Interpretation of Thermal Response Tests in the Presence of Groundwater Flow

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Interpretation of Thermal Response Tests in the
Presence of Groundwater Flow

• October 2, 2007
• Hikari FUJII, Hiroaki OKUBO, Ryuichi ITOI
• Kyushu University, Fukuoka, Japan

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Background

• In ground heat exchangers (GHEs), the increase of
  groundwater flow velocity improves heat exchange
  rates and apparent thermal conductivity (la).
• Relationship between groundwater velocity and
  heat exchange rates has not been examined though
  field tests.

Critical Peclet Number
Convection -dominated
Conduction -dominated
Groundwater Velocity 10 m/day

• (Conditions of Numerical Simulation)
• Calculation Period 5 days
• 1 Single U-tube
• l of formation 1.5. W/m/K
• as3.0x10-7m2/s

Peclet Number vs. Heat Exchange Rates by
Numerical Simulation (Fujii et al., 2005)
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Contents

• To estimate vertical distribution of la using
  fiber optical thermometers in TRTs.
• To carry out in-situ measurement of groundwater
  velocity and investigate relationship between
  groundwater velocity and la.

Optical Fiber Thermometer - Resolution 0.1K
- Accuracy 1.0K - Minimum Depth Interval
1.0m - Minimum Time Interval 60sec
Installation of Optical Fiber Sensor
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Experimental Well and Geological Column
Sensor Position
Location Karatsu City, Saga, western Japan
Ex-coal mine in a mountainous area gt Possible
fast groundwater flow Type of Formation Upper
Reclaimed soil, sand and coal Lower Weathered
sandstone Type of GHE Double U-tube with
silica sand grout
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Measurement of Groundwater Velocity Direction
Packer
Casing
Oriented Rod
60mm
Packer
Silica Sand (10-36 mesh)
Sensor
Groundwater Flow
Central Electrode (Spot pure water)
Photo of Measurement Tool
12 x Peripheral Electrode
Schematic of Tool
Plan View of Sensors
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Results of Survey

• _at_ 36.0m
• uw0.223 m/d, Pe0.29 gt 0.1

• Peclet number is larger than 0.1 at 36.0m.
• la could be enhanced by groundwater flow around
  this depth.

N
N
3min 6sec
7min 45sec
(ii) _at_ 44.3m uw0.040 m/d, Pe0.086 lt 0.1
N
N
17min 24sec
43min 30sec
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Information on Thermal Response Test
Conditions Circulation Period --- 10 days
Heat Load --- 4 kW (80W/m) Recovery
Period --- 10 days Heat exchange rate
fluctuated due to the unstable power supply and
insufficient insulation.
Heat Medium Temperatures and Heat Exchange Rates
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Analytical Model for Heterogeneous Formations
Tin
Tout
Apply cylindrical source function to each
sub-layer
q1
l1
q2
l2
Tave
lave


qave
qn
ln
Multi-layer Model (Fujii, et al., 2006)
Single Layer Model
l can be estimated for each sub-layer.
Average l is obtained.
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Interpretation Results of TRT
l3.14 W/m/K
Matching of Outlet Temperatures using Multi-layer
Model
Semi-log Plot during Circulation
A clear straight line could not be obtained due
to unstable heat load.
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Temperature Profiles during TRT

• Fast groundwater flow may exist in this interval
  between 25m and 40m.
• Fast groundwater flow was observed at 36.0m in
  groundwater flow survey

Good Agreement
Recovery Period
Circulation Period
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Matching of Vertical Temperature Profiles
Result Measured groundwater velocity and
estimated la showed a good correlation. Problem
Extremely quick recovery between 25m and 40m
could not be matched using late-time recovery
data after 1 day.
Estimated Distribution of Thermal Conductivity
Matching during Recovery Period
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Cross Flow in Permeable Grouts (?)
Nearly uniform temperature was observed between
25m and 40m. Cross flow between aquifers
through the permeable grout could be the
reason. In the GHE, fast cross flow of
groundwater may have accelerated temperature
recovery.
Measured Temperature Profile in Neighboring Water
Well
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Effect of Cross Flow on Heat Exchange rates
(Conditions) Well Depth 50 m Crossflow
Interval 25-40 m Well Radius 0.08m
Heat exchange rates are increased by cross flows
in permeable grouts. We may be able to enhance
heat exchange rates by charging water into
permeable grouts from wellhead to cause water
flow in grouts.
This study
Cross Flow Velocity vs. Heat Exchange Rates
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Summary
Through interpretations of TRTs and groundwater
velocity/direction surveys, following results
were obtained

• The estimated distribution of thermal
  conductivity well agreed with the measured
  groundwater velocity.
• Possible vertical cross flow in the wellbore
  improved heat exchange rates.