1. Numerical simulation of the Alum lakes geothermal outflow J. Newson and M. J. O’Sullivan

2. 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

3. TAUPO VOLCANIC ZONE

4. WAIRAKEI-TAUHARA

5. ALUM LAKES

6. CONCEPTUAL MODEL

7. Pirorirori (Alum Lake) Ceased flowing in late 1990’s Photo taken Nov 2004

8. AVAILABLE DATA Mass flow (including streamflow) Temperature Chemistry Water level (recent)

9. 3 SPRINGS WITH DATA Pirorirori

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

11. RESERVOIR SIMULATION: GRID Design a 2-D or 3-D block structure that will represent the system:

12. VERTICAL SECTION Alum Lakes Eastern BorefieldWestern Borefield Te Mihi 0 mrsl

13. DETAIL, 2-D GRID

14. RESERVOIR SIMULATION: PARAMETERS Give each block properties such as permeability, porosity, thermal conductivity… SURFACE FOLLOWS TOPOGRAPHY Pirorirori Butterfly Spring Lower Devil’s Eyeglass

15. RESERVOIR SIMULATION: B.C.’s Assign boundary conditions: 10% AV. ANN. RAINFALL HEAT HOT WATER SIDE BOUNDARIES CLOSED

16. 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

17. 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

18. NATURAL STATE MODEL Reservoir temperature vs depth for Wairakei before production (1953) the mass flow data for Alum Lakes

19. 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

20. 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

21. PRODUCTION HISTORY Eastern BorefieldWestern Borefield Enthalpy time history Reservoir pressure time history

22. ALUM LAKES MASS OUTFLOW

23. NATURAL STATE LIQUID FLOWS PIRORIRORI BUTTERFLY SPRING LOWER DEVIL’S EYEGLASS

24. NATURAL STATE GAS FLOWS PIRORIRORI BUTTERFLY SPRING LOWER DEVIL’S EYEGLASS

25. 1975 LIQUID FLOWS PIRORIRORI BUTTERFLY SPRING LOWER DEVIL’S EYEGLASS

26. 1975 GAS FLOWS PIRORIRORI BUTTERFLY SPRING LOWER DEVIL’S EYEGLASS

27. 2003 LIQUID FLOWS PIRORIRORI BUTTERFLY SPRING LOWER DEVIL’S EYEGLASS

28. FINAL PERMEABILITY STRUCTURE

29. NEW CONCEPTUAL MODEL NATURAL STATE

30. CONCEPTUAL MODEL 2003

31. 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

32. FUTURE WORK Model chloride component Model the water level change