Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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THERMOMECHANICAL ELECTRICAL GENERATOR/POWER SUPPLY
FOR A DOWNHOLE TOOL
Background of the Invention -
This invention relates generally to downhole tools ener-
gized by self-contained power supply apparatus and methods
for energizing downhole tools and more particularly, but not
by way of limitation, to electrical power supplies and
5 methods incorporating an external combustion engine in
powering an electrical circuit in a downhole tool.
In drilling and completing oil or gas wells, various
activities need to be performed downhole. For example,
downhole pressure and temperature readings need to be taken
10 when conducting a drill stem test, and perforating guns need
to be activated when perforating a casing prior to fractur-
ing a formation. These two specific operations, as well as
many others, are performed by tools which need to be ener-
gized when the tools are at their downhole locations. This
15 energization is typically electrical energization, at least
during some phase of the downhole operation.
In the past and at present, such electrical energization
has been and is provided through a wireline from a source at
the surface or through a self-contained battery pack located
20 within the downhole tool. The cells in the battery pack
have been chemical batteries, such as silver oxide or lith-
ium types. The use of fuel cells ~e.g., containing liquid
hydrogen and oxygen) in a self-contained pack has been con-
templated, but to my knowledge has never been commercially
25 implemented.
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One shortcoming of the wireline energization techniqueis the relative difficulty in using the wireline rather than
merely using a "slick line~ or retaining cable, which rela-
tive difficulty is well recognized in the industry. Addi-
5 tionally, because of the length of the wireline, electricallosses occur which would not occur if the power supply were
wholly contained within the downhole tool. Finally, the
requirement of a wireline does not lend itself to long-term
tests, as the wireline truck or skid and power supply must
10 remain at the well site. Moreover, the presence of a wire
line or any cable in the well bore prohibits quickly closing
off the well in an emergency unless one is willing to cut
the wireline or cable and then "fish" it out at a later
time.
Although battery packs overcome the two aforementioned
shortcomings of wireline energization, the battery packs
have relatively limited operating lives and electrical capa-
cities whereby the operation of the downhole tool, both as
to how much can be driven by a battery pack and as to how
20 long energization can be sustained, is limited. When
testing multiple parameters or conducting a long-term test,
e.g., weeks or months, such limitations become particularly
apparent. Although more batteries can be added to provide
more capacity, such additional batteries at some point can
25 no longer be accommodated because of the size constraints
which are imposed upon all downhole tools by the size of the
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well bore and other known factors. Also, even though bat-
teries can be replaced so that operations can be continued,
such replacement requires a trip of the pipe string in which
the battery packs are incorporated out of and back into the
5 well bore, thereby increasing the expense of the operation.
Such battery packs also have limitations as to the types of
wells in which they can be readily used; this is specifi-
cally referring to deep wells (e.g., wells from two to five
miles deep) because of the high pressures and temperatures
10 which are encountered in these wells and which can detrimen-
tally affect the chemical operations within the battery
cells.
Thus, there is the need for an improved power supply for
a downhole tool, which power supply is self-contained and
15 wholly mounted within the downhole tool for obviating the
necessity of a wireline, thereby achieving an advantage
similar to that of the battery packs. Furthermore, however,
such an improved power supply should overcome the short-
comings of the battery packs by providing for a longer
20 operating life and by providing for more output capacity
within a smaller volume than are provided by the battery
packs known to me and by providing for reliable usage even
in deep wells where temperatures are greater than those in
which presently available batteries can operate.
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Summary of the Invention
The present invention overcomes the above-noted
and other shortcomings of the prior art and meets the
aforementioned needs by providing a novel and improved
power supply apparatus and method for a downhole tool.
The present invention combines nuclear, mechanical and
electrical aspects into an overall combination having
the following features and advantages: very high power
density, stable operation, long operating life, and
minimal moving parts which neither rotate nor require
a lubricating system or valves. The present invention
is also simple, reliable and relatively inexpensive.
The present invention provides a power sùpply for
an oil or gas well downhole tool having a system re-
quiring energy from the power supply and having a re-
ceptacle for receiving the power supply. The power
supply comprises housing means for being received with-
in the receptacle of the downhole tool; a fuel capsule
removably retained inside the housing means, the fuel
capsule including means for generating thermal energy;
$tirling cycle engine means, disposed in the housing
means, for converting the thermal energy to mechanical
motion; conversion means, disposed in the housing
means, for converting the mechanical motion to energy
usable by the system within the downhole tool; and
connecting means, communicating externally of the
housing means, for connecting the energy from the con-
version means to the system of the downhole tool.
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The present invention provides a downhole tool
comprising a power supply including a housing; thermal
energy source means, disposed in the housing, for gene-
rating thermal energy; an electrlcal energy generatcr
disposed in the housing; external combustion engine
means, disposed within the housing, for actuating the
electrial energy generator in response to the thermal
energy from the thermal energy source means; and means
for connecting the electrical energy generator with an
electrical circuit, disposed in another housing in which
the first-mentioned housing is disposed, for performing
a function in a downhole environment of a well in res-
ponse to electrical energization from the electrical
energy generator. In the preferred embodiment the ther-
mal energy source means includes a radioisotope, the
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external combustion engine means includes a Stirling cycle
engine, and the electrical energy generator includes a
linear alternator.
The present invention also provides a method of ener-
5 gizing an electrical circuit contained in a downhole tool.
This method comprises inserting a fuel capsule into a
housing in heat transfer relationship with a Stirling cycle
engine retained in the housing, the fuel capsule including a
radioisotope; inserting the housing into the downhole tool;
lO and electrically connecting the electrical circuit to a
linear alternator disposed in the housing and mechanically
connected to the Stirling cycle engine. This method further
comprises generating, at the output of the linear alternator
and in response to the radioisotope, an electrical output
15 within the range between approximately 0.5 watts and appro-
ximately 2.5 watts for application to the electrical circuit
of the downhole tool. In the preferred embodiment this out-
put is achieved while constraining the linear displacement
of a movable member of the linear alternator to approxi-
20 mately 1/8 inch relative to a stator of the linear alter-
nator. The step of inserting the housing into the downhole
tool includes retaining the housing within a receptacle
region having a diameter of approximately one inch and a
length of not greater than approximately twenty-four inches.
Therefore, from the foregoing, it is a general object of
the present invention to provide a novel and improved power
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supply apparatus and method for a downhole tool. Other and
further objects, features and advantages of the present
invention will be readily apparent to those skilled in the
art when the following description of the preferred embodi-
5 ment is read in conjunction with the accompanying drawings.
Brief DescriPtion of the Drawinqs
FIG. 1 is a schematic illustration of a well having a
downhole tool including a power supply constructed in accor-
dance with the present invention.
FIG. 2 is a diagram of the preferred embodiment of the
10 power supply of the present invention.
Detailed Description of the Preferred Embodiment
Schematically illustrated in FIG. 1 is a well 2, par-
ticularly an oil or gas well, at the mouth of which is dis-
posed a conveyancing means 4, such as a winching system of a
more complex design than is schematically shown in FIG. 1,
for lowering and raising a downhole tool 6. Conveyancing
means 4 is also contemplated to include a pipe or tubing
string in which downhole tool 6 is incorporated or disposed.
The downhole tool 6 has an electrical circuit or system
8 for performing a function of whatever type might be needed
20 in the downhole environment of the well 2. Specific
examples of the downhole tool 6, but not by way of limita-
tion, are an electronic pressure and temperature gauge or an
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electrically-actuated perforating gun. The electrical cir-
cuit 8 is disposed or contained in a suitable housing 10 of
a type known to the art.
The housing 10 has a receptacle 12 for receiving a power
5 supply 14 which provides the electrical energization to
which the electrical circuit 8 responds, thereby enabling
the function to be performed. In the preferred embodiment
subsequently described, the power supply 14 is a radioiso-
tope thermomechanical electrical generator (thus the label
10 R T E G d FI
. . . . use ln G. ).
In the preferred embodiment the receptable 12 is sized
to accommodate the size of the power supply 14 subsequently
more particularly specified. The size specifications or
limitations are important in the preferred embodiment of the
15 present invention in that they provide a more compact self-
contained power supply than is provided by battery packs
known to me for similar applications. This is of consider-
able significance to a downhole tool designer who must work
within some absolute size constraints imposed by the size of
20 the well bore, the tubing disposed in the well bore, and the
formation with which the tool is to be used, for example.
With reference to FIG. 2, the preferred embodiment of
the power supply 14 will be described. Broadly, the power
supply 14 includes housing means 16 for being recelved
25 within the receptacle 12 of the housing 10 of the downhole
tool 6. The power supply 14 also includes thermal energy
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source means 18, disposed in the housing 16, for generating
thermal energy which powers an external combustion engine
means 20, also disposed within the housing 16, for con-
verting the thermal energy into mechanical motion. In the
preferred embodiment the external combustion engine means 20
is defined as a Stirling cycle engine having a dri-~ing out-
put coupled to conversion means 22, also disposed within the
housing 16, for converting the mechanical motion from the
engine 20 to energy usable by the system 8 within the down-
10 hole tool 6. Since the system 8 is an electrical system inthe illustrated preferred embodiment, this energy is, of
course, electrical energy; however, other suitable output
energies could be derived in correspondence with the nature
of some other type of functional system which might be used
in the downhole tool 6 in place of the electrical system 8.
Also included in the power supply 14 are transfer means for
transferring thermal energy from the thermal energy source
18 to the engine 20 and means for connecting the output from
the conversion means 22 to the system 8.
The housing 16 of the preferred embodiment is defined by
a tubular member 24 having a cylindrical side wall 25 with a
~ maximum outer diameter of, preferably, not greater than
approximately one inch and an outer length of, preferably,
not greater than approximately twenty-four inches. More
25 broadly, the maximum outer diameter is less than the inner
diameter of the downhole tool 6 and the outer length is less
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than the length of the downhole tool so that the housing 16,
and thus the entire power supply 14, can be fully received
entirely within the downhole tool 16. The specific dimen-
sions are particularly advantageous because they are signi-
5 ficantly less than typical dimensions of battery packs whichare now accommodated in downhole tool designs. Thus, with
the present invention a more compact overall downhole tool
is provided, thereby saving material and fabrication costs.
At one end of the tubular member 24 there is an opening
10 26 through which the thermal energy source 18 can be moved
into and removed from the housing 16. In the FIG. 2 embodi-
ment, this opening 26 is defined through an end wall 28 of
the tubular member 24. This end wall 28 is disposed trans-
versely to the cylindrical side wall 25 of the tubular
15 member 24. The opening 26 is closable by means of a closure
cap 30 which is connected by mating threads within the
opening 26. Because the thermal energy source 18 is re-
ceived in this end of the tubular member 24, the interior
surface of the end wall 28 and this portion of the tubular
20 member 24 are lined with a suitable insulation material 32.
Alternatively, the tubular member 24 can be constructed
in two sections which are threadedly connected as at refer-
ence numeral 33 shown in FIG. 2. With this construction no
end opening 26 and closure cap 30 are needed so that the end
25 wall 28 is continuous across the entire end area of that
portion of the tubular member 24; this permits better insu-
lating of the thermal energy source 18. Other suitable con-
structions of the housing 16 and other suitable techniques
for inserting and removing the thermal energy source 18 can,
of course, also be used as would be well known in the art.
The other end of the tubular member 24 has an end wall
34 disposed transversely to the side wall 25. This end wall
34 is spaced linearly from the end wall 28 at the opposite
end of the cylindrical side wall 25.
The thermal energy source 18, movable into and out of
10 the housing 16 through the opening 26 (or other suitable
alternative construction), is in the preferred embodiment a
unitary member constructed in the form of a fuel capsule 36
made of, at least in part, a suitable radioisotope having a
half-life sufficient to provide a sufficiently long-lived
15 primary energy source for the power supply 14 so that power
source replacements are not needed once an operation com-
mences, thereby making extra trips out of and into the well
unnecessary. There is a sufficient quantity of the radio-
isotope in the fuel capsule 36 so that the power supply 14
20 has an overall electrical output within the range of appro-
ximately 0.5 watt to approximately 2.5 watts. In the pre-
ferred embodiment it is specifically contemplated that the
electrical output from the conversion means 22 need be only
something less than approximately one watt, which output is
25 ultimately the result of the capacity of the radioisotope
primary power source contained in the fuel capsule 36. The
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fuel capsule 36 is removable from the housing 16 indepen-
dently of any of the other components of the power supply 14
so that this primary fuel source can be readily replaced if
ultimately needed. The fuel capsule 36 is surrounded by a
5 suitable heat transfer medium, such as a heat pipe, defining
the transfer mean~ for transferring the thermal energy gen-
erated by the radioactive decay of the radioisotope within
the fuel capsule 36 to the external combustion engine 20.
As previously mentioned, the external combustion engine
10 means 20 of the preferred embodiment includes a stirling
cycle engine. The Stirling cycle is a well known thermo-
dynamic cycle and various engines operating in accordance
with this cycle are well known. In general, these engines
have two pistons: one of which is referred to as a dis-
15 placer for moving a working gas between hot and cold cham-
bers, and the other of which is referred to as a power
piston for providing a mechanical motion output. The move-
ments of these pistons are in response to thermal energy, or
heat, applied from a suitable source, which in the preferred
20 embodiment of the present invention is the radioisotope of
the fuel capsule 36. As shown in FIG. 2, the Stirling cycle
engine is disposed adjacent the fuel member 36 so that the
heat generated by the radioactive decay of the radioisotope
in the fuel capsule 36 is transferred to the Stirling cycle
25 engine through the heat transfer medium within the volume
surrounding the capsule 36. In FIG. 2 the Stirling cycle
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engine is specifically identified by the reference numeral
38, and the mechanical motion is provided through A coupling
member 40, such as the piston rod of the power piston known
to be contained within the Stirling cycle engine 38.
The conversion means 22 is connected to the coupling
member 40 so that the conversion means 22 is actuated by
that motion, which motion is derived in response to the
thermal energy from the thermal energy source means 18. In
the preferred embodiment shown in FIG. 2, the converAion
10 means 22 is an electrical energy generator (specifically
identified as a linear alternator 42) which generates a
voltage across two terminals 44, 46. For the specific embo-
diment including a linear alternator, this form of the con-
version means 22 includes a stator with which the two
15 terminals 44, 46 are associated and a movable member con-
nected to the coupling member 40 so that relative ~ovement
between the stator and the movable member is achieved when
the Stirling engine 38 operates. That is, the stator is
fixed in a stationary manner relative to the housing 16 and
20 the movable member is fixed relative to the power piston
within the Stirling engine 38 so that movement of the power
piston moves the movable member relative to the stator.
This relative movement generates the electrical voltage by
the electromagnetic relationship between the stator and the
25 movable member as is well known in linear alternators. In
the preferred embodiment wherein size constraints are impor-
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tant factors, the Stirling engine 38 and the linear alter-
nator 42 are constructed so that this relative movement is
constrained to not more than approximately 1/8 inch, but
also so that such limited displacement still generates an
5 electrical output sufficient to provide power within the
range between approximately 0.5 watt and approximately 2.5
watts. As shown in FIG. 2, the linear alternator 42 is
disposed on the side of the Stirling cycle engine 38 oppo-
site the fuel member 36.
The electrical output from the linear alternator 42,
which is provided across the terminals 44, 46, is communi-
cated externally of the housing 16 by the connecting means,
schematically illustrated in FIG. 2 as including conductive
members 48, 50 and output contacts or terminals 52, 54.
15 These elements can be included in a single unitary member
which provides both mechanical and electrical coupling of a
suitable type for connecting with the circuit 8 to be ener-
gized by the power supply 14. This connecting, or coupling,
means is preferably connected to or through the end wall 34
20 of the housing 16 so that the connection is made within the
confines of the maximum outer diameter of the tubular member
24.
The above-described preferred embodiment of the appara-
tus defining the power supply 14 is also comprehended within
25 a method of energizing an electrical circuit contemplated by
the present invention. This method comprises inserting the
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fuel capsule 36 into the housing 16 in heat transfer rela-
tionship with the Stirl ng cycle engine 38, inserting the
housing 16 into the downhole tool 6, and electrically con-
necting the electrical circuit of the downhole tool to the-
linear alternator 42. The importance of this method is inutilizing the fuel capsule 36, having the radioisotope, with
a Stirling cycle engine in a downhole tool so that an im-
proved technique of energizing such a downhole tool is pro-
vided. In particular, this method includes within the step
10 of inserting the housing into the downhole tool the step of
retaining the housing within a receptacle region having a
diameter of approximately one inch and a length of not
greater than approximately twenty-four inches. This method
also comprises generating, at the output of the linear
alternator 42 and in response to the radioisotope in the
fuel capsule 36, an electrical output within the previously
defined range of between approximately 0.5 watt and approxi-
mately 2.5 watts for application to the electrical circuit
of the downhole tool. This power generating is achieved in
20 the preferred embodiment of the method in conjunction with
constraining the movement of the movable member of the
linear alternator 42 relative to the stator of the linear
alternator 42 to not more than approximately 1/8 inch. Al-
though one may consider these specific design parameters to
25 be merely matters of design choice, as comprehended within
the method of the present invention these parameters are
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specific critical limitations of the preferred methodology
by whicn an improved power supply technique is achieved
within the constricted downhole environment to which the
method is limited.
In summary, the radioisotope thermomechanical electrical
generator of the preferred embodiment power supply 14 uti-
lizes the energy released by the decay of the radioisotope
within the fuel capsule 36 to provide heat to operate the
Stirling engine 38 which will in turn drive the linear
10 alternator 42 to provide a suitable electrical power output,
uch as in the specific embodiment an output of less than
approximately one watt of AC or DC power for use in oil
field instrumentation. The power supply 14 will operate for
a long period of time, depending upon the half-life of the
15 radioisotope, and over a wide temperature range, from less
than 0C to over 200C because of the constant energy output
of the radioactive source. These operating parameters
define the invention in a manner which is particularly use-
ful in deep oil or gas wells.
Additional or alternative specific design criteria con-
templated for a specific implementation of the preferredembodiment includes an approximately one-inch maximum outer
diameter and a maximum length of preferably not greater than
approximately two feet, an approximately l/2-inch diameter
25 by approximately six-inch length well at the heat source end
of the housing for receiving a similarly sized fuel capsule,
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a suitable heat transfer mechanism, such as a heat pipe, to
transfer heat from the radioisotope capsule to the head of
the Stirling engine with suitable insulation as needed in
the side wall and end of this section of the housing, a
5 threaded end cap in a 1/2-inch well to secure the fuel cap-
sule in place and to provide maximum thermal contact between
the surfaces of the fuel capsule and the well, an overall
efficiency of 15% or better at 200C, isolated electrical
output terminals across which approximately 10-20 vac rms
10 are provided from DC to as high a frequency as possible with
a power output between approximately 0.5 watt and approxi-
mately 2.5 watts, and with a power drain two times normal
for two seconds out of 100 seconds.
Thus, the present invention is well adapted to carry out
15 the objects and attain the ends and advantages mentioned
above as well as those inherent therein. While a preferred
embodiment of the invention has been described for the pur-
pose of this disclosure, numerous changes in the construc-
tion and arrangement of parts and the performance of steps
20 can be made by those skilled in the art, which changes are
encompassed within the spirit of this invention as defined
by the appended claims.
