ngawha geothermal fieldgeothermal.jogmec.go.jp/report/file/session_160602_05.pdf · ngawha spring...
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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
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GNS Science
Auckland
Mokai
Wairakei
Rotokawa
Ohaaki
Tauhara
Kawerau
Locations for New Zealand Geothermal Power Plants
Ngawha
2
Ngatamariki
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GNS Science
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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GNS Science
Ngawha Expansion Project : pipeline layout
Units 1/2 10 MW -- --Units 3/4 15 MW
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GNS Science
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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GNS Science
NW-SE Cross-section of Geology
Reservoir is hosted in fractured greywacke basementwith cap-rock of low permeability sediments
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GNS Science
NgawhaGeothermal
System
DC and MT
resistivity ‘boundaries’
Key Well locations:
NG4, NG5NG9, NG11,
NG13, NG18
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GNS Science
Magneto-telluric – resistivity model
3D Leapfrog view
2D MT cross-sectionNW-SE
(from SKM, 2010)
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GNS Science
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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GNS Science
Temperature-depth profiles
Reservoir
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GNS Science
Conceptual model cross-section
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GNS Science
Conceptual hydrological model
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GNS Science
Resource Areas
Production expansion
Confirmed resource
NW springs and sinters
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GNS Science
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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GNS Science
Gas ratio (CO2/H2S) has also reduced over 15 years
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GNS Science
Geothermometer temperatures stable for 15 years
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GNS Science
Chloride content & Cl/B in total discharge:
show minor changes
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GNS Science
Environmental Issues:
Hot spring interference potential(flow-rate, temperature & chemistry)
Subsidence potentialInduced Seismicity potential
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GNS Science
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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GNS Science
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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GNS Science
Monitoring of Changes in Thermal features
Variable but NO significant response to production
Jubilee Universal
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GNS Science
Subsidence history
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GNS Science
Subsidence model - fit to history
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GNS Science
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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GNS Science
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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GNS Science
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