Characterization of Geothermal Reservoir Conditions Using Electrical Surveys: Some Preliminary Resul

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Characterization of Geothermal Reservoir
Conditions Using Electrical Surveys Some
Preliminary Results

• Sabodh K. Garg, John W. Pritchett, and Jim Combs

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Motivation

• Based on a theoretical study, Pritchett concluded
  that electrical surveys (DC resistivity, MT, and
  SP) may be used to explore for hidden
  geothermal resources.
• Present work Use existing data sets for the
  operating BR reservoirs to test the utility of
  electrical surveys for characterizing the
  subsurface.

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Data Sets

• BR geothermal fields with electrical surveys in
  the public domain
• Beowawe
• Cove-Fort/Sulphurdale
• Dixie Valley
• Roosevelt Hot Springs
• Soda Lake
• Adequate Reservoir Information
• Dixie Valley
• Beowawe
• Roosevelt Hot Springs

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Dixie Valley Geothermal Field (1)

• Reservoir model
• McKenna and Blackwell (2004) 2D model
• Electrical Surveys
• Roving Dipole DC resistivity, Meidav (1975)
• Frequency-domain electromagnetic soundings,
  Wilt and Goldstein (1983)
• MT survey (3 lines transverse to Stillwater
  Range), Wannamaker (2003)

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Dixie Valley Geothermal Field (2)

• Wilt and Goldstein (1983), Wannamaker (2003)
  Data interpreted using a 3-layer resistivity
  model.
• - A moderately conductive (resistivity 10-25
  ohm-meter) shallow layer
• - A conductive middle layer (resistivity 2-5
  ohm-m)
• - Except in a narrow band paralleling the range
  front fault, a deep resistive layer

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Computational Grid (Reservoir)

• Origin of co-ordinates at the earth surface in
  the middle of a north-south oriented valley in
  the middle of two mountain ranges. The terrain is
  flat between x0 and x/- 4500m it then rises
  to 500m at x/- 5500m.
• The 2-D grid (reservoir) extends 15 km (x-7500 m
  to x7500 m) in the east-west direction. A single
  grid block (15 km thick) is used in the
  north-south (y) direction. The grid extends from
  500 m to -5000m in the vertical (z) direction.
• A uniform spacing of 500 m used in both the
  horizontal and vertical directions.

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Fluid Mass/Heat Transfer Electrical Grids
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Subsurface Structure Computational Flow Volume
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Fluid and Formation Properties

• Reservoir fluid treated as pure water with a
  tracer used to track dissolved solids
• Valley Fill 1, cap rock low vertical
  horizontal permeabilities
• Valley Fill 2, main aquifer, high horizontal
  permeability
• Valley Fill 3, low vertical horizontal
  permeabilities
• Fault Zone, high vertical permeability
• Bulk rock, low permeability crystalline rocks
• Formation resistivity modeled using Archies law

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Boundary Conditions

• Vertical boundaries no mass or heat flux
• Top boundary P 1 bar, T 20 oC, Tracer 100 ppm
  (incoming fluid).
• Heat loss simulated by an energy sink in the top
  layer of grid blocks.
• Bottom boundary
• Conductive heat flux 90 mW/m2
• Fluid influx 30 kg/s with an internal energy of
  1228kJ/kg, and Tracer 1000 ppm.

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Computed Stable Subsurface Temperature
Distribution
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Computed Stable Subsurface Tracer (ppm)
Distribution
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Computed Subsurface Resistivity Distribution
Below Valley Fill
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Computed Subsurface Resistivity Distribution VF3
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Computed Subsurface Resistivity Distribution VF2
(layers 8-9) and VF1(10)
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Computed Results for Electric Surveys

• DC Resistivity Survey
• North-south oriented Wenner Array
• Electrode spacing 500m, 1000m, and 2000m
• Depth of penetration increases with increasing
  electrode separation.
• MT Survey
• Frequency range 0.01 Hz to 10 Hz
• Depth of penetration is inversely proportional
  to the square root of frequency.

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Computed Results for Electric Surveys
Wenner DC Survey
MT Survey
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Conclusions (1)

• Study demonstrates that it is feasible to
  correlate observed electric signals with
  subsurface conditions.
• Archies law assumes that the formation
  resistivity is directly proportional to fluid
  resistivity, which in turn is a function of fluid
  temperature and salinity.
• By matching the subsurface resistivities, it may
  be possible to infer subsurface temperature and
  salinity distribution.

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Conclusions (2)

• Both the MT and DC Resistivity surveys provide no
  direct information about fluid transport within
  the geothermal reservoir.
• Surface SP measurements reflect fluid upflows and
  downflows.
• Work currently in progress on a 3-D study of the
  Beowawe geothermal field for which DC
  resistivity, MT, and SP surveys are available.

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Acknowledgment

• This work was sponsored by the U.S. Department
  of Energy under a contract between Idaho National
  Laboratory (INL) and Science Applications
  International Corporation (SAIC).