heat flow measurements in geothermal...
TRANSCRIPT
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TRAINING ON SURFACE EXPLORATION STUDIES FOR GEOTHERMAL RESOURCES AND
DEVELOPMENT OF CONCEPTUAL MODELS
UNDER THE AUSPICES OF INTERIM PROJECT COORDINATION UNIT OF THE AFRICA
GEOTHERMAL CENTER OF EXCELLENCE
HEAT FLOW MEASUREMENTS IN GEOTHERMAL MAPPING
Antony Wamalwa, Geothermal Development Company-KenyaEmail:- [email protected]
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OUTLINE
INTRODUCTION
HEAT TRANSFER
HEAT FLOW SURVEY
METHODOLOGY
EQUIPMENT
CHALLENGES
CASE STUDIES
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GEOTHERMAL SYSTEM
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Geothermal energy is
heat from the earth; Geo - Earth
Thermal – Heat
•The Earth is composed of a number of
different layers:-
The core (7000km)
The mantle (2900km)
The crust (Oceanic crust
(5-100km), Continental
Crust (20-100km)
INTRODUCTION
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Earth’s interior temperature remain relatively constant over time
except near the surface
Temperature difference between cool crust and hot molten magma
establishes temperature gradient
Normal temp gradients range between 15 – 30 °C per km
Geothermal active regions exhibit higher temperature gradient
Areas with elevated temperature gradient - targets for geothermal
utilization
EARTHS TEMPERATURE
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RIFTING PROCESSES
Geophysical tomographic studies have
shown that the asthenosphere rises
beneath the rift hence the temperature
anomaly
The convective process in the
asthenosphere promotes the rifting
process
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•Tectonic plate movement results in faulting
•Hot rocks are brought close to the surface
•Water seeps through deep faults
• It’s heated by the hot rocks
•Faults serve as plumbing systems for fluid
and magma
•Results to geothermal manifestations
WHY HEAT FLOW MEASUREMENTS
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• To estimate amount of heat energy being lost naturally
• To analyze the distribution of heat loss features - hidden
fracture zones
• To help prioritize geothermal prospects for development
• It serves as an input for conceptual and volumetric models
• To estimate the extent of leakage of the reservoir
WHY HEAT FLOW MEASUREMENTS
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HEAT LOSS MEASUREMENTS & Other studies
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HEAT LOSS MEASUREMENTS & Other studies
Assist in quantifying amount of heat being lost on the surface
Complements other disciplines in determining the reservoir temperature
Assists or complements other discipline in identification of active
structures
Suggest possible orientation of fractures in the prospect area
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Heat is transferred from the geothermal system to the atmosphere
•Mechanisms include:
Conduction
Convection
Radiation
•Heat loss in geothermal systems is mainly through
Conduction
Convection
HEAT LOSS PROCESSES
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Free electron diffusion or proton vibration
No flow of medium or material
Occur mainly in solids
Transfer of energy is from high to low temperature
Hot grounds experience conductive heat loss
Boreholes serve as temperature gradient holes
Results can be used to estimate temperatures at depth
CONDUCTIVE HEAT FLOW
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Q = Ak dT/ dy
(One dimension heat equation
Where;
•Q = Conductive heat flow (watts),
•A = Surface area of hot ground (m2),
• k = Thermal conductivity of rock (w/m°C),
•T = Temperature (0C),
•y = Depth (m)
CONDUCTIVE HEAT FLOW
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Natural convection triggered by density variations due to heating
Involve the transfer of heat energy by fluids
Convective heat flow is often faster than conduction
Fluid moves from high pressure to low pressure due to pressure diff
Fluid transports the heat from one point to another
In geothermal systems hot springs, fumaroles, steaming grounds
transport heat & mass to the surface
CONVECTIVE HEAT FLOW
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Equation used; Qc = Vf ρf hf
Qc – convective heat flow (watts)
Vf - volumetric flow rate (m3/s)
ρf - density (kg/m3) (steam ~0.4753, water ~1000kg/m3 )
hf – enthalpy of fluid (~117kj/kg water at ambient and 2670kJ/kg for
steam )
f - denotes the fluid
d- Venturi Diameter~ 0.054m and dt – Diameter throat 0.00635m
Cd- Coef of discharge ~0.96
CONVECTIVE HEAT FLOW
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Desk top study
•Conduct literature review (gather existing relevant
information - geological, geochemical, heat loss,
geophysical) and generate necessary maps
(topographical, base maps).
This helps in;
•Avoid duplication of work.
•Defining objectives and scope of fieldwork.
DATA COLLECTION
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Spacing of the gradient holes depend on the
surface geothermal activity in the prospect
Area, time and resource available
Geologic formation of the area
Social constraints
DATA COLLECTION PLAN
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Global Positioning System (GPS) device
•Digital thermometer probe
•Mechanical soil auger
•Winch
•Venturi meter
•Manometer
•V-notch (weir box)
•Pitot tube
•Jembe, spade, panga, plastic basins
•Field map
•Satellite phone
FIELD TOOLS
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Global Positioning System (GPS) device
•Digital thermometer probe
•Mechanical soil auger
•Winch
•Venturi meter
•Manometer
•V-notch (weir box)
•Pitot tube
•Jembe, spade, panga, plastic basins
•Field map
•Satellite phone
FIELD TOOLS
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Hostile Climate
Poor communication (no roads)
Remote and undeveloped places
Equipment failure
Heavy equipment to be carried
Community acceptance of the project
Likely Challenges
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CASE STUDIES
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Temperatures range 23 –
96°C
•Anomalous thermal area
was approximately 35km2
•Total conductive heat loss
estimated at 1,130MWt
SILALI PROSPECT
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• Temperatures at 1m
depth varied 30 – 84°C
• Anomalous thermal area
was approximatelt 90km2
• NE-SW orientation of
thermal features
• Estimated heat loss
3,420MWt
• 10MWt convective
component
PAKA PROSPECT
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Temperatures range
from 23 - 97°C
•Anomalous thermal
area covered
approximately 85km2
•Total heat loss
2,870MWt
KOROSI_CHEPCHUK PROSPECT
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• Presence of anomalous heat flow supports existence of a
heat source
• Thermal anomalous areas can be used to infer presence
faults and geologic structures
• The use of thermal Gradient – Approximate Reservoir
temperature
• Thermally anomalous areas suggests resource area
CONCEPTUAL MODEL INPUT