Global occurrence of geothermal systems in different ... · Geothermal Systems, Settings,...

Global occurrence of geothermal systems in different geologic settings: their

identification and utilization

Mar-2016

William Cumming

Cumming Geoscience, Santa Rosa CA [email protected]

Office: +1-707-546-1245 Mobile: +1-707-483-7959 Skype: wcumming.com

Geothermal Systems, Settings,

Identification and Utilization

Agenda

Global Geothermal Occurrence of Geothermal Systems

• Thermodynamics implications for geothermal systems

• Classifications of geothermal systems

• What type of system is characteristic of the Western EARS?

• Implications of fault –based systems for resource capacity

• Implications of fault-based systems for Western EARS

• Exploitation options for fault-hosted geothermal development

• Priorities for volcano-hosted and fault-hosted exploration

• Conceptual model elements

© Cumming (2013)



Geothermal Geoscience Conceptual Context

Basic physics of permeable geothermal reservoirs (non-EGS)

• Geothermal reservoirs lose energy to surface through any rock by heat conduction and through leaky rocks by buoyant advection of hot fluid

• In proportion to stored energy, a geothermal reservoir emits energy at a rate orders of magnitude higher than O&G reservoirs

• The geothermal emphasis on “seeps” does not indicate primitive technology relative to O&G but a difference in resource physics

Implications for geothermal exploration strategy

• Geothermal reservoirs with vertical permeability “leak” heat upward, so “hidden” systems without near-surface manifestations are “special”

• Most cost-effective reduction of risk for geothermal resource with thick vertical permeability is to demonstrate permeability and temperature using water chemistry, if not from springs then from shallow wells

© Cumming (2013)



Geochemistry and Resistivity Methods Dominate Exploration For Permeable Geothermal Reservoirs

Why?

• All geothermal systems with extensive vertical permeability leak hot water or heat to surface or near-surface

• For leaky systems, geochemistry cost-effectively indicates

• Likelihood of economic temperature?

• Significant permeability at that temperature?

• Resistivity detects the clay cap, cost-effectively answering

• What is the geometry of the reservoir top?

• How big might the reservoir be?

• What well casing design is optimum?

© Cumming (2013)



Geothermal Resource Setting

Moeck (2015, Geothermics) Geologic setting • Divergent (rift) • Convergent (arc) • Transform (pull-apart) • Major volcanism • Intracontinental rifts Moeck play type • Magmatic volcanic • Magmatic plutonic • Extensional • Non-convecting plays Others argue • >230°C flash • <180°C pumped • 150 to 230°C gassy

flash



Geothermal Resource Power Density



Best Geothermal Prospects • Regional correlation with arcs,

rifts, transform step-over extension, Recent volcanics, magma detected at depth by imaging, radiogenic heat flow, earthquakes, etc

• Power density >20 MW/km2 correlates generally with Recent volcanics and specifically with water and/or gas geochemistry >240°C

• Hinz “Extension rate decisive”

• < 240°C common and can be “hidden”, so look for extension

U. Wisconsin, 2015



East versus West EARS Exploration

• Eastern EARS • Gas chemistry shows >230°C on most

current prospects

• Most targets volcano-hosted with significant Recent volcanism

• Recent volcanics tend to be thick

• Some reservoirs thinned by shallow magma and shallow 350°C isotherm (silica solubility)

• MT-TEM images clay cap with ambiguity from noise and limited smectite alteration in phonolite and trachyte

• Targeting general fault geometry favorable to “sweet spots” in distributed permeability of large reservoirs undergoing rapid extension

Cumming Geoscience

• Western EARS • Water and gas usually ambiguous but

more reliable indicators show <180°C

• Many targets more closely associated with structure than Recent volcanics

• Recent volcanics thin or absent

• Most Recent volcanics indicate deep eruption source, not shallow residence

• MT+TEM images possible clay cap and unsuitable host rocks - limited by noise and access

• Targeting specific fault geometry favorable to relatively low volume flow path in areas undergoing slower extension



If West EARS is Fault-Based Geothermal Exploration and Development Strategy

• Find locations with a coincidence of: – Relatively high temperature shallower than 5 km depth – Tectonic setting with a high rate of extension favorable to open-space fracturing – Rocks susceptible to open-space fracture permeability

• Targeting risk – Most affected by hot water and/or gas with suitable chemistry – High rate extension but faults are not enough, geometry and formation properties must be favorable – If no springs, TGW best indicator of permeability hosting hot water flow

• Mostly <180°C so pumped/gas-flash production to binary generation



East EARS Development Analogy

Melosh (2013, GEA)

5 - 7 km

2 -

3 k

m

350°C



Awibengkok Geothermal Field

Melosh (2013, GEA)

Awibengkok 377 MW



West EARS Development Analogy

12

Geothermex (2008)

BRADYS CROSS-SECTION

2 km

1 k

m



Bradys Geothermal Field

13 Geothermex (2008)

Bradys 15 – 20 MW



East versus West EARS Development Analogies

Melosh (2013, GEA)

Bradys 15 – 20 MW Awibengkok 377 MW



Western EARS Options Geothermal Exploration and Development

• Consider full range of conceptual models, including fault-based as well as volcanic-associated models

• Use several resource conceptual models to illustrate uncertainty with focus on structural target in suitable rocks

• Use TGWs or slim holes to test multiple models at lower cost

• Investigate acquisition of more complete versions of data collected for other purposes, including reflection seismic, airborne gravity , regional MT etc

• Assess viability of outcomes consistent with resource types likely to be found

Cumming Geoscience



“Conventional” Power Plants Dry Steam Single Flash

750 to 1300 MW Geysers Lund and Dipippo, 2003



Binary Power Plants

1 MW Wabuska 38 MW Puna

• Higher cost per MW • Modular and faster to install • Scalable 0.2 to 200 MW • Used for all pumped production

(<180°C) • Used for special cases of flash

production (>220°C) • Pumped production requires

economies-of-scale to support pump replacement

Lund and Dipippo, 2003



Geothermal Power Plant Types




Direct and Cascade Geothermal Uses

• Reservoirs shallower and lower temperature, 21-149°C, are used directly

• Spas, greenhouses, fish farms, industry and space heating

• Direct use since pre-history, power production 110 years

• Technology can add value, • Existing users often resist

sharing resource unless low impact can be guaranteed

• Cascaded use sometimes possible, but depends on chemistry, typically using heat exchange

GeoHeat Center, 2005



Direct and Cascade Geothermal Use

• Geothermal use is temperature dependent

• Heat transportation limited – longest in world is in Iceland, where high volume of 90°C water piped 27 km from Nesjavellir Field heat exchanger to Reykjavik for space heating

• Local resources must fit local needs

• For example, oil and mining applications must be near

• Cascaded use sometimes possible, but depends on chemistry




Agriculture and Aquaculture • Increasing growth rate of

flowers, vegetables, and other crops in greenhouses in cool climates

• Increasing growth rate and shorten time to maturity of fish, shrimp, abalone and alligators


http://geothermal.marin.org/GEOpresentation/sld081.htm



Hot Spring Bathing and Spas (Balneology)

• Since pre-history, all regions in which hot springs are found have used them for bathing

• Japan is world leader in balneology, with Beppu alone having 4,000 hot spring baths serving 12 million tourists a year

• Romans built cities around baths supplied by hot springs, such as the city of Bath, still a major tourist center



Svartsengi Geothermal Field

• 77 MWe power generation • 90 MWt district heating of Reykjanes suburbs of Reykjavik • Blue Lagoon tourist attraction




Geothermal Heat Pumps

• “Geothermal Heat Pump” also called “Ground Source Heat Pump” is not extracting heat from earth

• It makes energy use more efficient by heating or cooling more efficiently using the better heat exchange possible in saturated ground as opposed to in the air, as is done in conventional air conditioners.

• Ground source heat pumps are usually most effective in areas that use both heating and cooling cycles, that is, hot summers and cold winters

• There is almost no commonality between extractive and heat pump technologies




Exploration Data to Assess Geothermal Resource Uncertainty

Is it there? POSexpl = Ptemperature * Pchemistry * Ppermeability

• Temperature: Water and gas geochemistry on all features • Chemistry: Same as temperature but using process plots

• Permeability: Resistivity imaging to base of impermeable clay cap. Structural model. Map of thermal features and altered ground. Gravity and structure.

If yes, how big is it? P10, P50, P90 area • Area: Conceptual model outlines from resistivity, geochemistry, alteration,

structure, geology etc. • Power Density: Analogous fields, plausible MW/km2 • Field analogies provide check on probabilistic approaches

What is lowest cost exploration strategy • Lowest cost well target order to failure or success

• Access and hazards: Review access and hazards

• Environmental etc: Assess risk for permit denial etc.

© Cumming (2013)



Geothermal Exploration of Rift and Arc Systems Top Data Priorities For Decision Risk Analysis

Base Maps Digital high resolution satellite 50 cm color images georectified and integrated with

an SRTM DEM. In some areas, Google Earth is adequate.

Digital topographic maps with culture at resolution better than 1:50000

All images and maps should be in UTM projection with datum specified

Old Boreholes Temperature logs, water/gas, cuttings descriptions, cuttings clay analyses

Geochemistry Water chemistry of all hot springs and water wells and gas chemistry of all

fumaroles, acid-sulfate features and boiling springs – support with CO2 flux

Active Alteration Based on visual review of <50 cm color images, identify candidate features and

ground check for alteration type and surface temperature

Geology Standard exploration geology: volcanic history, expected reservoir rock, formation

map, heat source, exposed brittle rocks like rhyolite dome as cold water source,

basin structure, sediments, hydrothermal processes, alteration mapping (not just

active alteration detection), eruption breccias, etc.

MT (+-TEM) Remote reference robust MT from .01-300 Hz, better .001-10000 Hz

QA using D+ editing, 1D Occam inversion AVG, likely 3D inversion

Add TEM if likely to detect top of conductor at <30% cost increment

Structure Map lineaments, ground check geometry and rate, review evidence of extension.

Implications for permeability of the interaction of structure with reservoir leakage,

vertical stress, formation properties, alteration, irregularity, etc.

MEQ Microearthquake monitoring for >3 months to detect magma below active basalt

shield or for hazard assessment of an active andesite volcano (<4 to >100 Hz)

GPS + DEM GPS makes other methods more cost-effective if quality is controlled.

Check datum and coordinate quality. Use DEM or dGPS for elevation. Cumming ©2013



Geothermal Exploration of Fault-Based Systems Top Data Priorities For Decision Risk Analysis

Base Maps Digital high resolution satellite 50 cm color images georectified and integrated with

an SRTM DEM. In some areas, Google Earth is adequate.

Digital topographic maps with culture at resolution better than 1:50000

All images and maps should be in UTM projection with datum specified

Old Boreholes Temperature logs, water/gas, cuttings descriptions, cuttings clay analyses

Geochemistry Water chemistry of all hot springs and water wells and gas chemistry of all

fumaroles, acid-sulfate features and boiling springs – support with CO2 flux

Active Alteration Based on visual review of <50 cm color images, identify candidate features and

ground check for alteration type and surface temperature

Structure Map lineaments, ground check geometry and rate, review evidence of extension.

Implications for permeability of the interaction of structure with reservoir leakage,

vertical stress, formation properties, alteration, irregularity, etc.

Geology Standard exploration geology: volcanic history, expected reservoir rock, formation

map, heat source, exposed brittle rocks like rhyolite dome as cold water source,

basin structure, sediments, hydrothermal processes, alteration mapping (not just

active alteration detection), eruption breccias, etc.

MT (+-TEM) Remote reference robust MT from .01-300 Hz, better .001-10000 Hz

QA using D+ editing, 1D Occam inversion AVG, likely 3D inversion

Add TEM if likely to detect top of conductor at <30% cost increment

TGW (or slim hole) If the geochemistry or old boreholes do not indicate commercial temperature

Gravity +- magnetics Gravity maps structure if geometry simple, like sediments over crystalline rocks

Incidental If available, use aeromagnetic and reflection seismic data – but probably not cost-

effective to acquire. Cumming ©2013



Western EARS Options Geothermal Exploration and Development

• Consider full range of conceptual models, including fault-based as well as volcanic-associated models

• Use several resource conceptual models to illustrate uncertainty with focus on structural target in suitable rocks

• Use TGWs or slim holes to test multiple models at lower cost

• Investigate acquisition of more complete versions of data collected for other purposes, including reflection seismic, airborne gravity , regional MT etc

• Assess viability of outcomes consistent with resource types likely to be found

Cumming Geoscience



Generic Geothermal Conceptual Model Elements

29

Distributed Permeability Upflow Small Outflow

Single Fault Zone Upflow Large Shallow Outflow

Cumming (2013)



Geothermal Conceptual Model Elements

• Hydrology, especially deep water table but also perched aquifers • Isotherm pattern consistent with pressure and permeability • Heat Source

• Deep benign hot buoyant upflow in fractures

• Formations and alteration favorable to open space fracture permeability (and often primary permeability at shallower depths)

• Smectite Clay Cap (commonly combined cap, rarely, non-smectite cap)

• Faults creating permeable zones, flow barriers and field boundaries

• Reservoir temperature outflow with buoyant flow updip below clay cap (in liquid systems)

• Sub-commercial outflow with buoyant flow updip below clay cap

• Cold meteoric water flow down-dip into reservoir

© Cumming (2013)



Anomaly Hunting

• Rationale • Works by analogy

• Pitfalls • Conceptual relevance to new targets not

considered, just outcomes

• Other data not conceptually integrated

• Not directly tested by wells

• Drill a 5 ohm-m anomaly and it remains 5 ohm-m

• Remedy • Use for early and low cost decisions

• For high cost decisions, use conceptual models

to support team risk assessment

© Cumming (2013)



Conceptual Models

• Rationale • Decisions based on analogous experience

• Conceptual differences considered

• Directly tested by wells

• Pitfalls • Who can integrate geophysics, geochemistry,

geology, reservoir engineering …

• A single model is always wrong

• Multiple models require risk assessment

• Proposed Remedy • Training on building conceptual models and

assessing risk using case histories Cumming Geoscience



Generic Geothermal Conceptual Model Elements

33

Distributed Permeability Upflow Small Outflow

Single Fault Zone Upflow Large Shallow Outflow

Cumming (2013)



Geothermal Conceptual Model Definition by Isotherm Pattern

• The natural state isotherm pattern is essential component of a geothermal resource conceptual model and the initial constraint for numerical reservoir simulations.

• For simulation models, the isotherms constrain: • the overall reservoir permeability pattern, bulk permeability values and boundary conditions

• the thermodynamic state of the reservoir – including phase (boiling), upflow, outflow etc

• The isotherm pattern is constrained by the geoscience data and is mutually constrained by the interpreted reservoir elements:

• Isotherms arch upwards where water, gas and heat leak through the clay cap to surface manifestations based on surface temperature, geochemistry, alteration and resistivity

• Isotherms are close together in the clay cap due to the impermeable clay from resistivity imaging separating cool meteoric water from buoyantly rising hot reservoir water

• In systems interpreted to be hydrostatically pressured (water dominated) based on geochemistry and resistivity, the resistivity and/or geochemistry should detect a buoyant outflow that should be correlated with laterally elongated isotherms dipping upward

• The isotherm pattern differs for distributed permeability and single fault-hosted systems

• Arrows aid isotherm interpretation and are essential for out of section flow description

© Cumming (2013)







• Smectite Clay Cap (commonly combined cap, rarely, non-smectite cap, very rarely for commercial systems, uncapped)


• Reservoir temperature outflow with buoyant flow updip below clay cap



© Cumming (2013)



Geothermal Permeability

Davatzes et al (2010) and Lutz et al. (2010)



Smectite Clay Interpretation Model for Resistivity in a Geothermal Context

• Hydrated smectite alteration is created over almost all Arc and Rift geothermal systems due to gas loss from hot water and it is deposited in sedimentary shales and siltstones

• Hydrated smectite causes low bulk permeability

• Hydrated smectite causes the lowest resistivity detected in all commercial geothermal systems.

• Archie’s Law is for clay-free rock. Assumption that low resistivity implies high temperature is incorrect in a geothermal context

• Smectite is temperature sensitive, converting to illite clay at higher temperatures and is complete near 200-250°C

• Low resistivity correlates with low permeability “cap” over high resistivity, high permeability, high temperature reservoir

© Cumming (2013)



“Standard” Geoscience Plan >200°C Arc or Rift Geothermal Exploration

• Gas and fluid geochemistry for existence and conceptual target

• MT to map base of clay “cap”

• Maybe TEM for MT statics

• Geology, alteration and structure for context

• Shallow hydrology for context

© Cumming (2013)

Global occurrence of geothermal systems in different geologic settings: their

identification and utilization

Mar-2016

William Cumming

Cumming Geoscience, Santa Rosa CA [email protected]

Office: +1-707-546-1245 Mobile: +1-707-483-7959 Skype: wcumming.com

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