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Numerical simulation of the Alum lakes geothermal outflow J. Newson and M. J. O’Sullivan
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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 1990’s 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)
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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 Eastern BorefieldWestern Borefield Te Mihi 0 mrsl
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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 Devil’s Eyeglass
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RESERVOIR SIMULATION: B.C.’s Assign boundary conditions: 10% AV. ANN. RAINFALL HEAT HOT WATER SIDE BOUNDARIES CLOSED
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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 fieldmodel Pirorirori11.311.91 Butterfly Spring 7.57.68 Lower Devil’s Eyeglass 1.62.07 Alum Lakes: mass flow data (kg/s) 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 BorefieldWestern 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 DEVIL’S EYEGLASS
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NATURAL STATE GAS FLOWS PIRORIRORI BUTTERFLY SPRING LOWER DEVIL’S EYEGLASS
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1975 LIQUID FLOWS PIRORIRORI BUTTERFLY SPRING LOWER DEVIL’S EYEGLASS
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1975 GAS FLOWS PIRORIRORI BUTTERFLY SPRING LOWER DEVIL’S EYEGLASS
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2003 LIQUID FLOWS PIRORIRORI BUTTERFLY SPRING LOWER DEVIL’S EYEGLASS
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FINAL PERMEABILITY STRUCTURE
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NEW CONCEPTUAL MODEL NATURAL STATE
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CONCEPTUAL MODEL 2003
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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
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