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GEOTHERMAL RESERVOIR ASSESSMENT
Benedikt Steingrímsson
Introduction lecture course 2004
May 24th 2004
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Reservoir Assessment Methods(regional or local applications)
• Surface Thermal Flux• Volumetric Methods
• Decline Curve Analysis
• Lumped Parameter Model• Distributed Parameter Models
– Natural State Models
– History Matching and Exploitation Models
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Surface Thermal Flux Method
Estimation of • the continuous thermal flux caused by heat
conduction to the surface.
• The thermal power of geothermal manifestation (hotsprings and fumaroles).
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The Volumetric MethodStored Heat Calculations.
Calculation of the total heat energy stored in avolume of rock as compared to some
reference temperature (i.e. the annual mean
temperature or 0°C).This will be the sum of the thermal energy
stored in the rock matrix and the thermal
energy of the fluid (water, steam) in the rockpores).
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Stored heat in an isothermal reservoir
Eres = Erock + Efluid
Erock = V(1-Φ)ρr Kr (Tres - Tref )
Efluid = VΦρf Kf (Tres - Tref )
or
Efluid = VΦρf (Hres - Href )
where:
Φ is porosity
K is heat capacity
ρ is density
H is enthalpy
Geothermal
reservoir
Volume = V
Temperature = Tres
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Volumetric Resource Assessment(some definitions, see hand-out)
Geothermal resources base:
All the heat in Earth’s crust below a specific area (ref. mean annualtemperature). Commonly to 10 km depth.
Accessible resources base:
Energy at shallow shallow depths that can be tapped by drilling.
Commonly to 3 km depth.
Geothermal resource:
Fraction of accessible resource base that can be extracted
economically and legally at some reasonable future time.
Geothermal reserve:
Identified part of geothermal resource that can be extracted today
at the cost competitive with other energy sources.
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Diagram illustrating subdivision of Geothermal Resources Base
Depth 3 km
Economics at
some future time
Economics at the time
of determination
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Iceland Resource Assessment 1984.
Division into zones
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High temperature fields in Iceland
Proven
Probable
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Iceland Resource Assessment 1984.Estimated crustal temperatures
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High temperature fields. Accessible Resources Base
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Iceland. Geothermal Resource Assessment
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Lumped Models of Geothermal Reservoirs
Simple reservoir models used for matching and predictingchanges in one reservoir parameter caused byproduction from the system. (The Theis model is an example ofa lumped model)
Input parameter:
• Total production from the field as a function of time(Production history).
Parameter matched
• Usually the pressure (water level).Model predictions:
• Future pressures for various production plans.
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A general lumped parameter model used to simulate water
level or pressure changes in geothermal systems(Guðni Axelsson, see hand-out)
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Water level in Hamar LT-system in Iceland
simulated by a lumped model
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Water level changes, past and predictions for two productions
scenarios, calculated by a lumped model
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Example demonstrating the reliability of a lumped model for
Gata geothermal system in S-Iceland
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Distributed Parameter ModelsWhat is numerical modeling??
• Break the reservoir volume into small pieces called
“gridblocks.
• Assign hydrological and thermal properties to each
gridblock.• Assign sinks and sources where there are inflow or
outflow zones in the system or production or injection
wells.
• Use a computer program such as TOUGH2 to
calculate the evaluation of the system
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The Nesjavellir grid 1986-2000
• The extent of the 1986 gridwas 12 km in all directions
but was extended to 100 km
in 1992.
• Each production well has itsown gridblock .
• The model consists of three
400 m thick layers and one
800 m thick reservoir layer.
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General approach to
reservoir numerical
modeling (after Bodvarsson
and Witherspoon)
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Distributed Parameter ModelsWhat is the purpose of numerical modeling??
• To guide the development of the field – estimate generating capacity
– estimate flow rate and enthalpy changes of
production wells
– estimate how many wells need to be drilled
– select proper location of new wells
– evaluate effectiveness of re-injection
– estimate how chemistry will change with time
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Distributed Parameter ModelsWhat is the purpose of numerical modeling??
• To help to evaluate different conceptual models ofthe field
– where are the up-flow and outflow zones?
– how is the permeability structure (barriers, flow
channels)?
– how much is the natural recharge?
– how many different conceptual models can explain
the data?
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Distributed Parameter Models
Main parameter matched:• Temperature distribution in the reservoir
• Pressure distribution in the reservoir
• Flow histories for production wells (Q and H)
• Observed pressure interference between
wells
• The presence of two-phase or vapor zones
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Nesjavellir-Iceland
The 1986 model:
Computed and
observedtemperatures in the
R-layer
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Reasonable match withflow data from mostwells
• Predicted pressure
draw-down similar toobserved
Consequently:Only minor changes in
model parameters inthe 1998 update
Recalibration of the Nesjavellir model in
1998
1980 1984 1988 1992 1996 2000
0
1000
2000
3000
Simulated enthalpy
Observed enthalpy
E n t h a l
p y (
k J / k g )
Well NG-6 Production history 1982-1998
F l ow ( k g / s )
Year
0
20
40
60
Simulated flow
Oberved flow
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Distributed parameter models
Main parameters adjusted during the simulation:• Permeability distribution of the reservoir.
• Porosity distribution in the reservoir.
• Productivity indices for wells (the relation between Qand pressure drop from reservoir into the well).
• Mass recharge to system.
• Heat recharge to the system.
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Permeability and porosity distribution inferred from a well-by-well model for Olkaria, Kenya
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Distributed parameter models
Main parameters predicted.
• Natural recharge to the system.
• Permeability and porosity distribution for thereservoir.
• Location of barriers and flow channels.
• Future pressure draw-down in all observation wells.
• Future flow histories for all production wells (Q andH).
• Expansion of two phase or steam zones due to draw-
down.• Production induced (draw-down) recharge from the
outer boundaries of the reservoir
• Effects of re-injection (pressure maintenance, thermal
breakthrough).
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Nesjavellir
modeling 1986
Main conclusions• Recharge rate in up-flow zone
under Hengill volcano is 65 kg/s
with enthalpy of 1850 kJ/kg
• Permeabilities vary between 1-50 md.
• Porosities vary between 1-10%
• The field can support 300 MW
thermal power plant for at least30 years.
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The end