A Strategy for Interpretation of Microearthquake Tomography Results in the Salton Sea Geothermal Fie

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A Strategy for Interpretation of Microearthquake
Tomography Results in the Salton Sea Geothermal
Field Based upon Rock Physics Interpretation of
State 2-14 Borehole Logs

• Brian Bonner
• Lawrence Hutchings
• Paul Kasameyer
• Lawrence Livermore National Laboratory
• September, 2006
• GRC, San Diego

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Geophysical Setting
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Microearthquake Data
CalEnergy Data
State 2-14 Borehole
Microearthquakes
Microearthquake stations
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Reservoir Recent Seismicity
Depth - km
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• Purpose of Project
• Utilize CalEnergy microearthquake data
• Analyze borehole data to identify strategies for
  interpretation of tomography results
• Develop interpretations that can be obtained from
  surface recordings
• Use rock physics interpretations
• Identify permeability and alteration in the
  reservoir
• Obtain higher resolution of geologic structure
• Improve resolution for drilling targets

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Typical 3-D Tomography Results
Develop strategies to interpret Tomography results
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State 2-14 Borehole Data
Typical of Inversion Results
Daley et al., 1988 Tarif et al., 1988
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• Rock Physics Observations
• Closing of cracks (filling) due to mechanical
  pressure and chemical alteration
• Increase of velocity with depth due to closing of
  small cracks
• Extreme temperature gradient works to decrease
  velocity with depth
• Variations in sediment properties with depth

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• Lithology
• Elders and Sass, 1988 Paillet, 1988
• Four Mineralogic Zones
• Near-surface unaltered sediments
  0 lt depth lt 1200 m, 100o190o C
• Illite zone 1200 lt depth lt 1900 m, 190o250o C
• Chlorite zone 1900 lt depth lt 2500 m, 250o300o C
• Feldspar zone 2500 lt depth lt 3220 m, gt300o C

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• Poissons Ratio
• Longitudinal strain divided by transverse strain
• Obtained by relationship between Vp and Vs
• Perfectly elastic material PR 0.5
• There is not a linear relation between Vp/Vs and
  PR

Poissons ration
Vp/Vs
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Poissons Ratio identify features
Daley et al., 1988
Monotonic decrease of PR due to compaction and
lithification of sediments
Permeable zone 915 m 960 m loss of
circulation Paillet et al., 1986
Hydrothermal alteration sulfite,
chlorite,epidote 1220 m Elders and Sass, 1986
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Interpretation of Permeable zone
Is the Poissons ratio anomaly at 900 m
indicative of a permeable zone?

• Loss of circulation 915 m 965 m
• (Paillet et al., 1986)
• Fractures at 915 m (Daley et al., 1988)
• Sandstone layer between 884 m and 962 m
• (McKibben and Andes, 1986)
• The entire area is saturated, so fracturing
  causes
• permeability
• We have a problem with the sandstone permeability

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Poissons Ratio Anomaly
We hypothesize that the permeable zone is evident
in Poissons ratio

• We observe a monotonic decrease in PR due to
  consolidation
• We hypothesize that the anomaly is due to a
  further decrease is PR due to mechanical effects
• Crack closure caused by an increase in effective
  pressure tends to increase PR and is a purely
  mechanical (Bonner and Schock, 1982)
• Therefore, crack opening due to fractures
  counters this and causes a decrease in PR, i.e.
  the anomaly

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Stable Background - monotonic decrease

• Laboratory measurements (Lin and Daily, 1988)
• Isolated effects of temperature and pressure on
  velocity
• Simulated depth range from 600 m to 1220 m
• Compared laboratory data with sonic log data
• At depths shallower than sample depth lab vel. lt
  sonic vel.
• At greater depths lab velocity gt sonic velocity
• Therefore, temperature and pressure alone are not
  causing the monotonic decrease in PR
• Chemical alteration is also contributing

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Conclusions

• There is evidence that Poissons ratio may be an
  indicator of permeability in the Salton Sea
  geothermal field
• High resolution hypocenter locations may also
  provide definition of permeable zones
• High resolution tomographic inversion results may
• provide supportive evidence for identifying
  permeability
• We hope these studies will improve likelihood of
  success in choosing drilling targets

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Acknowledgements

• Gail Wigget California Energy Commission
• Brian Berard - CalEnergy
• Dennis Kaspereit CalEnergy
• Tom Daley Lawrence Berkeley National Laboratory
• Bill Foxall Lawrence Livermore National
  Laboratory