Magmatic - Volcanic
Geothermal Systems

Andrew Rae and Greg Bignall
Department of Geothermal Sciences
Wairakei Research Centre, GNS Science
NEW ZEALANDGNS ScienceOutline• 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

2Department of Geothermal Sciences

GNS Science

Types of Volcanic Geothermal Systems
Continental Type• Silicic volcanics• Deep magmatic heat source• Relatively restricted acid fluids,shallow, one source

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

e.g.: NZ (TVZ); USA (Yellowstone);
Kenya (E Africa Rift)

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

Island Arc Type• Andesitic volcanism• Shallow magmatic heat source• Extensive zones of acid fluids, shallow
and deep, two possible sources
e.g.: Indonesia; Philippines
3

Department of Geothermal Sciences

GNS ScienceRotokawa Geothermal System• Cool inflows from surface, at~500 mRSL, via silicic lavas.
• Inflows discrete from deeperproduction, due to occurrenceof 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 thegroundwater creates acidic
sulphate and CO2-rich fluids.
4

Department of Geothermal Sciences

GNS ScienceRotokawa Geothermal System• Severe external casing corrosion in several
wells, prior to and following development of
Rotokawa for electrical power production.
• We modelled formation of corrosive fluids inthe shallow injection aquifer from alteration
mineralogy, microgravity, fluid chemistry andcorrosion product studies.
• Occurrence of CO2-rich/bicarbonate fluids
supported by analysis of casing, cement
samples and downhole fluid chemistry.

Hot Hole Casing Corrosion logs for RK5(340 – 358m MD), Sep. 1993 to April
2005. Phase shifts, measuring mass ofmetal are offset for each run, indicates
metal loss.
5

Department of Geothermal Sciences

Formation and Neutralisation of Corrosive Fluids in the Shallow
Injection Aquifer, Rotokawa Geothermal Field, New ZealandBowyer, Bignall and Hunt. GRC Transactions, 2008
GNS ScienceRotokawa Geothermal System• Occurrence of kaolinite, alunite, dickite andgoethite in cuttings reveals presence of acidsulphate and CO2-rich/bicarbonate fluids.
• Ascent of >250°C reservoir fluids, boilingand condensation of CO2 and H2S into thecool, shallow groundwater aquifer.
• Microgravity survey supports two-phasezone in shallow aquifer prior to the start ofproduction, and “neutralisation” by shallow
injection during first 8 years of production,slowing rate of casing corrosion.
• Model for the formation and neutralisation ofAlteration mineralogy 250– 650m MD in RK9corrosive fluids vital tool in the management
and RK14. Zones of corrosion in RK9of the Rotokawa Geothermal Field.
indicated.
6

Department of Geothermal Sciences

GNS ScienceNew Zealand Magmatic – Volcanic Systems
Andesite-hosted geothermal
systems in Taupo Volcanic Zone

White Island

Ketetahi Springs, Mt. Tongariro
7

Department of Geothermal Sciences

GNS ScienceIndonesian Magmatic-Volcanic Systems
Hochstein and Sudarman (2008): 6 system‘types’, dependent on presence/absenceof vapour zones, magmatic components

Lahendong

Sibayak

Tangkuban Perahu
Dieng–Sikidang

PatuhaKaraha–Telaga Bodas

Ulubelu
8Department of Geothermal Sciences

GNS ScienceIndonesian Magmatic-Volcanic Systems
Sibayak volcano

Surface Features
Sibayak volcano
crater– volcanic summit: warmacid crater lake (pH<1-3);
fumaroles; solfatara; acidsprings; steam vents(superheated, possible
SO2 gases) and steaming
ground– volcano flanks: thermal
springs (acid to neutral,Cl-SO4-HCO3)
Patuha crater lake

Tangkuban Perahu
volcano crater

Source: Google Earth9

Department of Geothermal Sciences

GNS ScienceSystem Characterisation / Chemistry
Discriminatory Diagrams
Classification: Cl-HCO3-SO4Cl

primary
chloridebrinesacidmagmaticwaters
surficialacid waters

TauharaRelative B-Cl-SO4

peripheralwaters

TH1 (1964)
TH1 (1967)
TH2
TH2 (1967)
TH3 (1967)
TH6
TH9
TH10
TH11
TH12
TH13 3
TH14

Wairakei
Relative B-Cl-SO4
WK26B
Wk30
Wk105
Wk243
Wk245
Wk247

Cl/10

Relative B-Cl-SO4
Rotokawa
OhaakiHCOSO4

Wairakei

80

Cl/B
1960'sTauhara

56
4030
20

Giggenbach (1988)
10SO4

10

Department of Geothermal Sciences

10B

GNS ScienceSibayak Geothermal System (13 MWe installed)

Hochstein and Sudarman (2015)

••••
11

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

Department of Geothermal Sciences

GNS ScienceSibayak – DC Resistivity

Suparno et al. (2010)
Lawless and Gonzalez (1988)
12Department of Geothermal Sciences

GNS ScienceKaraha-Telaga Bodas Geothermal System

MT profile
0.25 mm

250° isotherm

Habit of hydrothermal alunite
Above: supergene, shallow alunite,pseudo-cubic habit
Below : hypogene, deep alunite,tabular habit

250° isotherm + epidote
Moore et al.0.25(2008)mm

13

Department of Geothermal Sciences

GNS ScienceDrilling Considerations• understand the nature ofacidity (genesis), fluid
chemistry, pH, temperatures;• understand reservoir
geohydrology and where
acidity occurs;• avoid if possible

14

Department of Geothermal Sciences

GNS ScienceDrilling Considerations (pH < 5)
• particularly aggressive fluids;• hard on drilling equipment,
casings, liners, master valve;• necessary requirement foralloys, stainless steel,titanium?, sacrificial casing?
• expenses can quickly
escalate, and may jeopardise
project economics.

kmLahendong (after Hochstein and Sudarman, 2015)• even if aggressive acidity is avoided, be aware well testing andsteam production can draw acid fluid into the part of the reservoir
encountered by the well.
15

Department of Geothermal Sciences

GNS Science

Engineering Considerations• volcanic gases
SO2, H2S, HCl, HF:
corrosive, aggressive;• corrosion rates canbe controlled;• sulphide corrosion
products form
protective coating;• presence of O2,changes corrosion
products fromsulphides to nonprotective oxides.
Sanada et al. (2000)
16

Department of Geothermal Sciences

GNS ScienceKey Points• Island arc magmatic setting is potential host of high temperaturegeothermal systems– potential for geothermal power development• Genetic conceptual models developed– commonalties: heat source, chemistry, fluid temperatures, central
magmatic-dominant conduits, peripheral meteoric-dominantregions– also unique qualities regarding the geohydrology (temperature,permeability, chemistry)• Magmatic acid zones commonly constrained to narrow, centralpermeable conduits

17

Department of Geothermal Sciences

GNS ScienceKey Points• 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 thereservoir
18

Department of Geothermal Sciences

GNS ScienceThank you

Acknowledgements:
Keith Lichti, corrosion consultant
Manfred Hochstein, geothermal scientist
Hagen Hole, geothermal drilling engineer

19

Department of Geothermal Sciences

GNS Science