Note: Descriptions are shown in the official language in which they were submitted.
CA 02436480 2003-07-31
CANADIAN PATENT APPLICATION
File \o. 45074.60
P~ROUS MEDIA. G~-1S BURNER
FIELD OF THE INVENTION
The present invention relates to a porous media gas burner and methods for its
use. In
particular, the invention relates to a downhole gas burner used in formation
heat treatment
methods.
BACKGROUND OF THE TNVENTION
Combustion of gases in porous media is a process where a combustible gaseous
mixture
is injected into a porous matrix and is combusted within the porous matrix.
Flames in porous
media have higher burning velocities and Leaner flammability limits than open
flames. These
effects are well known and are the consequence of excess enthalpy combustion.
Essentially, heat
that is generated in the combustion zone is transferred by radiation and
conduction through the
solid phase of the porous media to unburned gases. As a result, it is possible
to achieve
temperatures higher than the adiabatic flame temperature, and the increase in
burning velocities
can be significantly higher than the open flame laminar burning velocity for
the same mixture in
an open space.
Formation heat treatment is a process that is intended to improve hydrodynamic
conditions around the wellbore. If formati~n temperatures reach an adequate
level, blocking
water may be vapourized, clay structure may be dehydrated, clay minerals may
be partially
destroyed, and microfractures may be induced in the formation near the
wellbore. As a result,
permeability around the wellbore may be significantly improved.
It is known to use a downhole electrical heater in a formation heating method.
The
electrical heater is placed as close as possible to the target zone, and an
inert gas such as nitrogen
is co-injected through the annulus. The temperature of the injected gas may
rise to as high as
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800° C before entering the formation. However, this method involves
large energy requirements
which makes it cost-prohibitive, particularly with rising electrical energy
costs.
Combustion stimulation is a known technique to promote fluid production in a
formation.
S A combustion front is initiated in a wellbore by means of a surface heater
or burner and the front
is propagated into the formation to a distance of up to about 6 meters. The
formation in this zone
is reduced to clean burnt sand, which is very fluid permeable. I~owever, the
well casing is
subjected to high temperatures, which is undesirable, and there is an elevated
risk of explosions
or well bumouts using this technique. It is necessary to maintain wellbore
temperatures below
600° C in order to prevent damage to the liner, which limits the
temperature which may be
reached in the formation.
Therefore, there is a need in the art for a gas burner which may be used
downhole in a
formation heating process.
1 S SUMMARY ~F THE IIVVEl~TI~lIT
In one aspect, the invention may comprise a gas burner comprising a tubular
housing
adapted to operate in a pressurized environment, the housing defining an
intake opf~ning and a
flue opening and comprising means for receiving a supply of fuel and air; a
mixing zone where
the fuel and air are mixed comprising a packed bed of porous media; an
ignition zone comprising
a packed bed of porous media, and a reaction zone comprising a packed bed of
porous media;
wherein the pore size of the mixing zone and the reaction zone is smaller than
a minimum
quenching distance of a fuel gas under standard conditions while the pore size
of the ignition
zone is larger than the minimum quenching distance.
2S In another aspect, the invention may comprise a downhole formation heating
system for a
wellbore including a well casing, the system comprising:
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(a) a gas burner comprising a cylindrical housing defining an intake opening
and a
flue opening, the housing comprising means for receiving a supply of fuel and
air;
a mixing zone where the fuel and air are mixed; an ignition zone comprising an
igniter and a reaction zone, each zone comprising a packed bed of porous
media;
(b) an igniter for igniting the fuel and air within the gas burner;
(c) fuel and air supply tubing for delivering fuel and air to the burner; and
(d) means for delivering pressurized air or an inert gas in an annular space
between
the well casing and the fuel and air supply tubing.
In another aspect, the invention may comprise a method of heat treating a
formation comprising
the steps of:
(a) inserting a gas burner comprising a cylindrical housing defining an intake
opening
and a flue opening, the housing comprising means for receiving a supply of
fuel
and air; a mixing zone where the fuel and air are mixed; an ignition zone
comprising an igniter and a reaction zone, each zone comprising a packed bed
of
porous media, into a wellbore;
(b) inj ecting a fuel gas and air into the gas burner to create a combustible
mixture and
igniting the mixture to create a combustion front; and
(c) causing the combustion front to travel out the gas burner and into the
formation.
BRIEF DESCRIPTIOly1 OF THE DI~IhTG~
The invention will now be described by way of an exemplary embodiment with
reference
to the accompanying simplified, diagrammatic, not-to-scale drawings.
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Figure 1 is a schematic representation of one embodiment of the present
invention.
Figure 2 is a schematic representation of one embodiment of a formation heat
treatment
system.
DETAILED DESCRIPTI~1~T ~F' TIIE IliT'VEI~TTI~l~T
The present invention provides for a gas bunler and methods of using a gas
burner. When
describing the present invention, all terms not defined herein have their
common art-recognized
meanings.
In one embodiment, the invention comprises a gas burner as shown schematically
in
Figure 1. The burner generally includes a body (10) which comprises a mixing
zone (I2), an
ignition zone (14) and a reaction zone (16) where combustion will take place.
The body may
have any shape. In one embodiment, the body may be cylindrical or conical or
have both
cylindrical and conical sections. The body is preferably Iined with a heat-
refractory material such
as a ceramic Iiner.
As shown in Figure l, the mixing zone (12) and ignition zone (14) comprise
cylindrical
portions of the tubular body (10) while the reaction zone (16) comprises a
truncated conical
portion, with an expanding diameter as the reactants flow away from the mixing
zone of the inlet
end. In one embodiment, the outlet end diameter of the reaction zone may be
about twice that of
the inlet end. The reaction zone may have a length about 4 to 5 times the
diameter of the inlet
end. The mixing and ignition zones may have a length approximately equal to
their diameter.
All three zones (12, 14, 16) may be packed with a porous media bed within the
burner.
The packed bed may comprise heat resistant ceramic spheres such as alumina
beads or any other
suitable particulate material to create a porous media bed for example, but
not limited to,
zirconia-alumina composites, silicon carbide, or mullite (alumina-silicon
dioxide). As shown in
Figure l, air or oxygen is provided to the burner in the mixing zone (I2),
along with the
combustible gas. The gases mix in the mixing zone (12) and are ignited in the
ignition zone (14)
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by means of an igniter (not shown), which may be a small open flame burner or
a spark device.
Once ignited, the flame front will be allowed to advance into the reaction
zone (16).
In one embodiment, the mixing zone (12) may be packed with small size
particles so that
the pore size in the mixing zone is smaller than the minimum quenching
distance (MQD). The
second section would the ignition zone (14) and may be packed with larger size
particles so that
the pore size is larger than the minimum quenching distance. Tl~e size and
nature of the reaction
zone (16) particles (pore size) would depend on energy and operational
requirements, type of fuel
gas and operating conditions and could be of either uniform size or a
combination of sizes.
As used herein, the phrase '"minimum quenching distance'° or
"I~iIQD°' shall mean the
minimum diameter or opening dimension through which a flame may travel under
standard
conditions. It may be observed that a flame in a mixture within a flammable
range will be
extinguished if forced to propagate through a constriction. The walls of the
constriction exert a
repressive influence on the flame. A flame is quenched in a constriction
because of two
mechanisms which otherwise permit flame propagation: the diffusion of species,
and the
diffusion of heat. The walls of the constriction may extract heat and the
smaller the restriction,
the greater the surface to volume ratio will be. Similarly, the smaller the
constriction, the greater
the number of collisions of the active radical species with the wall, and the
greater the number of
these species which are destroyed. Accordingly, one skilled in the art will
understand that
increased temperature decreases the quenching distance and that quenching
distance decreases as
pressure increases.
MQD is a physical property of each fuel and may be determined in the
laboratory or by
using the criteria of Peclet number equal to 65. The T'eclet number is a
dimensionless parameter
that is based on the specific heat, laminar burning velocity, density, thermal
conductivity and
thermal diffusivity of the gas mixture, however the heating of the porous
media bed will affect
the minimum quenching distance.
It is generally accepted that main driving factor in the combustion of gases
in a porous
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media is heat recirculation through the porous media to preheat unburned
reactants. This
preheating of the reactants may permit combustion even if the pore size is
smaller than the MQD
of the porous media under standard conditions.
In one embodiment, the burner (IO) is adapted to operate in a pressurized
environment.
The body of the burner (10) is designed to withstand the desired pressure.
This provides the
opportunity to integrate the burner in the middle of a process stream, so that
exhaust gases may
be recovered at pressure for further treatment, separation or other downstream
processes. It may
allow use in subterranean hydrocarbon formations as will be subsequently
described. The
porosity of the packed bed may be controlled for specific applications. In one
embodiment, the
packed bed in the mixing zone and the reaction zone has a pore size smaller
than the minimum
quenching distance, while the ignition zone pore size may be larger than the
minimum quenching
distance. The pore size, combustible mixture flux, concentration of fuel gas
in oxidant (air), type
of fuel and shape of the burner may be varied to permit and optimize the
process in a pressurized
environment. A person skilled in the art may determine these matters with
minimal and routine
experimentation.
The main effect of operating pressure in the gas burner is a reduction in the
combustion
front velocity. Maximum temperatures attainable when operating at elevated
pressures are
generally lower than those temperatures observed for the same gas mixtures and
fluxes at
atmospheric pressure. While the effect of pore size is almost negligible at
atmospheric operating
pressure, as the operating pressure is increased, the velocity of the
combustion front increases as
the pore size decreases. At elevated operating pressures, burning velocities
appear to increase as
the inlet gas velocity increases. Additionally, burning velocities appear to
increase as the fuel gas
concentration is decreased.
The relatively smaller pore size of the mixing and the reaction zone promotes
mixing of
the reactants and preheating of the reactants due to heat transfer from the
solid phase to the gas
phase. The relatively larger pore size of the ignition zone allows easier
ignition of the reactants
and propagation of the flame into the reaction zone.
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The gas burner (10) may be used as a downhole gas burner to be used in a
formation
heating method. Generally, the formation heating method may comprise two
stages. In a first
stage, the burner is placed in the wellbore at the level of the formation by
means of coiled tubing
or the like. The tubing also provides the means by which a combustible mixture
is provided to
the burner. The combustible mixture may be a lean mixture of natural gas and
air which is below
the flammability limits of natural gas at atmospheric pressure. The mixture
may sustain a
combustion front within the burner which expels hot flue gases into the
formation. :(f desired, the
combustion front may be controlled to travel outward from the burner into the
formation. The
combustion front is controlled by increasing or decreasing the fuel gas flux,
changing the
concentration of fuel gas in the mixture fuel-air (oxidant) and by using
different particle size or a
combination of particle sizes in the reaction zone. These variable elements
may be varied either
singly or in combination.
Depending on the composition of the formation hydrocarbons, the amount of
oxygen in
the flue gas and the flue gas temperature, some oxidation/combustion reactions
may start to take
place in the formation at the same time as gas combustion is occurring in the
burner. Once the
combustion front leaves the burner, it is preferred to increase the
concentration of the fuel, so that
the temperature of the reaction front increases. This may be done safely
because the temperature
in the burner and the wellbore will not be high enough to facilitate or
sustain combustion. In
other words, the burner becomes a flame arrester once the combustion front
travels outward into
the formation.
The burner described herein may be used as a downhole burner in alternative
methods.
As described above, the burner may be used in well stimulation method where
blocking water is
vapourized, clays may be partially destroyed, asphaltenic deposits may be
burned and
rnicrofractures in the formation may be propogated. In another example, the
downhole gas
burner may be used in a downhole steam generation method for cyclic or
continuous steam
injection in deep heavy oil wells. In another example, the downhole gas burner
may be used in a
high pressure air injection technique, where combustion is initiated in the
formation and air is
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then continuously injected into the producing layer. The combustion of in-
place oil may provide
thermal and gas drive to the oil reservoir.
In one embodiment, with reference to Figure 2, a downhole burner (10) is
positioned near
the producing formation (20) by means of continuous or coiled tubing (22) and
anchored to the
well casing by means of an anchor {24). Air is injected through the tubing
(22) and a
combustible gas such as natural gas is injected through a separate tubing (26)
to the burner. Air
or an inert gas such as nitrogen may be inj ected in the annulus between the
well casing and the
tubing (22). The burner has a mixing zone (120), an ignition zone (122) and a
reaction zone
(124), each packed with a suitable porous media. As described above, in a
preferred
embodiment, the particle size or minimum quenching distances in each zone may
be varied. The
combustion front (126) will be established in the reaction zone (124) and move
outward into the
formation. At the same time, hot flue gases (128) are pushed outward into the
formation.
In each case, the ignition and combustion downhole and/or in the formation may
be
initiated using a lean combustible mixture, which may include waste gases. The
lean mixture
reduces the risk of explosive mixtures accumulating in the system.
As will be apparent to those skilled in the art, various modifications,
adaptations and
variations of the foregoing specific disclosure can be made without departing
from the scope of
the invention claimed herein. The various features and elements of the
described invention may
be combined in a manner different from the combinations described or claimed
herein, without
departing from the scope of the invention.
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EXAMPLES
The following examples describe specific embodiments which are exemplary of
the
present invention. They are not intended to limit the claimed invention.
Example 1 Porous Media
In one embodiment, the porous media comprises relatively uniform alumina
spheres
(90% alumina and 10% silica) having the following physical and hydrodynamic
properties:
(a) specific heat capacity at 20 C, 920
J/kgK
(b) thermal conductivity at 20 C, W/mK 1607
(c) Density, kg/m3 3,600
(d) diameter, m ~.6 E-3 to 2.9
E-3
(e) Porosity, fraction 0.383
(f) Pore size, m 2.31 E-3 to 1.18
E-3
(g) Permeability (Carman-Kozeny equation)2.544 E-8 to 6.625
m2 E-9
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