Deep geothermal system exploration in Taupo
Volcanic Zone, New Zealand – A newtechnical approach for assessing supercritical
geothermal resource potential
JOGMEC-GNS Workshop, Tokyo, 2nd June, 2016

Chris Bromley & Warwick Kissling
GNS Science, New Zealand

GNS Science

CONTENTS
The deeper & hotter ‘roots’ of
volcanic geothermal systems
 Heat transfer processes Motivation: sustainability of
conventional reservoirs?
Much greater energy output fromsuper-critical T&P wells.
 Examples of on-going researchusing novel technology aimed at‘deep roots’ :New Zealand – TVZ ‘hot deep roots’tectonics, hydrology, geophysics,
THM modelling
Others from : Italy, Iceland Conclusions
Tongariro hydrothermal eruption ,2012, Photo : Karen Britten

GNS Science

HEAT TRANSFER FROM ROOTS
Transfer of heat up to shallower levels is a complicated process
involving flow of magma, flow of fluids (2-phase or supercriticalfluids and/or superheated steam), heat conduction and convection,
as well as thermo-elastic-plastic rock mechanics (brittle-ductile)and chemical deposition/dissolution processes…SO…difficult toexplore, hasn’t been drilled yet and can’t be modelled withconventional modelling tools …ie challenging..!

GNS Science

Concept of heat transfer by volcanic geothermal systems :main heat sources are deep-seated magma-chambers,cooling plutons or smaller intrusions (dykes, sills)

Super-critical targetSæmundsson (2014)
GNS ScienceTHE DEEP ROOTS “CHALLENGE”
 What are the heat sources, deep magma chambers or

partial-melt in shallower dykes/sills?
 How does heat transfer up into reservoirs occur?
 Is there sufficient deep permeability for extraction

through deep wells? Can stimulation by cooling help?
 Are supercritical conditions prevailing at drillable

depth? Is there superheated steam at shallower levels?
 How can fluid with high temperature, high pressure andcorrosive chemistry (HCl, HF) be utilized?

GNS Science

RELEVANCE FOR FUTURE UTILIZATIONGreater output from wells drilled into the roots, if sufficient
permeability can be found, because of higher temperature andpressure, especially if supercritical or producing super-heatedsteam – perhaps more than 5x normal output

Extends resources vertically rather than laterallyLess environmental effects expected because of greater depthand smaller horizontal extent

Potential for applying reinjection-production doublets, or EGStechnology, if permeability is limited

Potential for reinjecting deep (4-5 km) and extracting shallow(above 3 km depth)But still numerous technical problems to be overcome

Cuernavaca Workshop April 2016

GNS ScienceCooling creates :“ConvectiveDownward Migration of Fractures”
 Why is heat transfer from hot intrusions, or magma, into
geothermal systems so rapid?
 Cooling water migrates into hot rock through fractures that
open up by contraction; energy that is derived from thecooling is transported upwards by convection.

GNS ScienceNew Zealand
Hotter and Deeper Geothermal
Integrated ResearchA.
B.
C.
D.

Seismicity (Bannister, Sepulveda, Sewell…)
Magneto-tellurics (Bertrand, Caldwell, Weise…)
Gravity and Magnetics modelling (Soengkono)
3D geological modelling - greywacke basement(Milicich, Alcaraz..)
E. Stress determinations from acoustic borehole logging(Massiot)
F. Reservoir simulation modelling to ductile zone depths(Kissling, Ratouis, O’Sullivan, Croucher…)

But…..

NOT YET ANY DEEP DRILLING (>3.5km) !
GNS Science

The Importance of Full Impedance Tensor Analysis for 3D Magnetotelluric Imaging the Rootsof High Temperature Geothermal Systems: Application to the Taupo Volcanic Zone, NZKey Points••••••••••

Roots of extractable geothermal resources from high temperature sources (i.e. magma) .
Deep-seated permeability.
Untapped energy resources may become available with advances in deep drilling
technology.
TVZ has high heat flux: hot fluid plumes from depth ? Mechanisms of heat transport ?
Uncertain about basement structure and rock permeability below 3km ?
MT shows evidence of electrically conductive plumes from 10 km depth, with permeable
pathways for transporting geothermal fluids.
For MT data modelling of a complex 3D environment, it is necessary to analyze the full
MT impedance tensor, since the analysis using only principal component tensor data (theoff-diagonal components) can produce artifacts in the imaging process.
While using only off-diagonal elements might be justified when the imaging grid isaligned with preferred structural trends in the data, for truly 3D earth this may be difficult
to achieve.
Full MT tensor analysis has its own challenges, especially how one treats data noise forall components in the impedance tensor, where individual entries can vary by several
orders of magnitude at a fixed period.
In these instances, the accuracy of noise estimates in the data and weights assigned tothe components of the impedance tensor are very important.

Newman et al WGC2015

GNS ScienceMT Resistivity 3D inversion : conductive roots

GNS Science3-D Seismic Velocity and Attenuation in the Central Taupo Volcanic Zone, NewZealand: Imaging the Roots of Geothermal Systems
Key Points•••••Image of deep seismic properties in the 2-8 km depth range, beneath a 30 x 50
km area of TVZ: (Wairakei, Mokai, Rotokawa, Ngatamariki, and Ohaaki )38-site broadband seismic array deployed for 2 years : spacing ~4-7 km ,plus
4 other active and passive seismic experiments (1995 to 2004)
Combined dataset comprised 1347 well-recorded earthquakes and 9explosions, recorded at 925 stations, providing more than 120,200 differential
travel time measurements, calculated using the absolute times from pairs ofneighbouring earthquakes.
Inverted these data to derive the spatial and depth variation of seismic
properties Vp, Vs, Vp/Vs, and Q ( 1/attenuation).
The final 3-D Vp, Vp/Vs, Qp and Qs volumes show a high level of heterogeneity
at a range of length scales, especially for Q, illustrating that older simplified 2D models are inappropriate for the region.

Bannister et al WGC2015

GNS ScienceTaupo Volcanic
Zone
Shallow
Seismicity
1995-2013
relocated;co-located withsome geothermal
fieldsGNS ScienceRotokawa
Inducedseismicity to~4km depth(Sewell et al, 2015)

ductile >4km?

GNS Science(From Sepulveda et al 2013)

Earthquakes near
Wairakei-Tauhara, forthe period 2000-2009(right), 2009-2013(above), originatingfrom shallow crustal
depths (80%<9km)

Wairakei - significant seismicity (~1 felt /month)
GNS Science

NEW ZEALAND Understanding the deeproots of geothermal systems in the Taupo
Volcanic Zone (TVZ)
Taupo Volcanic Zone – young(~2Ma) rift above subducting plate
High natural heat flux ~4200 MW(mean 700 mW/m2)
Extension 16 mm/yr at coast,
decreasing to the south
Partial melt beneath TVZ imaged
with Magneto-tellurics (MT)This section is based on material given at GEORG ‘Deep Roots’ meeting
Reykjavik, February 2016 by Warwick Kissling & colleagues, of GNS ScienceGNS Science•Taupo Volcanic Zone
many high T, volcanictype, geothermal systems

Subducting plate

Crustal velocity structure

GNS ScienceLarge-scale TVZ fluid-flow model(attempted in 2005)• Impermeable base at 8km• Distributed heat sourceson ~10 km scale – nointrusions• Simple geology• Locates geothermalsystems ~correctly• Mass/heat flows approx.
within a factor of ~2• Temperatures ~50 oC
too low

Kissling & Weir, JVGR 145, (2005)

GNS ScienceNew Geophysical Results have changed our view of TVZ
Deep Roots : a) Seismic tomography‘Active’ source seismic explosion data
Subduction zone earthquakes at ~4090 km depth propagate near-vertically;
dense sampling of mid-crust.

M6.5 July2011

40 sites deployed for 2 years ~ 5 km apart
Derive : Vp, Vs, Vp/Vs, Qp, Qs
Compare with resistivity/MT
Conclusion: crustal V & Q inhomogeneousgeonet.org.nz
Bannister, Bourguignon, Sherburn & Bertrand, WGC, (2015)

GNS ScienceSW-NE strike profiles of Qp

Rotokawa at x=32 km
Anomalous Low Q
features at 5-15 km depth
Wairakei at x=23 km
Waimangu at x=70 km
Wairakei at x=20-25 km
Waikite at x=63 km

Bannister 2015

GNS ScienceNew Geophysical Results of TVZ Deep Roots:b) 3D MT inversion

Are these MT inversions
consistent with an intrusion
model of the TVZ heat sources ?

Bertrand et al., JVGR (2015)

GNS ScienceRotorua - Waimangu

Relocated seismicity (mag>2 over 7 years) highlighted
blind active fault, permeable
fluid conduit to reservoir ?

Heise et al., JVGR Lake Rotomahana special volume
Bertrand et al., GNS CR 2014/116
Caldwell et al., GNS CR 2014/93
Heise et al., GNS CR 2013/203
Hill et al., GNS CR 2013/160
Bannister et al., JVGR special volume
GNS ScienceOhaaki geothermal field

Bertrand et al., JVGR, (2015).

Inclined deep upflow ?
Mroczek et al., Geothermics, (2016).

Kissling and Bertrand (in prep)
GNS ScienceMT & Seismicity at Rotokawa Geothermal System

Vertical
deepupflow?
Bertrand et al.,JVGR, 2015.

Compartment of seismicity:
induced by fluid flowfrom injection to production
Ductile, >400 oC at ~4 kmdepth ?
Sherburn et al.,Geothermics, 2014

GNS ScienceGround Surface Deformation
InSAR and GPS ~25 mm/yr of subsidence
concentrated in the central TVZ
Geothermal subsidence (up to 50 mm/yr at
Wairakei )Consistent with cooling and contractionof ~0.06 km3/yr rhyolitic magma sill at 6km depth
Implied heat loss ~ 5000 MW-15015 (mm/yr)

Hamling, Hreinsdottir and Fournier, JGR, 2015

GNS Science

Brittle-ductile transition with depth (from seismicity)

Bibby et al., JVGR 68 (1995)
Bannister et al., WGC (2015)
1995: transition ‘at 8 km’…but now:~3.5km beneath Rotokawa> ~10-30 km outside TVZ
shallower in areas of high heat flux, deeper elsewhere (Kissling et al., 2009)
GNS ScienceRegional scale THM models
TM code SULEC (Ellis et al., 2011) toexplore under-plating, rifting,fluid flux from depth and magma intrusions
TH code TGNS (Kissling, NZGW, 2014) tomodel convective fluid and heat transfer tothe surface
New THM model : Detailed geology (to subfault scale) to characterise hydrologicalbehaviour of different fault architectures;
self-determining brittle-ductile zone; heatsources from MT model.
Large scale structure
beneaththe TVZ rift.
(after Stern et al., 2010)

GNS Science

Modelling fluid flow through fractured rockKissling, Massiot et al., NZGW, 2015
GNS Science

RELATED PROJECTS WORLDWIDE
 Italy, DESCRAMBLE (deep drilling andresearch) Switzerland ,COTHERM (high T explorationand modelling) Iceland Deep Drilling Project (IDDP)
 Krafla Magma Drilling Project (KMDP)
 Japan : Super-critical (JBBP) New Zealand : HADES (hotter and deeper) IPGT collaboration through high-T modellinggroup (US, Aust., NZ, Swiss, Iceland)
Cuernavaca Workshop April 2016

GNS ScienceITALY : DESCRAMBLE deep drilling project (Venelle-2)
into supercritical conditions (~450°C & 250 bars)

29

Present welldepth is2,2 kmTarget welldepth is3-3.5 km
Location of Venelle_2 well, Larderello, Tuscany, Italy

Bertani, 2015, http://www.descramble-h2020.eu/
GNS ScienceIceland Deep Roots Research•High-resolution seismic
monitoring, MT, deformation
monitoring (GPS, INSAR)•

Advancement of modelingmethods (new EOS for iTOUGH2 >800 oC, 100 MPa) to understandphysical processes and manage
reservoirs•

Material selection, component
design for wells, design of power
plant and material recovery
http://www.georg.hi.is/node/255•Cuernavaca Workshop April 2016

GNS ScienceExample of modeling project for GEORG (Iceland) :Temperature distribution (°C) at 5,000 years after
emplacement of an intrusion; calculated with a newsupercritical EOS for iTOUGH2 (Magnúsdóttir,2015).
Also see recent NZ modeling papers by : Kissling, Ratouis,
Croucher, O’Sullivan
http://internationalgeothermal.org/ IPGT modeling groupcollaborators
GNS ScienceCONCLUSIONS
 Immense energy resource occur in ‘deep roots’ because ofhigh temperature and pressure (>supercritical)
 CHALLENGES: Is there enough permeability for production ?
 Could deep reinjection be a feasible means of extraction(ductile to brittle transition thru cooling) ?
 TOOLS: Exploration methods are being upgraded to yield
sufficient resolution/accuracy at depths involved
 Modelling of super-critical processes and large-scaleregional models of systems are improving
 PROGRESS: Collaboration between projects and countries
has been beneficial and should be further encouraged ‘Deep Roots’ Annex of IEA-Geothermal can contribute

Cuernavaca Workshop April 2016

GNS ScienceTHANK YOU!

Acknowledgements: Gudni Axelsson &IEA-Geothermal Annex 12 participants,
GNS Science fellow geophysicists andmodellers
Tongariro hydrothermal eruption 2012, Photo: Bruce Christenson, GNS

GNS Science