Ngawha Geothermal Fieldgeothermal.jogmec.go.jp/report/file/session_160602_05.pdf · Ngawha Spring...

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Ngawha Geothermal Field :Geology, Geophysics, Conceptual Model,

Geochemical Monitoring Trends & Environmental Issues

JOGMEC-GNS Workshop, Tokyo, 2nd June, 2016

Chris Bromley and Greg Bignall

GNS Science, Wairakei Research Centre, Taupo, New Zealand

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Auckland

Mokai

Wairakei

Rotokawa

Ohaaki

Tauhara

Kawerau

Locations for New Zealand Geothermal Power Plants

Ngawha

2

Ngatamariki

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Introduction

Ngawha Geothermal FieldOperator : Top Energy

10 MWe (from 1998), 25 MWe (from 2008) – an example of a geothermal project designed and managed to avoid all

significant surface environmental impacts (ie. no adverse

effects from pressure/temperature changes on hot springs)• ORC (Ormat OEC) binary plants (4 units)

• Full reinjection of all fluid produced from 1998 (except ~1.2 % CO2 gas initially)

• Supplementary surface water injection after 2008 to sustain

pressure (to replace all mass, including vented CO2)• No production cooling so far from reinjectate returns

• In 2015, consent granted for expansion by 50 MW• Expect commissioning 2019-2022

OEC units 1&2

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Ngawha Expansion Project : pipeline layout

Units 1/2 10 MW -- --Units 3/4 15 MW

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Ngawhasurface geology,resistivity ‘boundaries’

mapped faults and wells

Few natural earthquakes

(GeoNet M>2), & no recorded induced

seismicity (‘quietest’ region in New Zealand)

so …active tectonic

stresses alone are probably not sufficient to

maintain permeability in the host reservoir of

fractured greywacke

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NW-SE Cross-section of Geology

Reservoir is hosted in fractured greywacke basementwith cap-rock of low permeability sediments

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NgawhaGeothermal

System

DC and MT

resistivity ‘boundaries’

Key Well locations:

NG4, NG5NG9, NG11,

NG13, NG18

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Magneto-telluric – resistivity model

3D Leapfrog view

2D MT cross-sectionNW-SE

(from SKM, 2010)

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Gravity Anomaly -heat source ? Model: ~ 5 x13 km

extent,-300 kg/m3 lower density, at 3-5 km depthperhaps cooling

rhyolitic pluton ? ~115 km3

If T ~500 oC, heat content =20 EJ,If 17 MW surface

heat-loss, age=~40,000 yr ?

Gravity from SKM (2011)

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Temperature-depth profiles

Reservoir

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Conceptual model cross-section

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Conceptual hydrological model

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Resource Areas

Production expansion

Confirmed resource

NW springs and sinters

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Geochemical Monitoring:

Gas emissions halved over 15 years production

01/01/98 01/01/03 01/01/08 01/01/13

Date

0

0.5

1

1.5

2

2.5

Well

NG4

NG9

NG12

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Gas ratio (CO2/H2S) has also reduced over 15 years

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Geothermometer temperatures stable for 15 years

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Chloride content & Cl/B in total discharge:

show minor changes

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Environmental Issues:

Hot spring interference potential(flow-rate, temperature & chemistry)

Subsidence potentialInduced Seismicity potential

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Main NgawhaGeothermal Springs : Tiger bath, Ginns’ bath, Waiarikibath.

Ngawha village is centre left and the power stations are ~1km from top left.

Tiger Bath Area

Waiariki Bath

(Universal)

Ginns’ Bath

(Jubilee) Viewed South-west ^

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Ngawha Spring hydrology is complex: 4 fluids:

deep Cl, HCO3, acidSO4+ NH3, &

rainfall recharge

High variability before

& after production commenced, influenced by low liquid flows, & high gas fluxes.

Tiger Baths - acid SO4 dilution

Universal (WaiarikiDomain) – HCO3

dilution

Jubilee (Ginn’s Spa)

highest Cl (1000-1200 ppm)

Christenson, 2003

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Monitoring of Changes in Thermal features

Variable but NO significant response to production

Jubilee Universal

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Subsidence history

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Subsidence model - fit to history

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Ngawha subsidence model - predicted results

Predicted

subsidence

(1998-2050)

for a 50 MWe

expansion in

2020/2023,

using the 3D

Tough2

reservoir

model to

compute

poro-/thermo-

elastic

subsidence.

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Ngawha subsidence model simulation to 2050

Contour of level change from the

geothermal development in 2050 for a 50 MWeexpansion in

2020/2023 using the 3D model (excluding any background

trend)

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Thanks

Chris Bromley, GNS Science, Wairakei

Acknowledgements: Top Energy, John Burnell, Ed Mroczek, Mark Simpson, Malcolm Grant, Tricia Scott, & SKM

1998 Ngawha 10 MW OEC 1&2 Power

Station, Top Energy, Northland