Use of Electrical Surveys for Geothermal Reservoir Characterization: Beowawe Geothermal Field

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Use of Electrical Surveys for Geothermal
Reservoir Characterization Beowawe Geothermal
Field

• Sabodh K. Garg, John W. Pritchett,
• Philip E. Wannamaker, Jim Combs
• GRC 2007 Annual Meeting
• Sparks, Nevada

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Motivation

• Based on a theoretical study, Pritchett (2004)
  concluded that electrical surveys may be used to
  explore for hidden geothermal resources.
• Existing data sets for the operating BR
  reservoirs may be used to test the utility of
  electrical surveys for characterizing the
  subsurface.
• Garg et al. (2006) presented a preliminary study
  of the Dixie Valley geothermal field to
  demonstrate the feasibility of correlating
  electrical signals with reservoir conditions.

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Outline

• Three-dimensional numerical model of the Beowawe
  geothermal field, north-central Nevada.
• Computed temperature and salinity distribution
  used to compute pore fluid resistivity. Archies
  law relates formation resistivity to pore fluid
  resistivity.
• STAR DC, MT and SP postprocessors used to
  calculate expected response corresponding to
  available electrical surveys.

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Topography, Wells, DC resistivity lines (yellow),
MT stations (green)
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Numerical Grid (x-y plane) for natural state
simulation
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Geologic Sectioneast-west plane (j6)
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Computed pressures and temperatures (j6)
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Computed tracer (salinity) distribution (j6)
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Comparison between computed and measured feedzone
pressures
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Computed (dashed) and measured (solid)
temperatures in well Ginn1-13
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Electrical Grid
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Dipole-dipole resistivity survey

• In the volume common to the STAR and electric
  grids, Archies law adopted to relate formation
  electrical resistivity to pore fluid resistivity
  and porosity. Results of dipole-dipole survey
  used to specify formation resistivity in parts of
  the electrical grid that are disjoint from the
  STAR grid.
• The DC resistivity survey maps the so-called
  apparent resistivity distribution. The computed
  profiles reproduce most of the important features
  of measured profiles.

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Dipole-dipole Line 2
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Dipole-dipole resistivity (line 2)n 1, 2, 4, 5
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Magnetotelluric (MT) Survey

• The magnetotelluric (MT) method uses naturally
  occurring electromagnetic (EM) waves as sources
  to map the resistivity structure. EM time-series
  data are decomposed into spectra, providing
  apparent resistivity as a function of
  frequency. The depth of penetration is inversely
  proportional to frequency thus lower frequencies
  can be used to map the deeper resistivity
  structure.
• Twelve tensor MT soundings were collected on and
  around the Beowawe KGRA in 1976 only two MT
  stations (Ben-3 and Ben-12) lie within the
  currently exploited geothermal field.

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MT Station Ben-3
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MT Survey (continued)

• Although the computed and observed MT sounding
  curves have the same general shape and show a
  decline in resistivity with increasing depth,
  there exists a quantitative discrepancy between
  the observed and predicted MT responses. The
  observed results need to be multiplied by a
  factor of 3. Possible reasons for this difference
  include static shift and anisotropic
  (horizontal and/or vertical) resistivity
  distribution.

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Self-Potential (SP) Survey

• The self-potential (SP) method measures
  variations in natural DC voltages over the
  surface of the earth caused by the movement of
  underground fluids. Anomalies of several hundred
  millivolts of this type have been observed at
  several geothermal fields.
• Two self-potential surveys were performed at
  Beowawe in the1970s. The SP anomaly at Beowawe is
  positive to the northwest and negative to the
  southeast of the Geysers Terrace.

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SP Survey (computed)
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SP survey (continued)

• The SP anomaly at Beowawe is positive to the
  northwest and negative to the southeast of the
  Geysers Terrace.
• Although the computed and measured SP
  distributions disagree in detail, both display
  the same general behavior (positive to the
  northwest and negative to the southeast).

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Conclusions (1)

• A suite of electrical surveys may be used to
  define the characteristics of hidden Basin and
  Range type geothermal systems.
• None of the methods taken alone can provide
  unambiguous indication of a geothermal system.
• Both the hot and permeable geothermal reservoir
  rocks, and impermeable clays and shales have low
  resistivities. However, clays and shales cannot
  support the vigorous upflow needed to sustain
  geothermal systems.

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Conclusions (2)

• A self-potential anomaly can be caused by
  topography driven cold water flow, but ordinary
  cold ground water aquifers do not normally
  exhibit low resistivities.
• Therefore if the regional heat flow is high, and
  the electrical resistivity and self-potential
  anomalies coincide, the possibilities of finding
  a productive geothermal reservoir are enhanced.

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Acknowledgment

• This work was supported by the U.S. Department
  of Energy Geothermal Technologies Program under
  Contract No. 18084 Amendment 8 between Battelle
  Energy Alliance, LLC (operator of Idaho National
  Laboratory) and Science Applications
  International Corporation.