Numerical simulation of the Alum lakes geothermal outflow

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Numerical simulation of the Alum lakes
geothermal outflow

• J. Newson and M. J. OSullivan

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BACKGROUND

• Part of a study on simulation of geothermal
  surface features
• If water is taken by geothermal wells, is there
  less for the springs?
• What about heat?
• Is this important?
• Used data from Alum Lakes, Wairakei

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TAUPO VOLCANIC ZONE
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WAIRAKEI-TAUHARA
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ALUM LAKES
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CONCEPTUAL MODEL
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• Pirorirori (Alum Lake)
• Ceased flowing in late 1990s
• Photo taken Nov 2004

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AVAILABLE DATA

• Mass flow (including streamflow)
• Temperature
• Chemistry
• Water level (recent)

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3 SPRINGS WITH DATA
Pirorirori
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RESERVOIR SIMULATION

• Simulator that represents heat and mass flow in
  porous and fractured media (rocks)
• Two phase (steam, water, water vapour, and air)

TOUGH2
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RESERVOIR SIMULATION GRID

• Design a 2-D or 3-D block structure that will
• represent the system

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VERTICAL SECTION
Alum Lakes
Te Mihi
0 mrsl
Western Borefield
Eastern Borefield
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DETAIL, 2-D GRID
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RESERVOIR SIMULATION PARAMETERS

• Give each block properties such as permeability,
  porosity, thermal conductivity

SURFACE FOLLOWS TOPOGRAPHY
Pirorirori
Butterfly Spring
Lower Devils Eyeglass
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RESERVOIR SIMULATION B.C.s

• Assign boundary conditions

10 AV. ANN. RAINFALL
SIDE BOUNDARIES CLOSED
HOT WATER
HEAT
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RUNNING A RESERVOIR SIMULATION

• Simulator calculates the temperature and pressure
  at the centre of each block
• T P differences lead to flows between blocks
• Control the flows by changing the permeability
  and porosity in each block

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RESERVOIR SIMULATION TELLS US

• If the hypothesis is possible
• Possible permeability, porosity distribution
• Information about the subsurface flow paths
• Information on the future behaviour of the system

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NATURAL STATE MODEL

• Reservoir temperature vs depth for Wairakei
  before production (1953)
• the mass flow data for Alum Lakes

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NATURAL STATE MODEL
Alum Lakes mass flow data (kg/s)
field model
Pirorirori 11.3 11.91
Butterfly Spring 7.5 7.68
Lower Devils Eyeglass 1.6 2.07
Eastern Borefield
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PRODUCTION PERIOD MODEL

• Use the natural state model as a starting point
  for production simulation
• Check the response of the Alum Lakes in the
  model, compare with known fiels data (mass flow
  over time)
• Production enthalpy, and reservoir pressure for
  the Wairakei borefields

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PRODUCTION HISTORY
Eastern Borefield
Western Borefield
Enthalpy time history
Reservoir pressure time history
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ALUM LAKES MASS OUTFLOW
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NATURAL STATE LIQUID FLOWS
PIRORIRORI
BUTTERFLY SPRING
LOWER DEVILS EYEGLASS
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NATURAL STATE GAS FLOWS
PIRORIRORI
BUTTERFLY SPRING
LOWER DEVILS EYEGLASS
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1975 LIQUID FLOWS
PIRORIRORI
BUTTERFLY SPRING
LOWER DEVILS EYEGLASS
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1975 GAS FLOWS
PIRORIRORI
BUTTERFLY SPRING
LOWER DEVILS EYEGLASS
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2003 LIQUID FLOWS
PIRORIRORI
BUTTERFLY SPRING
LOWER DEVILS EYEGLASS
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FINAL PERMEABILITY STRUCTURE
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NEW CONCEPTUAL MODELNATURAL STATE
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CONCEPTUAL MODEL2003
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SUMMARY

• Behaviour of Alum Lakes Flows linked to reservoir
  changes
• Low permability zones control the shallow
  subsurface flow
• Groundwater now flows down into the reservoir
• Groundwater diverted from Alum Lakes springs, and
  from flowing further eastward

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FUTURE WORK

• Model chloride component
• Model the water level change