Characterization of Fracture Patterns in The Geysers Geothermal Reservoir by Shearwave Splitting

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Characterization of Fracture Patterns in The
Geysers Geothermal Reservoir by Shear-wave
Splitting

• J.A. Rial (P.I.)
• M. Elkibbi, M. Yang, G. Vlahovic, A. Gero
  
• Wave Propagation Lab
• UNC- Chapel Hill

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Introductory Remarks

• Characteristics of split shear-waves
• Anisotropy-inducing stress-aligned microcracks
  cause shear-waves to split into a fast and a slow
  component a phenomenon analogous to optical
  birefringence.
• The leading shear-wave is polarized parallel to
  the strike of the predominant fracture system,
  while the slow shear-wave is polarized
  perpendicular to it.
• The time delay between the arrivals of the fast
  and slow S-waves is proportional to crack density
  (number of cracks per unit volume).

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The Geysers seismicity
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Project Objectives

• Test the applicability and limitations
  (observability) of shear-wave splitting (SWS) in
  imaging anisotropic fractured reservoirs.
  Application to The Geysers and Coso.
• Detect potential directions of subsurface
  fluid-flow through stress-aligned cracks and
  regions of high crack density.
• Develop software for the processing, modeling,
  inversion, and interpretation of SWS.

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Methodology

• Data processing
• Forward Modeling
• Inversion

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Anisot X, a software package for SWS

• Anisot X is a multi-objective program to
  image fractured reservoirs using shear-wave
  splitting data.

PROCESSING
FORWARD
INVERSION

• Anisot X is Matlab-based
• with three operation modes
• PROCESSING
• FORWARD MODELING
• INVERSION

HELP
CONVENTIONS
EXIT
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Data Processing
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Modeling assumption

• Observed SWS is solely induced by
  crack-anisotropy in an otherwise isotropic
  medium, since the fabric of the reservoir rocks
  in both The Geysers and Coso lacks significant
  lithology-induced anisotropy.

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The forward problem
Theoretical polarizations
Theoretical time delays
Theoretical polarizations
Rose diagram
Equal-area plot
Equal-area plot
The forward problem is solved by simulating
propagation of a plane S-wave through a
transversely isotropic (TI) model.
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Inversion Scheme

• Both polarization and time delay observations are
  used to invert for the most likely crack geometry
  and fracture density in the subsurface.
• The method includes optimization-based and
  trial-and-error inversion schemes.
• Residual functions (RMS of observed
  calculated) for both polarizations and time
  delays are minimized in the least-squares sense.
• The starting model to simulate crack-induced
  anisotropy is HTI (horizontal transverse
  isotropy).
• Deviations from HTI are modeled as
  non-vertically dipping cracks or biplanar
  intersecting crack systems.

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Inversion Scheme
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Residual functions
strike55, dip81
complement crack dip
crack strike
Polarizations
Time delays
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Inversion Examples SE GeysersNW Geysers
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NW Geysers
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Models of Fracture Geometry and Crack
Densities
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SE Geysers
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Concluding remarks

• The observability test has been very successful.
  The SWS method has proven to be effective in
  imaging subsurface crack geometry and density in
  geothermal reservoirs.
• The inversion of SWS data provides a 3D view of
  the crack structure not obtainable by any other
  imaging method.
• Modeled crack geometries are in good to excellent
  agreement with
• drill core data, tracer tests, locally mapped
  fractures, and the regional tectonic setting,
  both in The Geysers and Coso.
• Models of fracture geometry and crack density can
  potentially reduce the risk of exploratory
  drilling and help increase geothermal
  productivity.

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Impact of work

• Coso Results dramatically corroborated by
  production engineers.
• A strong and important variation in crack
  orientation detected by measurements of SWS at
  station CE6 was confirmed by site drilling
  (Sheridan et al.,2003 S. Petty, personal comm.,
  2003).

CE6
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Impact of work (contd)
The UNC shear-wave splitting analysis
resulting from the 2002 injection studies at the
Coso east flank has added significant value to
the DOE-funded EGS program at Coso.The analysis
identified sets of fractures that became
conductive upon injection of cold condensate at
modest injection rates and pressures With
these results in hand, it would be very
interesting to conduct a similar shear-wave
splitting analysis in conjunction with the
massive-hydraulic-fracturing experiments planned
for the near future at Coso. Peter Rose,
PI, Coso EGS Project Geothermal Program
Coordinator Energy Geoscience Institute,
University of Utah
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Impact of work (contd)

• Upon a detailed presentation of our results,
  Icelands National Energy Authority (Orkustofnun)
  enthusiastically invited our research group to
  submit a proposal to map subsurface crack
  geometry and density in Hengill (Nesjavellir)
  and Krafla geothermal reservoirs.

Krafla
Nesjavellir
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Work supported by the U.S. Department of
Energy, Assistant Secretary Energy Efficiency and
Renewable Energy under DOE Idaho Operations
Office, Financial Assistance Award
DE--FG07-00ID13956.DISCLAIMER This
information was prepared as an account of work
sponsored by an agency of the U.S. Government.
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