GNS Science

Rotorua Geothermal Field: The role of science on the development and recovery of the geothermal system

Rob Reeves Brad Scottr.reeves@gns.cri.nz b.scott@gns.cri.nz

20/11/2017

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Outline of talkContents

• Introducing the Rotorua Geothermal Field(RGF)– Location– Geothermal features

• Timeline

• Key scientific summaries

• Moving forward

• Lessons

Photo: B Scott, GNS Science

GNS Science

Outline of talkThe Rotorua Geothermal Field (RGF)

Modified from Reeves et al., 2014

• Heat from volcanic source (230k y caldera) • Geothermal surface area approx. 12 km2

• Rotorua city built over the geothermal field.

• Over 1500 geothermal surface features.• Contains some of NZ’s last remaining

geysers.• Strong IWI links to the geothermal activity.• Relatively easy access to hot water through

shallow drill holes (e.g. 100°C water < 100m deep).

• Geothermal resources used for:• Cultural• Tourism• Heating (domestic and commercial)• Pools (domestic and commercial)

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Outline of talkExamples of surface geothermal features in the RGF

From BOPRC (2012)

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Pre 1800 Maori use

geothermal for cooking,

bathing, heating

1920’sFirst wells

drilled

194450 wells drilled

1974Summary of RGF science

1970-1980 Surface feature decline

observed

1982Monitoring of

the RGF commences

1985500 wells

in use. MoE

Science Report

1987Bore

closure programme begins

1992Post bore-

closure scientific

programme

1999RGF

regional plan

operative

2005Compilation of scientific monitoring

RGF timeline

Photograph from Stewart (2017)

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Outline of talk1974 - 1st Scientific summary of Rotorua

• Who:– 1974 DSIR report - NZ Geological Survey, Geophysics

division DSIR, Chemistry Division DSIR• Purpose:

– Investigate the potential geothermal resources of the Rotorua Geothermal Field

• What:– Largely an exploratory report that included:

• Geology - surface geology, bore geology, structural assessments

• Geophysics – resistivity, gravity, magnetic, heat flow, ground temperature

• Geochemistry – bores, springs– This report identified potential environmental issues with

large-scale development at Rotorua

Report: DSIR (1974)From DSIR (1974)

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Outline of talkFactors effecting RGF management 1970-1980

• Scientific– Changes in geyser activity

• Pohutu Geyser eruptions smaller and shorter• Te Horu Geyser stopped

– Changes in hot spring behaviour• Water level decreases (overflows stop/decline)• Change of character (pH neutral to pH acidic)

– Estimated heat flow reduction at Whakarewarewa reduced by 30% between 1967 and 1984– Estimated number of production bores 376– No regulation of bore use

• Social– Observations from large scale developments e.g. Loss of geyser valley, Wairakei associated with

the Wairakei Geothermal power Station in the 1950-60’s– Competing priorities developing between bore users vs tourist ventures vs conservation

groups

But …lack of quantifiable data:– Observational data from scientists not up to public scrutiny.– Some patterns difficult to assess due to increasing urbanisation + altered water drainage patterns.– Poor understanding of natural variation vs exploration effect making interpretation difficult.

Therefore, intensive scientific data collection and monitoring programmes were established 1982-1985 via a taskforce and ultimately used as a basis for government intervention (MoE, 1985).

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Outline of talkMoE, 1985

• Who:– 1985 Technical report - DSIR, Ministry of Works, Ministry of Energy, NZ Geological

Survey, University of Auckland• Purpose:

A. To develop a model of the shallow sections of the Rotorua Geothermal System using quantifiable data from which management plans can be developed.

B. Identify ways of reducing geothermal aquifer draw-off• What:

– Groundwater monitoring of multiple geothermal aquifers (water level, temperatures, chemistry)– Geothermal surface features (temperatures, flows, chemistry, descriptions)– Geothermal bore data (including location, use and reinjection, temperatures, chemistry)– Hydrology (groundwater, streams, structural, rainfall)– Geophysics (resistivity, heat flow, levelling, )– Geology (bores)– Numerical model– Bore testing– Water energy use– Engineering design– Waste geothermal water disposal

Note that many of the science areas were able to compile limited historical data from previous work to demonstrate some time-series trends.

The taskforce report provided a clear link between the resource development and decline in surface geothermal activity and aquifer levels

Report: Ministry of Energy (1985)

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Outline of talkExamples of data in MoE (1985)

From Kissling (2014)

pH and water temperature data from surface features

Aquifer level data

Descriptive data from 1967-69 (A) and 1984-85 (B)

Surface feature data

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Outline of talkGovernment Intervention (1987)

MoE (1985) data confirmed that geothermal development had influenced surface geothermal features + aquifer levels.

Measures introduced:

– Local city control over geothermal bores (>70°C, >61 m deep) was revoked

– All geothermal bores within 1.5km of Pohutu geyser closed and grouted

– Closure of all geothermal bores owned by a government department in Rotorua

– Promoting reinjection of geothermal fluids to moderate depths (>50m)

– All bores to be licenced

– Royalty system for geothermal energy use introduced

– RGF plan 1999

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Outline of talkImmediate post bore closure: science 1985-1992

• Who – DSIR Geology and Geophysics/DSIR Chemistry

• Purpose– Update geothermal science knowledge after the bore closures and update conceptual models of the Rotorua

system using monitoring and new data.• What

– Geological history (units, ages, structure)– Geophysics

• Resistivity to delineate geothermal field boundaries• Seismic reflection to investigate faulting and lakebed structure.• Aerial thermal infrared• Gravity• Heat flow

– Geochemistry• Cl fluxes to determine geothermal flow and heat flux• Characterise hydrology (water/gas/isotopes)• Age dating• Soil gas flux

– Drill holes to test geology, downhole conditions– Numerical models

Report: Geothermics (1992)

Key points:– Multidisciplinary science using state-of the art techniques– Intensive studies– Collates monitoring data with 1-off surveys– Data from bores drilled as part of development provide critical scientific information

From Bradford (1992b)

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Outline of talkImmediate post bore closure: Key science points 1985-1992

• Detailed conceptual models of the RGF developed• Numerical model improved• Summary of monitored changes

– Pohutu, Kereru geysers - enhanced activity within 2 years of bore closures– Many hot springs in the northern part of the RGF show good recovery– Increase in flow and cl water from Kuirau Lake by 1988– Changes from sulphate water to chloride type water in features– Increase in heat flow, mass and chloride from Roto-a-Tamaheke

• Overall – monitoring suggests large-scale field recovery (<4 years), although not all surface features have recovered.

Cl flow from Lake Roto-a-Tamaheke

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Outline of talkPost bore closure: Science 1992 -2005

• Who – Bay of Plenty Regional Council

• Purpose– Summarise science conducted by BOPRC since 2001 on the RGF with a focus

to inform the RGF management plan • What

– Surface feature monitoring at selected features– Chemistry (selected bores and surface features)– Soil gas survey– Fluid use (locations, quantities)– Numerical model updated

• Found 1992-2001 was the greatest period of feature recovery.

• Not all features recovered at the same rates. Some did not recover.

• The field is generally stable in terms of aquifer levels at current extraction/reinjection rates.

Reports: BOPRC (2001, 2005), Scott and Cody (2000), Scott et al. (2005)

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Outline of talkRGF Water level data

Water levels from Kissling (2014)

64 m deep

157 m deep

75 m deep

Bore closures

Min level

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Outline of talkRGF Current status

• Regular monitoring– Water levels at dedicated monitor bores– Natural feature monitoring from “indicator” features– Reviews of the above data

• Ad-hoc surveys– Event response (e.g. anomalous geothermal activity)– Bore use (# bores + take)– Numerical model updates– Aerial thermal infrared– Heat flow– Surface feature surveys

• RGF Plan still in place (no update since 1999)

From Reeves et al (2014)

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Outline of talkMoving forward

• Science– RGF has recovered to a large extent from earlier development

– Data indicates that many water levels have stabilised in surface features and monitor bores

– Some changes in surface thermal features are still occurring

• Social and political– RGF plan up for review in 2018

– There is increasing pressure to use the geothermal resource for development

– Can we use more, and if so, how much?

Can the science answer these questions considering lessons learnt?

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Outline of talkRGF Lessons - 1

• The Rotorua Geothermal System largely recovered quickly from development once large-scale action was taken.

• Quality, time series data that could provide baseline conditions were critical in establishing trends.

• Establishing dedicated, long-term monitoring sites is needed for reliable, long-term data. This can be difficult in geothermal conditions.

• Complex hydrological models were developed from intensive scientific studies that can provide management options.

• Research findings were maximised by using data (e.g. down hole temperatures) obtained from developers and bore owners.

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• Ongoing monitoring of surface thermal features and groundwater systems are needed to quantify changes of the development regime.

• Be aware of the limitations of the science – e.g. small hydrological changes maybe within the errors of computer models.

• It would have been good to have linked the science to regulatory frameworks earlier e.g. mapping of geothermal surface features can be linked to environmental or building regulations.

• Having expert teams easily communicating will assist with issue identification and solving.

RGF Lessons - 2

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Outline of talkReferences

Bay of Plenty Regional Council, 2001. Rotorua Geothermal Field management monitoring. Environment BOP Environmental Report 2001/22. ISSN: 1172-5850

Bay of Plenty Regional Council, 2005. Rotorua Geothermal Field management monitoring Update:2005. Bay of Plenty Regional Council Environmental Publication 2005/12. ISSN: 1175-9372.

Bay of Plenty Regional Council, 2012. Guideline for mapping and monitoring geothermal features. Guideline 2012/03. ISSN: 1179 9595.

Department of Scientific and Industrial research, 1974. Geothermal Resources Survey Rotorua Geothermal District 1974. D.S.I.R Geothermal Report No. 6.

Geothermics, 1992. Special issue: Rotorua geothermal Field, New Zealand. Vol 21, No. 1/2.

Kissling, W. 2014. Rotorua Geothermal Bore Water level Assessment – 2014, GNS Science Consultancy Report 2014/199. 39 p. Accessed from https://www.boprc.govt.nz/media/395573/rotorua-geothermal-bore-water-level-assessment-2014.pdf on 4/8/2017.

Ministry of Energy, 1985. The Rotorua geothermal Field. Technical Report of the Geothermal Monitoring Programme 1982-1985. Ministry of Energy.

Reeves, R.R.; Scott, B.J.; Hall, J. 2014. 2014 Thermal infrared survey of the Rotorua and Lake Rotokawa-Mokoia Geothermal Fields, GNS Science Report 2014/57. 28 p

Scott, B.J., Cody, A. D., 2000. Response of the Rotorua geothermal system to exploitation and varying management regimes. Geothermics 29 (4/5), pp 573-592.

Scott, B.J., Gordon, D. A., Cody, A. D., 2005. Recovery of Rotorua geothermal field, New Zealand: progress, issues and consequences. Geothermics 34 (2), pp 159-183.

Stewart, C, 2017. 'Geothermal energy - Heat from the earth', Te Ara - the Encyclopedia of New Zealand, http://www.TeAra.govt.nz/en/photograph/5419/maori-women-cooking-at-whakarewarewa (accessed 7 August 2017)