GNS Science

Magmatic - Volcanic Geothermal Systems

Andrew Rae and Greg BignallDepartment of Geothermal Sciences

Wairakei Research Centre, GNS ScienceNEW ZEALAND

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• Adaptation of Presentation by Andrew Rae at Western Pacific Branch of IGA, Taiwan, 2016

• Geothermal Systemso New Zealand and Indonesia

• Surface Features

• Geochemistry: discriminatory diagrams

• System examples

• Hydrothermal Mineralogy

• Drilling & Engineering Considerations

Outline

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Island Arc Type• Andesitic volcanism• Shallow magmatic heat source• Extensive zones of acid fluids, shallow

and deep, two possible sourcese.g.: Indonesia; Philippines

Types of Volcanic Geothermal Systems

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Acid fluids: shallow, supergene, “steam-heated”, H2S oxidation

Acid fluids: deep, magmatic, hypogene, HCl – SO2 dissociation

Continental Type• Silicic volcanics• Deep magmatic heat source• Relatively restricted acid fluids,

shallow, one sourcee.g.: NZ (TVZ); USA (Yellowstone);

Kenya (E Africa Rift)

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Rotokawa Geothermal System

• Cool inflows from surface, at ~500 mRSL, via silicic lavas.

• Inflows discrete from deeper production, due to occurrence of impermeable ignimbrite.

• Downhole temperatures reveal >250°C, deep-sourced fluids “leak” through the ignimbrite.

• As hot fluids ascend they boil create a two-phase zone, with CO2 and H2S to steam phase.

• Condensation of gases in the groundwater creates acidic sulphate and CO2-rich fluids.

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Hot Hole Casing Corrosion logs for RK5 (340 – 358m MD), Sep. 1993 to April 2005. Phase shifts, measuring mass of metal are offset for each run, indicates metal loss.

• Severe external casing corrosion in several wells, prior to and following development of Rotokawa for electrical power production.

• We modelled formation of corrosive fluids in the shallow injection aquifer from alteration mineralogy, microgravity, fluid chemistry and corrosion product studies.

• Occurrence of CO2-rich/bicarbonate fluids supported by analysis of casing, cement samples and downhole fluid chemistry.

Rotokawa Geothermal System

Formation and Neutralisation of Corrosive Fluids in the Shallow Injection Aquifer, Rotokawa Geothermal Field, New Zealand

Bowyer, Bignall and Hunt. GRC Transactions, 2008

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Alteration mineralogy 250– 650m MD in RK9 and RK14. Zones of corrosion in RK9 indicated.

Rotokawa Geothermal System

• Occurrence of kaolinite, alunite, dickite and goethite in cuttings reveals presence of acid sulphate and CO2-rich/bicarbonate fluids.

• Ascent of >250°C reservoir fluids, boiling and condensation of CO2 and H2S into the cool, shallow groundwater aquifer.

• Microgravity survey supports two-phase zone in shallow aquifer prior to the start of production, and “neutralisation” by shallow injection during first 8 years of production, slowing rate of casing corrosion.

• Model for the formation and neutralisation of corrosive fluids vital tool in the management of the Rotokawa Geothermal Field.

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Ketetahi Springs, Mt. Tongariro

White IslandAndesite-hosted geothermal systems in Taupo Volcanic Zone

New Zealand Magmatic – Volcanic Systems

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Sibayak

Karaha–Telaga Bodas

Tangkuban Perahu

Patuha

Dieng–Sikidang

Ulubelu

Lahendong

Indonesian Magmatic-Volcanic Systems

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Hochstein and Sudarman (2008): 6 system ‘types’, dependent on presence/absence of vapour zones, magmatic components

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Tangkuban Perahuvolcano crater

Source: Google Earth

Indonesian Magmatic-Volcanic Systems

– volcanic summit: warm acid crater lake (pH<1-3); fumaroles; solfatara; acid springs; steam vents (superheated, possible SO2 gases) and steaming ground

– volcano flanks: thermal springs (acid to neutral, Cl-SO4-HCO3)

Surface Features

Sibayak volcano crater

Sibayak volcano

Patuha crater lake

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Discriminatory DiagramsClassification: Cl-HCO3-SO4

System Characterisation / Chemistry

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Giggenbach (1988)10B

Cl/10

10SO4

TauharaRelative B-Cl-SO4

TH1 (1964)TH1 (1967)TH2TH2 (1967)TH3 (1967)TH6TH9TH10TH11TH12TH13TH14

WairakeiRelative B-Cl-SO4

WK26BWk30Wk105Wk243Wk245Wk247

Relative B-Cl-SO4RotokawaOhaaki

Wairakei

TauharaCl/B

20

30

40

56

80

1960's

Cl

SO4 HCO3

surficial acid waters

peripheral waters

acid magmatic

waters

primary chloride brines

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• active volcano (1881 eruption)• neutral and acid waters and fumaroles (SO2, HCl) in crater area• acid and neutral springs - indicate disequilibria• highest T and well productivity close to crater; some magmatic gases

Hochstein and Sudarman (2015)

Sibayak Geothermal System (13 MWe installed)

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Lawless and Gonzalez (1988)Suparno et al. (2010)

Sibayak – DC Resistivity

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Moore et al. (2008)

MT profile

250° isotherm

250° isotherm + epidote

Karaha-Telaga Bodas Geothermal System

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0.25 mm

0.25 mm

Habit of hydrothermal aluniteAbove: supergene, shallow alunite, pseudo-cubic habitBelow : hypogene, deep alunite, tabular habit

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• understand the nature of acidity (genesis), fluid chemistry, pH, temperatures;

• understand reservoir geohydrology and where acidity occurs;

• avoid if possible

Drilling Considerations

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• particularly aggressive fluids;

• hard on drilling equipment, casings, liners, master valve;

• necessary requirement for alloys, stainless steel, titanium?, sacrificial casing?

• expenses can quickly escalate, and may jeopardise project economics.

• even if aggressive acidity is avoided, be aware well testing and steam production can draw acid fluid into the part of the reservoir encountered by the well.

Drilling Considerations (pH < 5)

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kmLahendong (after Hochstein and Sudarman, 2015)

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• volcanic gases SO2, H2S, HCl, HF: corrosive, aggressive;

• corrosion rates can be controlled;

• sulphide corrosion products form protective coating;

• presence of O2, changes corrosion products from sulphides to non-protective oxides.

Sanada et al. (2000)

Engineering Considerations

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• Island arc magmatic setting is potential host of high temperature geothermal systems– potential for geothermal power development

• Genetic conceptual models developed– commonalties: heat source, chemistry, fluid temperatures, central

magmatic-dominant conduits, peripheral meteoric-dominant regions

– also unique qualities regarding the geohydrology (temperature, permeability, chemistry)

• Magmatic acid zones commonly constrained to narrow, central permeable conduits

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Key Points

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• Meteoric-dominant peripheral neutral pH fluids– may have T and permeability suitable for utilisation– however, permeability typically lower; steep T decreasing gradient

• Need for unique conceptual models for each prospect– based on rigorous multidisciplinary geoscience – geochemistry,

geology, geophysics

• Drilling and production– acidity (pH<5) can be managed (stainless steel, chemical dosing)– acidity (pH<3), very difficult to manage– need to think of long-term management: fluid withdrawal during

production can draw acid fluids (meteoric or magmatic) into the reservoir

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Key Points

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Acknowledgements:Keith Lichti, corrosion consultant

Manfred Hochstein, geothermal scientistHagen Hole, geothermal drilling engineer

Thank you

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