Presentation OutlineRole of Geothermal Geologist

Mapping, Stratigraphy and Structure

Hydrothermal Alteration

PermeabilityExploration Development

Summary

Role of Geology in Targeting Permeability for Geothermal Resource Utilisation

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Dr. Greg Bignall

Head of Department

Department of Geothermal Sciences

Wairakei Research Centre

GNS Science, NEW ZEALAND

Role of Geothermal Geologist (1)

By observing minerals and rocks, the geologist learns about processes, and controls on hydrology.

• Rock types• Permeability controls • Fluid-rock interactions

• Heat source • Age of the system• Geohazards

CalciteAlbite

Adularia

Illite

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Experience Matters !Pioneers of TVZ Geothermal Geology

Ted Lloyd

Peter Wood

Pat Browne

Alfred Steiner

Jim HealyGeorge Grindley, Jim Healy, Alfred Steiner, Don Rishworth, Bill Watters, Pat Browne, Peter Wood, Bruce Thompson, Ted Lloyd and others ….

New Zealand has long established geothermal research and consultancy expertise, acknowledged internationally.

George Grindley

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Wairakei Geothermal Field4

NorthIsland

(NZ)

North

0 20 40 60

kilometres

BAY OF PLENTY

38oS

39oS

176oE 177oE

177oE

Taupo Volcanic Centre

Rotorua Volcanic Centre

Okataina Volcanic Centre

Ruapehu

WHAKATANE GRABEN

Taupo Fault Belt

TAUPO VOLCANIC

ZONE

High temperature geothermal systems

Maroa Volcanic Centre

We’ve identified the “conventional”active, high-T geothermal systems in the TVZ – our challenge is to delineate the “deep-seated” (>4 km) resources (& low-enthalpy systems).

Average resistivity of upper 500 m

5

Recent TVZ Drilling Successes

Taheke

• In the last 10 years, ~200 wells (for geothermal exploration, production, injection & T-P-X monitoring) drilled in N. Island.

• 10 fields (incl. Ngawha).

• Utilising dynamic targeting strategies, New Zealand has explored targets to -3000 mRSL

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Permeability controls in TVZ systems

• In most TVZ systems, past drilling has targeted structurally-controlled permeability , associated with indurated rocks with low inherent permeability.

e.g. reservoirs hosted by (andesitic) lava & meta-sediments (greywacke) at Kawerau/Rotokawa.

• In ignimbrite-dominated systems (e.g. Wairakei, Mokai), formation-hosted permeability may have greater importance in providing fluid pathways.

Drilling ignimbrite-hosted high temperature

geotherrnal systems of the TVZ

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Formation Permeability

v

Fracture Permeability

Dominant control on fluid flow in TVZ systems ?

Role of Geothermal Geologist (2)

Geological activities divided into two parts:

(i) Geology which takes place before drilling (e.g. geological, alteration and surface feature mapping)

(ii) Geology undertaken during / after drilling

Geotechnical / geohazards geology important

CalciteAlbite

Adularia

Illite

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Surface Alteration Mapping

Adularia

By understanding physical-chemical processes, and mineral stability ranges, the geologist can predict the nature of the reservoir fluids

Secondary minerals deposited in pores and fractures – replace existing minerals.

Distinguish degree (intensity) and nature (rank)Silica sinter, Waiotapu Mud pool, Rotorua

calcite

illite

Calcite (illite overprint) Mineral exploration

gold

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Surface Geological Mapping

Used to infer geothermal field geology

•Regional mapping- indicator of subsurface stratigraphy

- analogy with other active volcanic areas

•Map thermal features/hydrothermal alteration

•Map stratigraphic relations to infer geological (eruptive/structural) history of the area

- Units may not have been horizontally deposited

- Units may have abrupt or irregular terminations

- Cautious interpretation of age dating

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Typical Reservoir Rocks

•Can be any type - most often volcanic or volcanogenic•Easiest systems to develop are those with thick sequence of volcanic rocks•Shallow, low-permeability basement: may not be favourable for production•Carbonate rocks (e.g. limestones or marbles) may point to future problems

granite basalt andesite lithic breccia

sandstone

sandstone

limestone schist

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Field Stratigraphy

Surficial Deposits

Volcaniclastic Sediments

Volcanic Lavas and Intrusives

Post 250ka Pyroclastic Units

Pre 250ka Pyroclastic Units

Basement greywackeWairakei Geothermal Field

+500

SL

-500

-1000

-1500

-2000

Ele

vatio

n (m

RL)

WK680 WK221

WK212WK253WK248 WK202 WK121 WK45 WK35

WK301WK305

WK315WK314WK302R

esis

tivity

Bou

ndar

yZ

one

Res

istiv

ityB

ound

ary

Zon

eWEST EAST

after Rosenberg et al. 2009Scale : 2 km

KA30 KA31 KA28KA36/37

KA21 PK3 PK4/4A PK5

SL

-500

-1000

-1500

-2000

Ele

vatio

n (m

RL)

Scale : 2 kmafter Milicich et al. in prep.

NORTH-WEST SOUTH-EAST

Kawerau Geothermal Field

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Petrology

Objective of petrological work during exploration of geothermal systems is identification of :

• Primary Rock Types- assist structural interpretation

- correlate/differentiate stratigraphic units

• Infer controls on permeability in the system

• Evidence for thermal (or chemical) change

• Help determine casing depths

• Hydrothermal Mineralogy - infer fluid chemistry, reservoir temperature and

system evolution

1 mmPorphyritic augite andesite

1 mmSubhedral granular gabbro

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Structure

Silica sinter covered fault scarps, Orakei Korako

Active faults map, Wairakei-Tauhara

To predict permeability controls in the geothermal reservoir – particularly fracture permeability.

Consider:

• Evidence of rejuvenated structural permeability

• Lateral outflows

• Detailed fracture and vein mapping

• Air photography – radar imagery

• Map structural lineations – thermal features

Within a sub-horizontal stratigraphy, the most productive zones are likely to coincide with wells that intersect steep dipping fractures .

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GNS Science Active Faults database

• Fault expressions eroded or obscured by recent volcanism

• Geodetic studies show NW-SE crustal extension in Taupo area of ~2-8 mm/yr

• Deformation provide fluid pathways along fault zones and diffusely through unconsolidated rock

Te Mihi

Wairakei Geothermal Field

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+500

SL

-500

-1000

-1500

-2000

Ele

vatio

n (m

RL)

WK680 WK221

WK212WK253WK248 WK202 WK121 WK45 WK35

WK301WK305

WK315WK314WK302R

esis

tivity

Bou

ndar

yZ

one

Res

istiv

ityB

ound

ary

Zon

eWEST EAST

after Rosenberg et al. 2009Scale : 2 km

X-section

Permeability“The state / quality of a material that causes it to allow liquids or gases to pass through it”

Bulk permeability (e.g. tuff, ignimbrite) Fracture permeability (welded ignimbrite, andesite, greywacke)

What is the dominant control on fluid flow in geothermal systems ?

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How does fluid move through rock?

Primary Permeability

POROSITYDistribution random, pores may not be interconnected

LITHOLOGICAL CONTACTSsub-horizontal or low angles of dip

DIATREMES, VOLCANIC VENTS Subvertical

COOLING JOINTS &AUTOBRECCIATIONLava Flows - random or sub-horizontal zones

after KML (1995) From Bignall et al. 2010

-2000

metres

2500

-1000

-15002000

1500

-5001000

+500

0Sealevel

500

0 mCHF masl

1km

MK5MK7 MK16

MK10 MK13 MK6

MK5

MK10MK7

MK16

MK13

MK

6

Permeable zones identified from completion test

Ignimbrite C

Ignimbrite D

WhakamaruIgnimbrite

Volcaniclastic Sediments

Ignimbrite F

VolcaniclasticSediments

Mokai Ignim.

Mokai Rhyolite

Rhyolite B

Rhyolite C

NW SE

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How does fluid move through rock?

Formation v Fracture permeability

FRACTURING dyke injection - likely to be subvertical

THERMAL CRACKINGRandom distribution

ROCK DISSOLUTIONRandom distribution

FAULTINGLikely to be subvertical

HYDROTHERMAL BRECCIATIONLikely to be subvertical

after KML (1995)

+ + + + + + + + + + + +

+ + + + + + + + + + + + +

+ + +

+ +

+ +

+ +

+ +

+

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Secondary Permeability

3

Conceptual (hydrological) Model

• Chemical / hydrological structure of the geothermal system

• Hydrological model evolves as more information comes available.

+ geophysically-defined

+ geological control on fluid flow

+ chemical structure (e.g. reservoir conditions, flow path, temperature, acidic fluids ?)

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2 0Ω m3 0

Ω m5 0

Ω m

>300oC

246oC

180oC

Upflow

Outflow

0

0 metres 2000 4000

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Geological input to exploration drilling

The exploration geologist has an important role, in combination with engineers and developers, to:

• design drilling strategy (define drill targets)• achieve objectives of the drilling programme• support interests of the developer/operator.

Goals of Exploration Drilling

• Test hydrological (conceptual) model• Confirm commercial temperatures / permeability• Refine capacity assessment• Part of development scheme (commercially productive)• Reasonable cost• Provide project confidence

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Well targeting at reduced risk (1)

Probable

Possible

Speculative

Drillhole

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0 metres 2000 4000

>300

oC250 o

C

20Ωm

MostProspective

Development Strategy

Magm

atic Acidity

SL

Fumaroles

Acid pools

Hot Water Flow

Cold Water FlowCOOLING MAGMATIC INTRUSIVE

Boiling NaCl Springs

Silica sinter terraces

250oC

BOILING ZONE200oC

300oC

Meteoric

Recharge

Steaming Ground

Na-HCO3-SO

4springs

Steam-heated

groundwater ELEVATION

mRSL

Springs

NaCl Brine Pyroclastic

Rocks

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>300oC

176oC

Conceptual Model

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Well targeting at reduced risk (2)

Conceptual model based on integrated geology, chemical and geophysical information

•Insights from conceptual hydrological model:

– upflow v outflow zones?

– fault/fracture permeability v formation permeability?

– expected reservoir temperatures?

– reservoir chemistry?

– geophysical information: resistivity-defined field boundary?

•“green-field” exploration: little subsurface knowledge:

– delineation drilling to test areal extent/resource potential

22Rotokawa

• Geologic prognoses: - stratigraphy and structure - permeable zones - temperature and pressure

• Recommend - depth of production casing shoe - coring depths

• Description of rocks - lithology - hydrothermal alteration

• Interpret - rock formations - stratigraphic context- fault structures - reservoir temperature - fluid chemistry - reservoir permeability

• Regular reporting - communication with on-site / office drilling staff - nature of formation (e.g. hardness, fracture permeability, acid

zones, swelling clays, correlate drilling parameters with geology) - maintain geologic log / completion report

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fracture permeability

formation permeability

Role of Geothermal Rig Geologist

Geothermal 3D Modelling

• 3D geological visualisation and modelling

• spatial integration of geoscientific data

– geological, geochemical, geophysical data

– DTM, air photos, maps– any x,y,z data

• only as good as the quality of input data

• conceptualisation of hydrological model

– assist with future well targeting (geological prognoses)

– reservoir management

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Hydrothermal Alteration

HYDROTHERMAL ALTERATION

OpalChalcedony

Quartz

K-feldsparAlbite

KaoliniteSmectite

Illite-SmectiteIllite

Chlorite

MordeniteLaumontite

WairakiteEpidote

Amphibole

100oC 200oC 300oC

Geothermal system type

Permeability

Hydrothermal fluid chemistry

Primary rock composition

Water-Rock Ratio

epidote and anhydrite

Press. Temp. Conditions

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Look for evidence of thermal-chemical change in the geothermal system

Leading to revised hydrological model

1 m

Fracture orientation, width & distributionAcoustic Formation Imaging Technology

Characterise stress orientationσ

1

σ3

σ3

σ2

Extensional regime

SV = σ1Horizontal stresses are Shmin (S3, σ3) & SHmax (S2, σ2)

High Temperature Acoustic Formation Imaging Technology (AFIT)

Fracture Imaging

Shmax

Shmin

Vertical Well Scenario

Shear failure & breakout generated in direction of Shmin

Tensile or drilling induced fractures

propagating in direction of Shmax

Infer Reservoir Geohydrology

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ROP: useful information in zones of blind drilling

Met

ers

MeB used to ground-truth resistivity data

thermal decline

Lateral

flow

Infer upflow or outflow? / reservoir margin ? / thermal change ?

Revise Geological / Conceptual Model

Tough2 gridNumerical (simulation) model

Hydrothermal alteration Stratigraphy

Wairakei Geothermal Field

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Structure

Engineered Geothermal Systems (EGS)

•Artificially create permeability via hydraulic and/or chemical stimulation of high temperature, low permeability rock mass.

•Transfer heat to surface by circulating water (or other fluid) via a fracture network linking injection and production wells.

•Permeability is influenced by Thermal-Hydrological-Mechanical-Chemical effects

•Need to understand the effects, and their timing, in order to engineer the reservoir

EGS Challenges :•Resource characterisation•Drilling Technologies (incl. cost)•Reservoir Creation (stimulation)•Longevity / Sustain reservoir•Environmental Issues

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Future Deep TVZ Drilling

The barrier to realising New Zealand’s deep geothermal potential is the ability to identify permeability that can be tapped by drilling.

Limit of shallow

seismicity

after Heise et al., (2006)

TVZ – deep science drilling project

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Summary

1. Design geoscience strategy that aids decision making.

2. Geology input ongoing in field exploration, delineation and development stages.

3. Identification of positive resource attributes, and issues that could have a detrimental impact on resource development / use.

4. Identifying / understanding controls on permeability is key !

5. Sound geological advice early (and ongoing) has potential to save time, resources and money later …

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Conceptual Model

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Dr. Greg Bignall

GNS ScienceWairakei Research CentreTaupo, NEW ZEALAND

http://www.gns.cri.nz

g.bignall@gns.cri.nz THANK YOU

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