Note: Descriptions are shown in the official language in which they were submitted.
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PATENT APPLICATION OF
JASMIN RAYMOND AND LOUIS LAMARCHE, MONTREAL, CANADA
TITLE: APPARATUS AND M ETHOD FOR IN SITU ASSESSMENT OF THERMAL
PROPERTIES
REFERENCE CITED
US Pat. Documents
3,668,297 06/1972 Howell etal.
3,864,969 02/1975 Smith, Jr.
3,892,128 07/1975 Smith, Jr.
3,981,187 09/1976 Howell
4,313,342 02/1982 Poppendiek
4,343,181 08/1982 Poppendiek
8,005,640B2 08/2011 Chiefetz et al.
Dutch Pat. Document
2009 011 600 Al 01/2006 Kietzmann et al.
European Pat. Documents
1,600,749 B1 05/2005 Rohner et al.
1,959,213 B1 02/2008 Rohner et al.
World Pat. Document
2010/058056 Al 05/2010 Martos Torres etal.
Other Documents
Austin III, 1998. Development of an in situ system for measuring ground
thermal
properties. Oklahoma State University, Master's Thesis, Oklahoma.
Eskilson, 1987. Thermal analysis of heat extraction boreholes. Lund Institute
of
Technology, Doctoral Thesis, Lund.
CA 02882008 2015-02-13
Gehlin, 1998. Thermal response test - in-situ measurements of thermal
properties in
hard rock. Lulea University of Technology, Licentiate Thesis, Lulea.
Raymond, 2010. Geothermal system optimization in mining environments. Laval
University, Doctoral Thesis, Quebec.
Rohner et al., 2005. A new, small, wireless instrument to determine ground
thermal
conductivity in-situ for borehole heat exchanger design. Proceedings of the
World
Geothermal Congress, Antalya, pp. 1-4.
BACKGROUND OF THE INVENTION
[0001] The present invention relates to an apparatus and method for assessing
the
thermal properties of a medium. More specifically, the invention relates to an
apparatus
used to measure temperature and inject heat in the medium and the method used
to
determine the undisturbed temperature and the thermal conductivity of the
subsurface
from the heat injection experiment performed in the field.
[0002] Several apparatus and method's have been proposed for in situ
measurements
of thermal properties. For example, US Pat. No. 3,668,927 discloses an
apparatus in
which a probe with a single heating section is lowered into a borehole to
perform
measurements of the surrounding thermal conductivity.
[0003] Modifications of the apparatus described in US Pat. No 3,668,927 and
further
methods to carry out thermal conductivity measurements in a borehole were
disclosed
in US Pat. Nos. 3,864,969, 3,892,128, 3,981,187, 4,313,342 and 4,343,181 as
well as in
Dutch Pat. No. 10 2009 011 600 Al. All these patents concern probes which have
to be
placed at a specific position in a hole surrounding the medium. A change of
probe
location along the borehole is therefore required to determine a thermal
conductivity
profile.
[0004] Other methods for measuring the thermal conductivity of a medium
involve the
use of a heat exchanger installed in a borehole, where heated liquid is
circulated to
disturb the thermal equilibrium of the medium. For example, US Pat. No.
8,005,64062
discloses an apparatus for performing thermal conductivity measurements, which
comprises a heat exchanger connected to a surface apparatus enclosing a pump,
a
heating element, temperature sensors and a computer for recording data. The
method
used to perform the test is itself based on the thermal response test method
described
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in the scientific literature by Gehlin (1998) and Austin III (1998). A high
potential
difference electric current is required to run the apparatus. Analysis of data
obtained
from the test provides a global measurement of the thermal conductivity of the
medium
over the length that is intercepted by the borehole. Mathematical development
of the
analytical solutions used to analyze the tests has been described by Eskilson
(1987).
[0005] Improvements of the thermal response test method have been further
described in various patents. European Pat. No. 1,959,213 B1 discloses a
method for
measuring the thermal conductivity of a medium surrounding a borehole in which
a heat
exchanger is installed, and where temperature profiles are recorded at
different times
following heat injection by means of circulation of a heated liquid in the
heat exchanger.
The evaluation of the thermal conductivity of the medium is carried out with a
finite
element numerical model.
[0006] World Pat. No. 2010/058056A1 discloses a method and apparatus for
conducting temperature measurements with a wireless probe flowing in a heat
exchanger installed in a borehole in which heated liquid is circulated. The
method
provides additional data for assessing the thermal conductivity of the medium
at
different depths with the thermal response test.
[0007] Thermal response tests to measure the thermal conductivity of a medium
at
different depths have also been performed with continuous heating cables
lowered
inside the pipes of a heat exchanger installed in a borehole. The continuous
heating
cables are powered with electric current and dissipate heat disturbing the
thermal
equilibrium of the medium. Long and continuous heating cables require a high
potential
difference electric current for the heat injection rate to be sufficient. The
temperature is
measured at different locations along the heating cables during the heat
injection and
during the following recovery period. The thermal response test method with
continuous
heating cables has been described in the scientific literature by Raymond
(2010).
[0008] An accurate temperature profile measured in a borehole revealing
variations of
the geothermal gradient can alternatively be used to determine the thermal
conductivity
of the medium surrounding the borehole if the Earth's natural heat flow is
known at the
studied site, as described by Rohner et al. (2005). European Pat. No.
1,600,749 B1
discloses a method for measuring the temperature profile in a U-shaped heat
exchanger
with a wireless probe that sinks into the pipe of the heat exchanger to
determine the
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geothermal gradient and infer the thermal conductivity. The Earth's natural
heat flow
has unfortunately not been measured with significant accuracy in all regions
of the
world, and the method is therefore restricted to specific areas.
[0009] All these patents and scientific publications had a significant impact
on the
development of apparatus and methods for in situ measurements of thermal
properties.
In view of the above, it will be apparent to those skilled in the art that
there exists a need
to improve such in situ measurements with: (1) a method that does not requires
that an
apparatus be moved from different positions to obtain a profile of the
medium's
properties, (2) an apparatus that requires low power source to decrease costs,
and (3)
the possibility of performing the measurements in any region of the world.
SUMMARY OF THE INVENTION
[0010] The objective of the present invention is to provide a simple apparatus
and
method for carrying out in situ measurements of the thermal properties of a
medium
using a low power source.
[0011] The apparatus of the present invention encloses short sections of
heating
cable along which heat is dissipated to disturb the thermal equilibrium of the
medium
where a hole has been drilled to insert the cable assembly. The use of short
heating
cable sections interchanging with longer length non-heating electric cable
sections is
the novel feature that makes the apparatus unique. Such an arrangement of
heating
cables has not been considered previously for thermal conductivity. The field
and
analysis methodology has been adapted to this unique feature of the apparatus,
which
has resulted in further innovations.
[0012] The heating cable sections allow measurements of the medium properties
at
different locations along the hole with decreased power requirements for
testing, as
compared to a conventional thermal response test with flowing liquid or a
thermal
response test with continuous heating cables. Measurements of thermal
properties do
not rely on knowledge of the Earth's heat flow, and so the new method can
therefore be
carried out just about anywhere.
[0013] An assessment of thermal properties is performed by reproducing
temperatures recorded during the thermal recovery period following heat
injection. An
analytical solution describing heat transfer from a finite length linear
source is used to
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reproduce the temperature observations. Mathematical solutions of the prior
art are
linear and cylindrical heat sources, of either infinite length or finite
length at one
extremity and an image heat source at the other extremity. The linear heat
source
solution with a finite length at both extremities and its use in the scope of
the present
invention constitute a unique feature of the methodology.
[0014] These and other objects, novel features, aspects, advantages and
further
scope of applicability of the present invention will become apparent to those
skilled in
the art from the following detailed description, which, taken in conjunction
with the
accompanying drawings, disclose a preferred embodiment and method of the
present
invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] Referring now to the attached drawings which form a part of this
original
disclosure:
[0016] FIG. 1 is a linear view of a first type of cable assembly comprising
the heating
and non-heating electric cable sections for performing the test in accordance
with the
present invention.
[0017] FIG. 2 is a linear view of a second type of cable assembly comprising
the
heating and non-heating electric cable sections for performing the test.
[0018] FIG. 3 is a drawing of the electric circuit formed by the cable
assembly when
resistances of the heating sections are connected in series.
[0019] FIG. 4 is a drawing of the electric circuit formed by the cable
assembly when
resistances of the heating sections are connected in parallel.
[0020] FIG. 5 is a vertical view of the apparatus installed in a ground heat
exchanger
consisting of a U-pipe inside a hole.
[0021] FIG. 6 is a vertical view of the apparatus installed in a ground heat
exchanger
consisting of coaxial pipes inside a hole.
[0022] FIG. 7 is a vertical view of the apparatus installed in a hole sealed
with a pipe
or a flexible liner.
[0023] FIG. 8 is a vertical view of the apparatus installed in an open hole.
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[0024] FIG. 9 is a plan view of the objects enclosed in the apparatus at the
surface of
the medium.
[0025] FIG. 10 is a flowchart of the field test method for the measurement of
thermal
conductivity in a hole.
[0026] FIG. 11 is a flowchart of the analysis method for the measurement of
thermal
conductivity in a hole.
DETAILED DESCRIPTION
[0027] Referring now to the invention in more detail, in FIGS. 1 and 2, an
elongated
cable assembly is shown, containing sections of heating electric cable 1
interchanging
with sections of non-heating electric cable 2. The sections of the heating and
non-
heating cables are placed one after the other, and the number and length of
the
sections are adjusted according to the length of the hole in which the cable
assembly is
installed and the number and depth of the measurements that have to be
completed.
The length of all heating cable sections is equal to another and additionally
shorter than
the length of the non-heating cable sections. The ratio of the length of the
sections of
non-heating cable over the length of heating cable sections is at least higher
than 2, and
can vary greatly with the borehole length. For example, this ratio is
preferably around 6
to 12 when the length of the borehole is approximately 150 m. The outer jacket
at the
ends of the sections of heating and non-heating cables can be attached to a
means of
connection 3, as shown in FIG. 1 or can be continuous, avoiding the means of
connection, as shown in FIG. 2. Both cable assembly types are terminated with
a
waterproof male electric connector at one extremity 4 and with a submersible
end-seal 5
that closes the electric circuit at the other extremity. At least one
perforated disk 6 is
located at the interface of each cable section. The size of the perforations
must be large
enough to allow water to flow through the disks when installing the cable
assembly into
a hole, and must be small enough to block water movements due to free
convection
when tests are conducted. For example, the size of the perforations should be
more
than 1 mm in diameter and less than 1 cm in diameter. Each cable interface can
enclose more than one disk 6, depending on the size of the perforations and
the
capacity of the disks to block water movements due to free convection.
Perforations
should preferably be offset if more than one disk is installed at each cable
interface.
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[0028] At least one means of measuring the temperature 7 is located near the
middle
height of each heating cable section. The location of the means of measuring
the
temperature near the middle height of the heating cable sections is such as to
facilitate
the reproduction of measured temperatures with an analytical solution of
finite length at
both extremities. There can be more than one means of measuring the
temperature per
heating cable section, all located at the middle height or at different
locations along the
heating cable section to duplicate or to allow more detailed measurements. The
means
of measuring the temperature can involve: (1) submersible capsules, enclosing
a
temperature sensor and a data logger, (2) thermistors connected to a data
logger at the
surface with wires, or (3) a fiber optic cable connected to an optical reader
and data
logger at the surface. The preferred means of measuring the temperature is the
submersible capsule, which avoids using the several wires required with
thermistors.
Furthermore, the submersible capsule can easily be placed near the middle
height of
the heating cable sections. Carrying out temperature measurements near the
middle
height of the heating cable sections can be complex with a fiber optic cable
whose
spatial resolution is commonly around 1 m.
[0029] Referring in more detail to FIGS. 3 and 4, the electric circuit of the
cable
assembly is shown to enclose a power source 8, electric wires or conductors 9
and
electric resistances 10, which constitute the heating cable sections. The
electric
resistances 10 are made with thin electric wires that can be connected in
series, as
shown in FIG. 3 or in parallel, as shown in FIG. 4. The electric resistances
of the
heating cable sections R1, R2, Rn
are equal to each other, and the difference in
potential at the power source is V. The power outputs Q1, Q2, ..., Qn for the
circuit with
series resistances shown in FIG 3 are equal to each other since the current
intensity / is
the same throughout the circuit. The power outputs Q1, Q2, Qn
for the circuit with
parallel resistances shown in FIG 4 are different from one another since the
current
intensities /1, /2, .../n are different for each section. Series connections
are preferred
over parallel connections for the resistances to have the same power output
for each
heating section.
[0030] When the electric resistances of the heating cable sections are
connected in
series, as shown in FIG. 3 or in parallel, as shown in FIG. 4, the means of
connection 3
between the heating and non-heating cable sections can be made with
submersible
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splices or connectors, as shown in FIG. 1. The use of splices or connectors
can be
avoided, as shown in FIG. 2, when resistances are connected in parallel, as
shown in
FIG. 4. In this case, the thin wire used for the parallel resistances of the
heating
sections can be rolled around the two conductors through the whole cable
assembly,
but cut to the desired length and brought in contact with the two conductors
at the
locations of the heating sections only. A continuous outer jacket is extruded
over the
thin wire for the cable assembly to be submersible. A cable assembly without
splices or
connectors, as shown in FIG. 2, can also be fabricated by extrusion of an
outer jacket
over the heating and non-heating cable sections connected in series, as shown
in FIG.
3. In this case, the inside jacket around the thin wires of the heating cable
is made
thicker to allow the heating and non-heating sections to have the same
diameter. The
preferred cable assembly is that of FIG. 1, and submersible connectors
constitute the
preferred means of connection 3 because the number of heating and non-heating
sections can be easily adjusted from one test to another.
[0031] Referring in more detail to FIGS. 5, 6, and 7, which illustrate the
installation of
the apparatus in a hole 11 cutting across the medium, we see the medium
surface 12,
an optional pipe casing 13 to hold the medium in place when it is non-
consolidated and
material 14 filling the hole 11. Most commonly, the medium is the ground or
the earth
subsurface, but it could also be a structure of various material types, such
as a concrete
dam. The hole 11 can be installed by drilling, which makes it a borehole, or
by any other
means, while constructing the structure that constitutes the medium. Material
14 that
fills the hole 11, which is shown to be vertical, and can also be inclined, is
most
commonly grout, but could also be water, sand, or backfill material of any
kind. The
cable assembly is connected to a waterproof electric junction box 15 with the
waterproof
male electric connector 4. The junction box can be hung to the pipe casing 13
with rigid
pipe straps 16. An electric cable 17 with waterproof female 18 and male 19
connectors
is used to connect the junction box 15 to a power source.
[0032] The cable assembly can be placed in the pipe of a hole where a ground
heat
exchanger made with a single U-pipe 20 or multiple U-pipes has been installed,
as
shown in FIG. 5. The pipe is filled with a liquid whose level is illustrated
by the dash
lines and triangles 21. The liquid is most commonly water, but could also be a
mix of
water and antifreeze or other aqueous solution of any kind. The ground heat
exchanger
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can alternatively be made with a center pipe of smaller diameter 22 surrounded
by a
pipe of larger diameter 23 in a coaxial fashion, as shown in FIG. 6. In this
case, it is
better to place the cable assembly in the smaller diameter center pipe 22. The
coaxial
pipes are filled with liquid whose level is illustrated by the dash line and
triangle 21. The
cable assembly can also be installed in a hole 11 that does not include a
ground heat
exchanger, as shown in FIG 7 and 8. The hole 11 can be blocked with a solid
pipe or a
flexible liner 24 filled with a liquid whose level is illustrated by the dash
line and
triangle 21, as shown in FIG. 7. The hole 11 can alternatively be in open
contact with
the medium, as shown in FIG. 8. The level of the liquid in the hole,
illustrated through
the dashed line and triangle 21, will be equal to that of the medium if there
is liquid in
the pores or fractures of the medium at an unconfined pressure. The level of
the liquid
in the hole could be different from that of the medium if the pressure of the
liquid in the
pores or fractures of the medium is confined. The heating cable sections are
placed
below the level of the liquid in the hole or pipe to avoid convective heat
transfer in the
air. The width of the perforated disks 6, which are used to block water
movements due
to free convection, are adjusted according to the inner diameter of the pipes
20, 22
(FIGS 5 and 6), liner 24 (FIG 7) or hole 11 (FIG 8). The width of the
perforated disk
should be small enough for the disks to enter the pipes, liner or hole, and
large enough
to block water movements due to free convection in the open space between the
disk
and the inner surface of the pipes, liner or hole.
[0033] Referring in more detail to FIG. 9, which illustrates the main
components of the
waterproof electric junction box 15, there is shown a breaker panel 25, an
automated
switch or a timer 26 for programming the system operation, a power meter and
data
logger 27 for recording measurements, a handle 28 for transporting the box,
and rigid
pipe straps 16 and bolts 29 to hang the box to the pipe casing of the hole.
Electric
power enters the box through the electric cable 17 with a waterproof female
connector
18 installed in a flanged inlet 30. An electric cable 31 links the flanged
inlet 30 to the
breaker panel 25 including at least two electric circuits. One of the circuits
32 supplies
the power meter and data logger 27. The other circuit exits the breaker panel
through
an electric cable 33 that goes into the automated switch or timer 26, then
through
another electric cable 34 connected to the reading ports of the power meter
and data
logger 27, and then through an electric cable 35 connected to a flanged outlet
36.
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Electric power exits the box through the waterproof male connector 4 and the
first non-
heating section 2 of the cable assembly installed in the hole to perform the
test.
[0034] Referring in more detail to FIG. 10, which illustrates the field method
for
carrying out thermal conductivity measurements of the medium, there are five
basic
steps to follow. The test begins by setting and starting the data loggers to
record
temperature measurements near the middle height of the sections of heating
cable 1 as
well as potential differences and the current intensity of the electric
circuit formed by the
cable assembly shown in FIGS. 3 and 4. The cable assembly shown in FIGS 1 or 2
is
installed inside the hole according to FIGS 5, 6, 7 or 8. The electric
junction box 15 is
fixed to the pipe casing 13 with pipe straps 16, the cable assembly is hooked
up to the
electric junction box 15, which is connected to a power source with the
electric cable 17.
The undisturbed temperature of the medium is measured and recorded at
different
depths inside the hole using the means of measuring temperature 7 before
starting heat
injection. The switches of the breaker panel 25 are turned on to set the
automated
switch or timer 26 and allow the electric current to flow in the cable
assembly to begin
heat injection. The difference in potential and the current intensity of the
electric circuit
of the cable assembly are measured and recorded with the power meter and data
logger 27. The duration of heat injection in the medium is approximately 50
hours. This
heat injection period can be longer for a medium of low thermal conductivity
and shorter
for a medium of high thermal conductivity. Heat injection is stopped and the
temperature
is measured and recorded at different depths inside the hole using the means
of
measuring temperature 7 for a duration that is roughly equivalent to that of
the heat
injection period. Equipment is retrieved after the thermal recovery, and all
recorded data
are downloaded, ending the field test procedure.
[0035] Referring in more detail to FIG. 11, which illustrates the analysis
method for
carrying out thermal conductivity measurements of the of the medium, there are
five
basic steps to follow, one of which is decisional. After the field procedure
is ended, the
undisturbed temperatures at the depths of the means of measuring temperature 7
are
evaluated from the measurements taken before heat injection. Temperature
increments,
defined as the temperature difference between a given time and the undisturbed
condition (AT=T-To), are determined from measurements at depth taken after the
beginning of the heat injection. Observed temperature increments at depth are
then
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reproduced over a selected time interval. Only the temperature increments
during the
late recovery period are reproduced. Data recorded early following the end of
the heat
injection must be removed from observations to reproduce until the temperature
through
the middle horizontal plane of each heating cable section becomes uniform near
the
cable. This period of time is about a few hours, and can be pre-determined
with
numerical simulations of heat transfer for selected thermal properties of the
medium and
borehole materials. Observed temperature increments at depth during the
selected time
interval are reproduced with an analytical model solving conductive heat
transfer from a
linear heat source of finite length. Calculated temperature increments are
fitted with
observed temperature increments by adjusting the thermal conductivity of the
medium.
A verification of the quality of the fit between observed and computed
temperature
increments is done. If the fit is satisfactory, the undisturbed temperature
and thermal
conductivity measurements at the given depths can be reported; otherwise,
analysis
starts over again at the third step.
[0036] The advantage of the present invention is that it offers the
possibility of
assessing the thermal conductivity of a medium at several locations without
moving an
apparatus at different depths into a hole, and conducting repetitive tests at
each
location. A single heat injection experiment with multiple heat sources and
temperature
sensors is carried out with the present invention. The power source required
to perform
the test can be of low potential difference because heat is injected along
short heating
cable sections with at least one temperature sensor per section. A test with a
continuous heating cable in a long hole would require a higher power source,
which
complicates installation of the apparatus in the field and safety procedures.
For the
present invention to be efficient, it is therefore important for the heating
cable sections
to be shorter than the non-heating cable sections, such as to minimize energy
consumption. Short heating cable sections allow a high number of measurements
that
can be performed, for example, every 10 m. The spacing of heating cable
sections is
however not restricted to such a specific value, and can be adjusted to site
settings. The
cable assembly of the present invention is consequently designed to facilitate
operation
of the apparatus and to maximize the number of measurements, distinguishing
the
cable assembly from those of other fields using heat tracing technologies.
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[0037] Data collected during the field experiment could not have been analyzed
with
mathematical solutions and methods of the prior art since temperature buildup
along a
linear heat source of short finite length is significantly different from that
of heat sources
of infinite length or of finite length with an overlying image source. The use
of a new
mathematical solution with a linear heat source of finite length at both
extremities in the
scope of the present invention makes the test analysis comprehensive, fast and
accurate. The solution is derived to describe temperature increments at the
middle
height of the heat source, and analysis is facilitated by positioning the
temperature
sensors near this location. =
[0038] The Earth's natural heat flow does not affect analysis when the
geothermal
gradient along the hole axis is small enough to be considered negligible.
Therefore, the
test can be carried out in about any regions of the world, even where
knowledge of the
Earth's heat flow is poor.
[0039] While the foregoing written description of the invention enables one of
ordinary
skill to make and use what is considered presently to be the best mode
thereof, those of
ordinary skill will understand and appreciate the existence of variations,
combinations,
and equivalents of the specific embodiments, methods, and examples herein. The
invention should therefore not be limited by the above described embodiments,
methods, and examples, but by all embodiments and methods within the scope and
spirit of the invention. It is therefore intended that the appended claims
encompass any
such embodiments, methods and examples.
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