Numerical simulation of the Alum lakes geothermal outflow

J. Newson and M. J. O’Sullivan

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

TAUPO VOLCANIC ZONE

WAIRAKEI-TAUHARA

ALUM LAKES

CONCEPTUAL MODEL

•Pirorirori (Alum Lake)

•Ceased flowing in late 1990’s

•Photo taken Nov 2004

AVAILABLE DATA

• Mass flow (including streamflow)

• Temperature

• Chemistry

• Water level (recent)

3 SPRINGS WITH DATA

Pirorirori

RESERVOIR SIMULATION

• Simulator that represents heat and mass flow in porous and fractured media (rocks)

• Two phase (steam, water, water vapour, and air)

RESERVOIR SIMULATION: GRID

Design a 2-D or 3-D block structure that willrepresent the system:

VERTICAL SECTION

Alum Lakes

Eas

tern

Bo

refi

eld

Wes

tern

Bo

refi

eld

Te

Mih

i

0 mrsl

DETAIL, 2-D GRID

RESERVOIR SIMULATION: PARAMETERSGive each block properties such as permeability,

porosity, thermal conductivity…

SURFACE FOLLOWS TOPOGRAPHY

Pirorirori

Butterfly Spring

Lower Devil’s Eyeglass

RESERVOIR SIMULATION: B.C.’sAssign boundary conditions:

10% AV. ANN. RAINFALL

HEATHOT WATER

SID

E B

OU

ND

AR

IES

CLO

SED

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

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

NATURAL STATE MODEL

• Reservoir temperature vs depth for Wairakei before production (1953)

• the mass flow data for Alum Lakes

NATURAL STATE MODEL

field model

Pirorirori 11.3 11.91

Butterfly Spring

7.5 7.68

Lower Devil’s Eyeglass

1.6 2.07

Alum Lakes: mass flow data (kg/s)

Eastern Borefield

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

PRODUCTION HISTORYEastern Borefield Western Borefield

Enthalpy time history

Reservoir pressure time history

ALUM LAKES MASS OUTFLOW

NATURAL STATE LIQUID FLOWS

PIRORIRORI

BUTTERFLY SPRING

LOWER DEVIL’S EYEGLASS

NATURAL STATE GAS FLOWS

PIRORIRORI

BUTTERFLY SPRING

LOWER DEVIL’S EYEGLASS

1975 LIQUID FLOWS

PIRORIRORI

BUTTERFLY SPRING

LOWER DEVIL’S EYEGLASS

1975 GAS FLOWS

PIRORIRORI

BUTTERFLY SPRING

LOWER DEVIL’S EYEGLASS

2003 LIQUID FLOWS

PIRORIRORI

BUTTERFLY SPRING

LOWER DEVIL’S EYEGLASS

FINAL PERMEABILITY STRUCTURE

NEW CONCEPTUAL MODELNATURAL STATE

CONCEPTUAL MODEL2003

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

FUTURE WORK

• Model chloride component

• Model the water level change