Note: Descriptions are shown in the official language in which they were submitted.
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PULSED COMBUSTION DEVICE AND METHOD
Background of the Invention
The invention relates to a pulsed combustion
device and method of using such a device.
Pulsed combustion devices are known, for example
from US patent Nos. 2,899,287; 2,860,484 and 5,044,930, from
European patent Nos. 550401 and 636229 and from
International patent application PCT/EP93/00961 published as
WO 93/21477 on October 23, 1993.
The known devices generally comprise a combustion
chamber having an open downstream end and an upstream end
which is periodically closable by a one-way valve.
European patent No. 636229 and International
patent application PCT/EP93/00961 published as WO 93/21477
on October 23, 1993 disclose downhole pulsed combustor
devices that have cylindrical combustion chambers into which
small quantities of air are periodically injected to ignite
a fraction of the volume of natural gas in the chamber so as
to enhance the flow of natural gas to the wellhead.
A disadvantage of these known devices is that they
require complex procedures to start up and control the
pulsed combustion process and that they have a rather low
pumping efficiency.
The pulsed combustion device disclosed in
US patent No. 2,860,484 can be used to generate driving
and/or heat energy. The known device comprises a tubular
combustion chamber having an upstream end which is equipped
with a non-return valve and an open downstream end which
defines a tailpipe section which is slightly narrower than
the rest of the combustion chamber. The combustion chamber
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is arranged co-axially within a tube in which another non-
return valve is arranged upstream of the combustion chamber.
The second non-return valve is closed.
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periodically by high pressure fronts which are reflected
from the downstream end of the combustion chamber back
through the annulus surrounding the chamber. The presence
of two non-return valves which must open and close
sequentially is not attractive for downhole use since
varying wear, friction and pollution of the valves can
easily result in an incorrect and out of phase opening
and/or closing of the two valves which may eventually
result in stalling of the device.
US patent No. 2,899,287 discloses a pulsed combustor
which comprises either a single tubular combustion
chamber or two parallel tubular combustion chambers. In
each case each combustion chamber has an open tailpipe
having a slightly smaller internal diameter than the rest
of the combustion chamber and a fuel injection pump which
injects accurately defined quantities of fuel into each
combustion chamber to control the combustion process.
If the known device has a single combustion chamber
then it is equipped with a mechanical non-return valve
and if it has two parallel combustion chambers then it is
equipped with a pair of aerodynamic non-return valves.
These aerodynamic valves comprise U-shaped regenerative
tube systems, which have an inlet close to the downstream
end of the combustion chamber and which convey combustion
gas pressure pulses back to the inlet and which tend to
adjust themselves into phase-opposition.
A disadvantage of the pulsed combustor known from
US patent 2,899,287 is that it is not suitable for
downhole use since it is not feasible to install downhole
a fuel injection pump which remains stable over a period
of several years and there is no room available to
install two parallel combustion chambers with associated
non-return valves and a U-shaped regenerative tube
system.
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{ French patent No. 1252585 discloses another
9g
oscillating heating device with a Helmholz oscillator and
a U-shaped regenerative tube between the downstream and
the upstream end of the combustion chamber, which is not
suitable for use in a well because of the lack of space
available for such a U-shaped regenerative tube.
It is an object of the present invention to provide a
pulsed combustion device and method which are able to
operate safely and efficiently under varying downhole
conditions and which comprise a minimum of wear prone
components so that only minimal maintenance and
inspection is required.
Summary of the Invention
The pulsed combustion device according to the
invention thereto comprises a substantially tubular
combustion chamber having an upstream and a downstream
end, separate fuel and oxidant supply conduits for
supplying fuel and oxidant to the combustion chamber, one
of said conduits having a fluid discharge port debouching
into the combustion chamber between the upstream and
downstream ends thereof, the other of said conduits
having a fluid discharge port located at the upstream end
of the chamber which discharge port is equipped with
return flow limitation means which limit flow of
combustion fluids from the combustion chamber into the
fluid supply conduit and wherei-n the coqjbUs.tion chamber
is shaped as a Helmholz resonator having a tailpipe
section near the downstream end of which the smallest
cross-sectional area is between 0.15 and 0.30 times the
average cross-sectional area of the other parts of the
combustion chamber.
It has been found that by properly shaping the
combustion chamber as a Helmholz resonator the pulsed
combustor device becomes self-aspiring and discharging
without requiring a U-shaped regenerative tube.
AMENDED SHEET
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Preferably the tailpipe and the other parts of the
combustion chamber have a cylindrical or conical shape.
Experiments revealed that the above combustion
chamber geometry is optimal since it transmits a
significant part of the pressure fluctuations from inside
the combustion chamber to the outlet of the tailpipe
without destroying the pulse combustion process.
If the pulsed combustor according to the invention is
used to compress natural gas downhole in a gas production
well then it preferably is installed inside a production
tubing by means of a pair of expandable packers and air
or another oxidant such as oxygen is fed to the device
via a supply conduit in the casing-tubing annulus, which
conduit is connected to an orifice in the production
tubing which is located between the two packers. The air
or oxidant is then allowed to flow into the combustion
chamber from the annular space between the packers via an
oxidant supply port which debouches into the combustion
chamber between the upstream and downstream end thereof.
In that case it is preferred that the return flow
limitation means comprise one or more flapper-type
discharge or non-return valves.
Alternatively, the pulsed combustor device according
to the invention is used to heat the underground
formation which surrounds the welibore in which one or
more pulsed combustion devices-are operqtj~,d.
In that case the method according to the invention
comprises feeding fuel and oxidant to each pulsed
combustor device via fuel and oxidant supply conduits
which extend from the wellhead into the well and
repeatedly allowing in each pulsed combustor device the
MD09/TS6044PCT
AMENDED SHEET
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oxidant to react with a fraction of the fuel fed into the
combustion chamber thereby generating a high pressure
wave front which is inhibited at the upstream end of each
combustion chamber by the return flow limitation means
and which is enhanced at the downstream end of said
chamber by the tailpipe section. At the downstream end of
the tailpipe section the high pressure wave front is
reflected and followed by a low pressure wave front which
induces oxidant and fuel to flow into the combustion
chamber.
It is preferred that the return flow limitation means
of the heater device comprise one or more aerovalves
which do not comprise any movable parts or a regenerative
tube system extending between the downstream and upstream
ends of the combustion chamber.
Preferably a string of pulsed combustor devices is
suspended from the wellhead from the oxidant and fuel
supply conduits such that the devices are axially spaced
in the well.
Such a string of axially spaced pulsed combustion
devices is particularly suitable to heat underground
shale or heavy oil reservoirs such that the reservoir
temperature in the region of the wellbore is between 600
and 800 K.
Experiments have revealed that the pulsed combustor
device is able to operate in a stable manner at such high
temperatures over periods of many years and provides a
cost-effective alternative to existing electrical and
catalytic flameless combustion downhole heating devices.
Brief description of the drawings
The invention will be described in more detail with
reference to the accompanying drawings in which:
Fig. 1 is a longitudinal sectional view of a pulsed
combustor device according to the invention in a
production tubing of a natural gas production well;
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Fig. 2 is a longitudinal sectional view of two pulsed
combustor devices according to the invention which are
used to heat an underground formation; and
Fig. 3 is a graph in which the fraction of combusted
methane is plotted against the ratio AT/AC between the
minimum cross-sectional areas of the tail pipe and other
parts of the combustion chamber.
Detailed Description of the Invention
Referring to Fig. 1 there is shown a pulsed combustor
device 1 which is located in a production tubing 2 in a
natural gas production well 3 which traverses an
underground formation 4.
The pulsed combustor device 1 is sealingly secured
inside the production tubing 2 by means of a pair of
expandable packers 5.
Air or another oxidant, represented in the drawing as
02, is fed to the device 1 via an air supply tube 6 which
extends from the wellhead (not shown) through the
tubing/casing annulus to an orifice 7 in the tubing 2
between the packers 5.
The air flows from the orifice 7 via annular spaces 8
to a series of air discharge ports 9 which debouch into a
combustion chamber 10 of the device 1 at a location
between an upstream end 11 and a downstream end 12 of
said chamber 10.
A series of flapper-type discharge or non-return
valves 13 is arranged at the upstream end 11 of the
combustion chamber 10 which valves allow natural gas,
represented in the drawing as CH4, to flow from the
production tubing 2 below the device into the combustion
chamber 10, but which prevent natural gas and/or
combustion products, represented in the drawing as C02 +
H20 to flow back from the combustion chamber 10 into the
production tubing 2 below the device 1.
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In accordance with some embodiments of the present
invention the combustion chamber 10 is shaped as a Helmholz
resonator wherein the chamber 10 is provided with a narrow
and elongated tailpipe 15 which has a smallest diameter DT,
which preferably is between 0.3 and 0.5 times the average
diameter DC of the cylindrical lower part of the combustion
chamber. Experiments and computer calculations have
indicated that this DT/Dc ratio is optimal since the highest
pressure fluctuations and the highest massflow of natural
gas through the device 1 are achieved at lowest fuel
consumption as will be explained in more detail with
reference to Fig. 3.
In some embodiments, the Helmholz resonator has a
tailpipe section near the downstream end of which the
smallest cross-sectional area is between 0.15 and 0.30 times
the average cross-sectional area of the other parts of the
combustion chamber.
The device 1 of Fig. 1 is equipped with a glow
plug 16 to which electrical power is supplied via a power
cable 17. The glow plug 16 is continuously activated during
operation of the device 1 and is generally not switched off
when the device 1 has reached its normal operating
temperature since if the device 1 is used as a downhole gas
compressor its operating temperature is maintained at such a
low level that there is no spontaneous combustion of the
natural gas.
During normal operation of the device 1 pulsed
combustion takes place in the combustion chamber 10.
The frequency of the pulsed combustion process is
dictated by the Helmholz effect and is typically between 10
and 50 cycles per second.
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During each cycle a high pressure wave front is
generated which is followed by a low pressure wave front.
Both wavefronts are enhanced by the Helmolz effect so that a
maximum amount of natural gas is sucked into the chamber 10
when the low pressure wave front reaches the upstream end
thereof and also a maximum amount of natural gas and
combustion gases are pressed via the tailpipe 15 through the
downstream end of the chamber 10 as a result of the high
pressure wave front. The divergent shape of
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the tailpipe 15 further enhances the mass flow through
the combustion chamber.
If the device 1 is used as a downhole compressor in a
natural gas production well only a relatively small
amount of air or other oxidant, such as pure oxygen, is
supplied to the combustion chamber such that less than
10% of the natural gas flowing through the production
tubing 2 is combusted. The presence of a small fraction
of combustion gases only provides insignificant pollution
of the produced natural gas.
Referring to Fig. 2 there is shown a heat injection
well 20 which traverses an underground shale or heavy oil
bearing formation 21.
In the well 20 a string of pulsed combustion devices
22 according to the invention is suspended.
The devices 22 are suspended from a central methane
injection tube 23 which passes through the centre of each
of the devices 22. An air injection tube 24 is connected
to an air inlet chamber 25 of each device 22 via an
orifice 26.
The air inlet chamber 25 is connected to the
combustion chamber 27 via a number of aerovalves 28,
which allow air to flow up from the air inlet into the
combustion chamber but which inhibit combustion gas to
flow back from the combustion into the air inlet chamber.
During normal operation of the devices 22 methane
(CH4) or another fuel is injected via the methane
injection tube 23 and a series of methane discharge ports
29 into the combustion chambers 27. At the same time air
is injected into the chambers 27 via the aerovalves 28
which causes at the elevated temperature in the
combustion chambers 27 a pulsed combustion process to
take place.
If the devices 22 are used as heaters the combustion
process is only assisted by a glow plug (not shown)
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during start-up, whereas during normal operation
spontaneous combustion of the methane occurs in the
combustion chambers as a result of the prevailing
pressure and temperature in the chambers 27.
During each combustion cycle high and low pressure
wave fronts develop in the combustion chambers 22 at a
frequency which is dictated by the Helmholz effect, which
is induced by the presence of a tailpipe 30 at the
downstream end 31 of each combustion chamber which is
relatively narrow compared to the upstream part 32 of
each combustion chamber.
In the example shown the cross-sectional area of the
tailpipe is represented as AT and the cross-sectional
area of the upstream part 32 of the combustion chamber as
AC.
It will be understood that the cross-sectional area
AM of the methane injection tube 23 at the centre of the
devices 22 does not count as part of the cross-sectional
areas AT and AC of the tail pipes and upstream parts 32
of the combustion chambers 22. In the example shown the
ratio AT/AC is selected between 0.15 and 0.25 on the
basis of the following analysis.
Experiments revealed that the onset of thermo-
acoustical pulsations in a pulse combustor may be studied
by linear analysis of the one-dimensional conservation
equations for mass, momentum and energy. It was found
that the pulsations get the more damped,
a) the larger the gas velocity through the combustion
chamber 27 is;
b) the shorter the upstream part 32 of the combustion
chamber is relative to the length of the tail pipe 30;
c) the smaller the diameter of the tail pipe 30 is
relative to that of the upstream part 32 of the
combustion chamber.
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On the other hand it has been found that the pressure
build up in the combustion chamber 27 is the larger, the
more closed-off the combustion chamber 27 is. So there
must be an optimum tail pipe diameter at which the
highest pressure fluctuations are achieved.
The standard geometry ratio between the cross-
sectional areas of the tail pipe and the other parts of
the combustion chamber deviates from common dimensions of
pulse combustors in industrial and scientific
applications. A set of computer simulations has been done
to investigate whether a change in the cross-sectional
area ratio AT/AC can improve the performance of the pulse
combustor. The minimum tail pipe diameter is the only
parameter that is changed in these simulations.
The results of these computer simulations and
experiments are shown in Fig. 3.
Fig. 3 shows that an optimal tail pipe cross-
sectional area does indeed exist for a given compression
ratio at which the combusted fraction of methane is
minimal. A minimal methane combustion at a given
compression rate is a clear indication that the pulsed
combustion process performs in an optimal manner. Fig. 3
indicates that an optimum AT/AC ratio is between 0.15 and
0.25. If the tailpipe and other parts of the combustion
chamber are tubular and have an open centre as shown in
Fig. 1. then the ratio between their diameters DT/DC
should be between 0.3 and 0.5. The chosen diameter for
the standard geometry is in both cases reasonably close
to the optimal diameter. Nevertheless, for a compression
ratio of 1.15 the massflow can be increased by 20% by
choosing a somewhat broader tail pipe.
Also for the heater assembly shown in Fig. 2 it is
important to have an optimal compression ratio since this
ensures a stable operation of the device 22.
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The string of devices 22 may extend along the entire
depth of the shale oil formation. If required the heat
injection well 20 may be inclined or horizontal and may
be an open or a cased hole.
