. End of Presentation