bignall - role of geoscience - nz · pdf filesilica sinter deposited by boiling, neutral,...

Presentation OutlineSuccessful Low Risk DevelopmentGeothermal Resource EvaluationGeology – Chemistry – GeophysicsHydrologic ModelsResource ParametersGeothermal Risk Assessment

Application of Geoscience for Geothermal Resource Evaluation and Mitigation of Development Risks

1

Dr. Greg Bignall

Head of DepartmentDepartment of Geothermal Sciences

Wairakei Research CentreGNS Science, NEW ZEALAND

3

Outline of the presentation

• This presentation reviews geoscientific disciplines used in exploration stages of geothermal project development, and assessment of risk.

• Highlights exploration techniques used for resource characterisation.

• Stresses the value of integrating all geoscience disciplines as the key to successful development of geothermal resources.

2Adapted from “Overview of

Resource Assessment” by C. Harvey “Start to Steam”, Taupo June 2012

3

Key to Successful Low Risk Development

Literature Review

Geology Geochemistry Geophysics

Environmental, Hazards and Risk

• Rigorous scientific studies at reconnaissance and exploration stages

• Integration of data from all disciplines (geology, geochemistry, geophysics)

• Recognition of hazards or barriers to development

• Conceptual models to be tested and refined by more detailed work

3

3

How does fluid move through rock?

• Government dedicated to greater use of renewable energy resources

• ~830MWe base load electricity generation; ~15% of generation

Sixth for installed geothermal electricity generation capacity

• Increasing uptake of direct heat use – esp. GSHP

New Zealand Geothermal Scene

4

TVZ Geothermal Systems

Geothermal Energy, New Zealand~ 1200 MWe available~ 1600 MWe protected

5

3

Large, high temperature system* Good permeability Shallow reservoir Accessibility (grid / market)

Benign fluid chemistry Environmental issues

2 km

2 km

Geothermal System : A transfer of heat energy to the earth's surface.Geothermal Energy : A resource utilised for heating or other direct uses (residential, industrial) or electricity generation.

Anatomy of TVZ Geothermal Systems

All geothermal systems have features that make for easy development, and other features that are a disadvantage

Integrate all resource data, to understand the system

Does the area have geological, geochemical and geophysical characteristics consistent with a prospective geothermal system?

Establish geoscience strategy that aids decision making

No geothermal resources are identical

7

3

Sustainable Geothermal Development

Primary considerations :

1. sustainability of the geothermal reservoir…., avoiding detrimental impacts by maintaining the reservoirand surface character of the field

and

2. maximising the use of the geothermal energy, whilst minimisingrisk factors(generating the highest possible income at the lowest operation and maintenance cost to the developer).

8

Wairakei Geothermal Field

3

Geothermal Development Flow Chart

(after Gislason & Arnason, 2005)

9

By combining geological, chemical and geophysical survey data, thegeoscience team able to establish a conceptual hydrological model,which is updated through exploration and field development stages.

?

North South

AT-1Thermal Seeps

Thermal SeepsHot Pools

WAIKATO RIVERLAKE ATIAMURINORTH ARM

LAKE ATIAMURI

Mamaku Igm /Ohakuri Igm

Ohakuri Igm

Maroa Deposits ?

Taupo & Oruanui Igms& associated sediments

Post - Whakamarucaldera fill

Whakamaru-typeIgnimbrites

Whakamaru-typeIgnimbrites

Whakamaru Igm ?

Whakamaru Igm ?

Position of boundary

zone unknown

Permeability enhancedby hydro-fracturing Permeability focussed

lithological boundaries?

Structural control on fluid flow? Horizontal

flow good in some units

Possible High-Temperature

Resource

LOW PERMEABILITY

HIGH - MODERATEPERMEABILITY

Lucastrine sediments

?Hot thermal fluids

Cool meteoric waters

Isotherm

Old Ignimbrites

ATIAMURI GEOTHERMAL FIELDconceptual hydrological model

~1 km

Exploration drilling is the final step to prove economic temperature and permeability, and resolve the deep stratigraphy and reservoir chemistry.

Conceptual Model of typical (TVZ-style) geothermal system)

10

3

Geochemical data important to help define system boundaries and identify possible up-flow zones

• Mapping thermal features (geodiversity)

• Characterise the fluid & gas chemistry (fluid sources)

• Obtain baseline data on non-thermal fluids

• CO2 flux and ongoing monitoring surveys

• Identify development-limiting chemicalcomponents (scaling? corrosion?)

Obtain first temperature estimates of the resource (c.f. geothermometry)

Build hydrological model of the system

1. Chemical Surveys

11

3

Cl

SO4 HCO3

surficial acid waters

peripheral waters

acid magmatic waters

primary chloride brines

seawater

Photo: GNS SciencePhoto: http://www.nps.gov/

Fluid Types – Surface Manifestations

Interpretations of water chemistry requires an understanding of end-member types, and methods by which they were formed

Silica sinter deposited by boiling, neutral, alkali-Cl waters, SiO2 >350 mg/kg, >200oC source fluids.

Travertine – CO2-rich, Na-HCO3 springs, moderate to high gas contents?Turbid, grey acid-SO4 pools, near-surface processes

Photo: GNS Science Photo: GNS Science

Sulphur deposition & superheated fumaroles, may imply SO2 / magmatic (acid) fluid conditions at depth

3

• Estimate reservoir temperature using water / gas chemistry data

• Based on field-observed correlations and theoretical data

• Each geothermometer has limitations

• Used to monitor changes in developed geothermal fields

Geothermometry

• Solute - Quartz (TQz)- Na/K (TNa/K)- Na/K/Ca (TNa/K/Ca)- Na/Li (TNa/Li)

• Gas - Fischer-Tropsch (methane breakdown)- H-Ar geothermometer

13

2. Geothermal Geology

Geological activities divided into two parts:

(i) Geology which takes place before drilling (e.g. map geology / stratigraphic relationships, surface hydrothermal alteration and manifestations)

(ii) Geology undertaken during / after drilling

Identify geotechnical issues / geohazards

+500

SL

-500

-1000

-1500

-2000

Ele

vatio

n (m

RL)

WK680 WK221

WK212WK253WK248 WK202 WK121 WK45 WK35

WK301WK305

WK315WK314WK302R

esis

tivity

Boun

dary

Zone

Res

istiv

ityBo

unda

ryZo

neWEST EAST

after Rosenberg et al. 2009Scale : 2 km

Structure – Fracture ImagingTo predict permeability controls in the geothermal reservoir

Silica sinter covered fault scarps, Orakei Korako

Evidence of rejuvenated structural permeability

• Lateral outflows• Air photography – radar imagery – fracture mapping• Map structural lineations – thermal features

Within the sub-horizontal (TVZ) stratigraphy, the most productive zones coincide with wells that intersect steep dipping fractures.

1 m

Fracture orientation, width & distributionAcoustic Formation Imaging Technology15

Evidence for system change

• Change in surface thermal activity (time and space)(e.g. surficial features that are not in equilibrium with current fluids)

• Record in hydrothermal alteration mineralogy• Changes in fluid inclusion data with time (e.g. salinity and

temperature - depth of erosion, fault displacement etc)

• Alteration minerals out of step with current T-X conditions

20m

Leading to revised hydrological model

16

OpalChalcedony

Quartz

K-feldsparAlbite

KaoliniteSmectite

Illite-SmectiteIllite

Chlorite

MordeniteLaumontite

WairakiteEpidote

Amphibole

100oC 200oC 300oC

3

3. Geophysical Investigations

• Mainly to assess the dimension (extent, thickness) of reservoir

• Likely to postdate initial chemistry / geology surveys

• May provide information on:

- reservoir structure (shallow or deep,upflow zones, lateral outflows)

- likely location of productive zones- natural heat balance

and, in a more regional sense…,

- the geological setting of the system

Schlumberger apparent resistivityMokai geothermal system, AB/2=500m

17

3

Geophysical Methods

• Heat flow surveys• Remote sensing • Gravity • Magnetics• Resistivity • Magneto-tellurics• Seismic surveys• Borehole geophysics

The success of any geophysical investigation depends on applying the best combination of techniques in the correct sequence to explore a prospect

Understanding heat balance of the system

Measure heat discharges (convective, conductive, evaporative) from active manifestations

18

3

Geophysical Methods



Distribution of surface/shallow temperaturesGeological setting and structures (local/regional)

IR imagery (satellite data, aerial surveys), Satellite and aerial photos, Spectral imaging, Radar altimeter, LIDAR

19

3

Geophysical Methods



Small variations in the earth gravitational field

Mokai

A

B

Mokaifield

Edges detectionof anomalies

2 km

20 km

Soengkono (unpubl)

18

3

Geophysical Methods



Mapping local disturbance of geomagnetic field

Mokai

A

B

(Soengkono, unpublished result 2009)

Mokaifield

Edges detectionof anomalies

2 km

20 km

(Soengkono, unpublished result 2012)

Interpretation:

Interpretation:

18

3

Geophysical Methods



TVZ Schlumberger apparent resistivityAB/2=500m (GNS data)

Mokai Geothermal Field

18

3

Geophysical Methods



3D MT model, Coso Geothermal Field (Newman et al., 2008)

Recording natural electromagnetic waves at ground surface over a wide range of frequency

18

Geophysical Methods



“Active seismic” (shot point) - “Passive seismic” (natural seismic signal) surveys - micro-earthquake surveys.

18

From Yukutake et al. (2008)

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, magmatic fluids)

440 440

480

480

500

500500

520560

560

560

580

580

580 580

600

600

600

620

620

620

620

640

660

660

660

680

700

700

700

720

740

760

800

800

840

860 720740

760

780

780800

820

580

760

Mt. Tan

540

840

>300oC

246oC

180oC

Upflow

Outflow

0

0 metres 2000 4000

19

Which Hydrological Model ?

From Harvey(“Start to Steam”, June 2012)

Hot Water Flow

Cold Water FlowCOOLING MAGMATIC INTRUSIVE (Magmatic vapour with HCl)

Boiling NaCl SpringsSilica sinter terraces

250oC

BOILING ZONE 200oC

NaCl Brine 1000‐2000ppmCl 300oC

MeteoricRecharge

Na‐HCO3‐SO4springs

Steam‐heatedGroundwater

FumarolesAcid‐SO4 pools

PyroclasticRocks

OldVolcanics

0

ELEVATION

mRSL

?

OUTFLOW

Deep Reservoir

BOILING ZONE

Magmatic source(with HCl, CO2, SO2, H2O)

Acid‐SO4‐Cl springsand fumaroles

Alkali‐chloride Springs

Dilute Cl‐SO4springs

350oC

300oC

250oC 180oC

Hot Water Flow

Cold Water Flow

ANDESITE VOLCANO

Acid‐SO4 Fluids

MeteoricRecharge

Basement Rocks

Volcanic Rocks

Andesite

UPFLOW

Faults

0

?Appropriate

model

From Barnett (“Start to Steam”, June 2012)

3

Resource Capacity Assessment

Resource AreaSurface Heat FlowResource TemperatureControls on Fluid FlowReservoir Chemistry

Estimate total resource capacity

• Heat Flux MethodNatural heat flux of the system, derived from physical estimates

• Areal MethodEstimate development size from areal extent, multiplied by power density factor (8-10 MW/km2)

21

440 440

480

480

500

500500

520560

560

560

580

580

580 580

600

600

600

620

620

620

620

640

660

660

660

680

700

700

700

720

740

760

800

800

840

860 720740

760

780

780800

820

580

760

Mt. Tan

540

840

>300oC

246oC

180oC

Upflow

Outflow

0

0 metres 2000 4000

3

Geothermal Risk Assessment

• Assess / mitigate risks that could threaten viability of development

• Consequences of some risks, may prove fatal to development(s).

• As exploration progresses, level of confidence in resource increases

• How probable that a constraint will apply during project lifetime?

440 440

480

480

500

500500

520560

560

560580

580

580 580

600

600

600

620

620620

620

640

660

660

660

680700

700

700

720

760

800

800

840

860 720740

760

780

780800

820

760

Mt. Tan

540

840

Upflow

Outflow

0 metres 2000 4000

440 440

480

480

500

500500

520560

560

560580

580

580 580

600

600

600

620

620620

620

640

660

660

660

680700

700

700

720

740

760

800

800

840

860 720740

760

780

780800

820

580

760

Mt. Tan

540

840

0 metres 2000 4000

Most Prospective

Prospective

Development StrategyConceptual Model

22

3

Exploration Drilling Programme

440 440

480

480

500

500500

520560

560

560580

580

580 580

600

600

600

620

620620

620

640

660

660

660

680700

700

700

720

740

760

800

800

840

860 720740

760

780

780800

820

580

760

Mt. Tan

540

840

0 metres 2000 4000

Most Prospective

Prospective

Development Strategy

• Test surface exploration model - confirmation of resource extent and potential, at reasonable cost (ideally commercially productive).

• First well sited on basis of hydrological model, with clear objectives (e.g. test high temperature zone, permeability structure of the field)

• Outcomes /drilling strategy assessed (a) drill second / third well as planned(b) change strategy of next well,(c) postpone / abandon project.

• Drilling costs reduced by drilling :(a) shallow (“temperature gradient”) holes(b) slimholes (later drill full diameter wells)

23

Resource Evaluation Outputs

• Conceptual (hydrological, and geological framework) Model

• Assessment of Energy Reserve and Sustainable Resource Capacity.

• Steady State Model (if possible, based on well data).

• Models for various development scenarios (include effect of resource use on existing field activities and surface features).

24

Wairakei Geothermal Field

Tough2 gridNumerical (simulation) model

Geothermal Resources of New Zealand

Environmental EffectsLow Enthalpy Systems

SustainableDevelopment

Geomicrobiology

ResourceCharacterisation

NZ$4.4M/annumProgamme Leader: Greg Bignall

14

25

Summary

1. Design geoscience strategy that aids decision making.

2. Geoscience 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 geoscience advice early (and ongoing) has potential to save time, resources and money later ….

580

860

760

480

560

560580

580

580

600

600

620620

620

640

660

660

680700

720

760

800720 740

780820

Mt. Tan

540

246oC

840

440 440

480500

500 500

520 560600620

660

700

700

800840

760

780

800>300oC

176oC

Conceptual Model

26Greg Bignall ([email protected])

Dr. Greg Bignall

GNS ScienceWairakei Research CentreTaupo, NEW ZEALAND

http://www.gns.cri.nz

[email protected] THANK YOU

bignall - role of geoscience - nz · pdf filesilica sinter deposited by boiling, neutral,...

Documents