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GNS Science
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
GNS Science
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