Geothermal Sustainability A Review with Identified Research Needs

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Geothermal Sustainability- A Review
withIdentified Research Needs

• Ladislaus Rybach
• Vice Chairman IEA GIA and
• GEOWATT AG, Zürich, Switzerland
• and
• Mike Mongillo
• Secretary, IEA-GIA and
• GNS Science, Wairakei, New Zealand

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IEA GEOTHERMAL IMPLEMENTING AGREEMENT (GIA)

• Framework for international geothermal
  cooperation under auspices of IEA.
• Began operation 7 March 1997.

• Members include the EC and
• 10 Countries Australia, Germany, Iceland, Italy,
  Japan, Mexico, New Zealand, Republic of Korea,
  Switzerland, the United States and the EC.
• 3 Sponsors Geodynamics Limited, Green Rock
  Energy Limited, ORMAT Technologies, Inc.
• Mission To advance and support the use of
  geothermal energy on a worldwide scale by
  overcoming barriers to its development.
• Major Objectives
• Compile and exchange information on worldwide
  geothermal energy research and development
  concerning existing and potential technologies
  and practices
• Develop improved technologies for geothermal
  energy utilization
• Improve the understanding of the environmental
  benefits of geothermal energy and ways to avoid
  or mitigate environmental impacts

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IEA GIA (contd)

• Current Research
• Annex I Environmental Impacts of Geothermal
  Development
• Annex III Enhanced Geothermal Systems
• Annex IV Deep Geothermal Resources
• Annex VII Advanced Geothermal Drilling
  Techniques
• Annex VIII Direct Use of Geothermal Energy
• Membership Benefits
• Increases RD capabilities through combined
  effort of several nations and industries
• Provides appropriate focus for RD, hence avoids
  costly duplication
• Improves RD cost effectiveness by sharing
  research costs and pooling of skills, knowledge
  and financial resources
• Regular scientific interaction and opportunity
  review current issues, ongoing research and need
  for future research
• Helps develop technical standards and
  methodologies
• Provides impartial information to help guide
  national policies and programmes

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Outline

• Introduction
  
• Renewability and Sustainability
• Effects of Production from a Geothermal Reservoir
• Geothermal Regeneration Time- Scales
• Key Issue - The Sustainable Production Level
• Research Needs
• Conclusions

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Introduction

• Ultimate source of geothermal energy is the
  immense heat stored within the earth.
• 99 of the earths volume has temperatures gt1000
  C
• Only 0.1 at temperatures lt100 C
• The total heat content estimated to be 1031 J.
• Take over 109 years to exhaust via global
  terrestrial heat flow
• The internal heat of earth mainly provided by
  decay naturally occurring radioactive isotopes.
• At rate of 860 EJ/y
• About twice worlds 2004 primary energy use of
  463 EJ/y
• Thus, the geothermal resource base is
  sufficiently large and basically ubiquitous.

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Renewability and Sustainability (1)
Renewability

• Geothermal generally classified as a renewable
  resource, therefore included with such
  alternative energy sources as
• Solar
• Biomass
• Wind
• Renewable describes attribute of energy resource
• the energy removed from a resource is
  continuously replaced by more energy on time
  scales similar to those required for energy
  removal and those typical of technological/societa
  l systems (30-300 years) rather than geological
  times

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Renewability and Sustainability (2)
Sustainability (1)

• Original definition of sustainable from
  Brundtland Commission Report (1987, reinforced at
  Rio 1991, Kyoto 1997 and Johannesburg 2002 UN
  Summits)
• development that meets the needs of the
  present without compromising the ability of
  future generations to meet their own needs

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Renewability and Sustainability (3)
Sustainability (2)

• Definition of sustainable production from a
  geothermal system (Axelsson et al, 2001)
• For each geothermal system, and for each mode
  of production, there exists a certain level of
  maximum energy production, below which it will be
  possible to maintain constant energy production
  from the system for a very long time (100-300
  years)
• This applies to total extractable energy (heat in
  the fluid and rock) and depends upon
• Nature of system
• Not on load factors or utilization efficiency
• Not consider economic aspects, environmental
  issues or technological advances.

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Effects of Production (1)

• Geothermal resources commonly used by withdrawing
  fluid and extracting its heat.
• Balanced fluid/heat production, i.e. not
  producing more than the natural recharge
  re-supplies is fully sustainable.
• These rates are limited and often NOT economical
  for use
• High production rates exceeding long-term rate of
  recharge can lead to depletion, especially of
  fluid content, with most of heat remaining in
  rock matrix.
• Therefore apply reinjection, which replenishes
  fluid and helps to sustain or restore reservoir
  pressure

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Effects of Production (2)

• Excessive production often used to meet economic
  goals like quick payback of investments for
  exploration and equipment, resulting in reservoir
  depletion.
• The Geysers most prominent example.
• Reinjection halted production decline only
  temporarily

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Effects of Production (3)

• Geothermal heat/fluid extraction is NOT mining.
• Mining an ore removes the material forever.
• Geothermal resources (heat fluid) will always
  be replenished, sometimes over very long times.

Chuquipit mine, Chile
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Geothermal RegenerationTime Scales (1)

• Geothermal resource regeneration process occurs
  over range of time scales depending upon
• Type and size of production system
• Rate of extraction
• Attributes of resource
• The Good News is-
• Production continuously creates hydraulic/heat
  sink resulting in
• Pressure temperature gradients
• Both during and after cessation of production
• Fluid heat inflows working to re-establish
  pre-production state

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Geothermal RegenerationTime Scales (2)

• Time-scales for recovery following cessation of
  major utilization schemes (geothermal heat pumps,
  aquifer-based doublet, conventional use low
  enthalpy resources w/o reinjection, high-enthalpy
  reservoir for power generation) have been
  investigated by numerical simulation.
• Recovery shows asymptotic behaviour.
• Strong at the beginning, subsequently slowing
  down
• Original state re-established theoretically after
  infinite time
• However, practical regeneration (e.g. 95
  recovery) reached, generally on a time-scale of
  the same order as the lifetime of geothermal
  production systems

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Geothermal RegenerationTime Scales (3)

• Bore hole heat exchanger with geothermal heat
  pump.

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Geothermal RegenerationTime Scales (4)

• Geothermal doublet for space heating.

Production-recovery cycles for a fractured
reservoir model at Riehen/CH (from Mégel Rybach
2000)
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Geothermal RegenerationTime Scales (5)

• Power plant producing from high-enthalpy
  reservoir (Heber/USA).

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Geothermal RegenerationTime Scales (6)

• Enhanced geothermal systems (EGS).
• Extracts heat by semi-open circulation through
  fractured rock, at considerable depth, between
  injection and production wells.

• Thermal output depends on efficiency of heat
  exchange in fractured reservoir.
• EGS lifetime considered be several decades.
• Expected that recovery period, which depends
  reservoir rocks rate of thermal recovery after
  production stops, will be of similar duration,
  though this time-scale could be beyond economic
  interest.
• More studies using numerical simulation are
  needed.

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Key Issue -The Sustainable Production Level

• Sustainability will depend upon
• Initial heat and fluid content
• Their regeneration rates
• Largely on the rate of heat/fluid extraction
• High initial extraction (fluid/heat) rates will
  yield correspondingly high energy at the
  beginning (and economic reward), but energy
  delivery will decrease significantly with time,
  and can cause the breakdown of a commercially
  feasible operation.
• Lower initial production rates can secure the
  longevity of production, i.e. relatively constant
  production rates can be sustained.
• In addition, sustainable production rates can
  provide similar total energy yields to those
  achieved with high extraction rates as
  illustrated by following example.

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Power Generation from an EGS System (1)
Production stop

• High circulation rate (500 l/s) starts with 45
  MWe capacity terminates after 20 years total
  generation of 245 MWeyears (from Sanyal and
  Butler, 2005).

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Power Generation from an EGS System (2)

• Lower circulation rate (126 l/s) yields
  long-lasting power production total
  generation of 250 MWeyears (from Sanyal and
  Butler, 2005).

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Research Needs

• Authors believe there a clear need for
  significantly more research in geothermal
  production sustainability, focusing on the
  following areas
• Determination of true sustainable production
  levels and techniques for defining them at
  earliest stages of development
• Compilation and analysis of successful examples
  for stabilizing reservoir performance during
  production for high and low enthalpy systems
• Synoptic treatment of numerically modelled
  production technologies through unified approach
  looking at regeneration time-scales
• Numerical modelling of EGS considering long-term
  production/recovery for different production
  scenarios e.g. combined heat and power
  production, load-following, etc.
• Determination of dynamic recovery factors,
  which must account for the enhanced recharge
  driven by the strong hydraulic and thermal
  gradients created by fluid/heat extraction

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Conclusions (1)

• Any balanced fluid/heat production (i.e.
  production does not exceed natural recharge) by a
  geothermal utilization scheme can be considered
  fully sustainable.
• A new and sustainable equilibrium condition can
  be established through dynamic recovery, where
  natural recharge rate is increased by the
  pressure and temperature sinks created by
  production. BUT, production rates that exceed
  long-term recharge rate will eventually lead to
  reservoir depletion.
• Unlike for mining (e.g. mining out an ore body),
  there will always be geothermal resource
  regeneration, with recovery typically asymptotic,
  being strong at the beginning and slowing down
  subsequently.
• Practical replenishment (e.g. 95 recovery) will
  be reached generally on a time-scale of the same
  order as the lifetime of geothermal production
  systems.

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Conclusions (2)

• Recovery of high-enthalpy reservoirs is
  accomplished at the same site at which the
  fluid/heat is extracted. In addition, for the
  doublet and heat pump systems, truly sustainable
  production can be achieved.
• Geothermal resources can be considered renewable
  on time-scales of technological/societal systems,
  not requiring geological times.
• For geothermal energy utilization, sustainability
  means the ability of the production system
  applied to sustain the production level over long
  times. Sustainable production therefore secures
  the longevity of the resource, at a lower
  production level.
• Production from geothermal resources should be
  limited to sustainable levels.
• The level of sustainable production depends on
  utilization technology and the local geothermal
  resource characteristics. Its determination
  requires specific studies, especially model
  simulations of long-term production strategies,
  for which exploration, monitoring and production
  data are needed.
• Further sustainability research is needed in
  several areas.

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International Energy Agency Geothermal
Implementing Agreement

• One of the aims of this paper is to stimulate
  discussion of sustainable geothermal utilization
  amongst the geothermal community.
• A major outcome is planned to be the development
  of an IEA-Geothermal Implementing Agreement
  position on sustainability.
• The authors encourage and invite your comments
  and ask that you send them, before 30 November
  2006, to

Mike Mongillo at mongillom_at_reap.org.nz