U.S. patent application number 14/185411 was filed with the patent office on 2014-09-04 for throttling boiler for fouling mitigation.
This patent application is currently assigned to CONOCOPHILLIPS COMPANY. The applicant listed for this patent is CONOCOPHILLIPS COMPANY. Invention is credited to David William LARKIN, James P. SEABA.
Application Number | 20140246196 14/185411 |
Document ID | / |
Family ID | 51420348 |
Filed Date | 2014-09-04 |
United States Patent
Application |
20140246196 |
Kind Code |
A1 |
LARKIN; David William ; et
al. |
September 4, 2014 |
THROTTLING BOILER FOR FOULING MITIGATION
Abstract
Methods and systems generate steam for thermal oil recovery,
such as a steam assisted gravity drainage (SAGD) operation.
Feedwater is first pressurized to a pressure above that desired for
steam injection in the SAGD operation before being heated to avoid
at least some nucleate boiling. After being throttled, the local
boiling regime is beyond the nucleate boiling regime due to the
local pressure drop and the enhanced mixing caused by the
throttling process. Two-phase liquid may continue through the
boiler generating higher quality steam.
Inventors: |
LARKIN; David William;
(Tulsa, OK) ; SEABA; James P.; (Calgary,
CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
CONOCOPHILLIPS COMPANY |
Houston |
TX |
US |
|
|
Assignee: |
CONOCOPHILLIPS COMPANY
Houston
TX
|
Family ID: |
51420348 |
Appl. No.: |
14/185411 |
Filed: |
February 20, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61771202 |
Mar 1, 2013 |
|
|
|
Current U.S.
Class: |
166/272.3 ;
166/61 |
Current CPC
Class: |
E21B 36/005 20130101;
F22B 27/00 20130101; E21B 43/2406 20130101; F22B 1/22 20130101 |
Class at
Publication: |
166/272.3 ;
166/61 |
International
Class: |
E21B 43/24 20060101
E21B043/24; E21B 36/00 20060101 E21B036/00 |
Claims
1. A method of producing steam for oil production, comprising: a)
providing feedwater through a steam generator system under
sufficient pressure to heat said feedwater and prevent formation of
steam in an output of heated feedwater; b) passing said heated
feedwater to a pressure reducer where said heated feedwater is
flashed to steam; and c) conveying said steam into a wellbore for
mobilizing oil.
2. The method of claim 1, wherein the steam generator is a
once-through steam generator.
3. The method of claim 1, wherein the feedwater in step a) is
pressurized to at least 14,000 kilopascals.
4. The method of claim 1, wherein the feedwater in step a) is
pressurized to at least 20,000 kilopascals.
5. The method of claim 1, wherein the pressure reduction in step b)
decreases pressure by at least 3500 kilopascals.
6. The method of claim 1, wherein the pressure reduction in step b)
decreases pressure by at least 6500 kilopascals.
7. The method of claim 1, wherein said feedwater in step b) is
pressurized by a pump.
8. A steam generator system for oil production, comprising: a) an
economizer for preheating feedwater; b) a radiant section for
heating said feedwater from said economizer; c) a pressurizing
element for increasing the pressure of said system enough to
prevent nucleate boiling of the feedwater at least within the
economizer; d) a pressure-reducing element for reducing the
pressure and producing flash steam; and e) an injection assembly
for injecting the flash steam into an oil reservoir, wherein
elements a though e are fluidly connected.
9. The steam generator system of claim 8, further comprising a
flash vessel.
10. The steam generator system of claim 8, wherein said
pressure-reducing element is at least one of an orifice and a
throttling valve.
11. The steam generator system of claim 8, wherein said
pressure-reducing element reduces the system pressure of said
feedwater from said radiant section.
12. The steam generator system of claim 8, wherein said
pressure-reducing element reduces the system pressure from said
economizer.
13. The steam generator system of claim 8, wherein said steam
generator system is a once-through steam generator.
14. The steam generator system of claim 8, wherein said
pressurizing element is a pump.
15. The steam generator system of claim 8, wherein said
pressurizing element increases the pressure of said feedwater to at
least 14,000 kilopascals.
16. The steam generator system of claim 8, wherein said
pressurizing element increases the pressure of said feedwater to at
least 20,000 kilopascals.
17. The steam generator system of claim 8, wherein said
pressure-reducing element reduces the pressure by at least 3500
kilopascals.
18. The steam generator system of claim 8, wherein said
pressure-reducing element reduces the pressure by at least 6500
kilopascals.
19. An improved method of producing oil, the method comprising
generating steam to inject into a wellbore and mobilize oil for
production, the improvement comprising heating feedwater under a
pressure sufficient to prevent nucleate boiling, reducing said
pressure to make flash steam prior to any nucleate boiling of the
feedwater and injecting said flash steam into a wellbore to
mobilize the oil for production.
Description
PRIOR RELATED APPLICATIONS
[0001] This application is a non-provisional application which
claims benefit under 35 USC .sctn.119(e) to U.S. Provisional
Application Ser. No. 61/771,202 filed 1 Mar. 2013, entitled
"THROTTLING BOILER FOR FOULING MITIGATION," which is incorporated
herein in its entirety.
FIELD OF THE INVENTION
[0002] The invention relates to methods and systems for generating
high pressure steam with minimal or eliminated fouling otherwise
resulting largely from boiling nucleation.
BACKGROUND
[0003] Steam Assisted Gravity Drainage (SAGD) is an enhanced oil
recovery technology for producing heavy crude oil and bitumen. It
is an advanced form of steam stimulation in which a pair of
horizontal wells are drilled into the oil reservoir, one a few
meters above the other. High pressure steam is continuously
injected into the upper wellbore to heat the oil and reduce its
viscosity, causing the heated oil to gravity drain into the lower
wellbore, where it can be pumped to the surface.
[0004] As in all thermal recovery processes, the cost of steam
generation is a major contributor to the cost of oil production.
Historically, natural gas has been used as a fuel for Canadian oil
sands projects, due to the presence of large stranded gas reserves
in the oil sands area. However, with the building of natural gas
pipelines to outside markets in Canada and the United States, the
price of gas has become an important consideration. Other sources
of generating heat are under consideration, notably gasification of
the heavy fractions of the produced bitumen to produce syngas,
using the nearby deposits of coal, or even building nuclear
reactors to produce the heat.
[0005] Considering oil production volume, and the fact that
approximately 3 barrels of water are needed for every barrel of
oil, the water requirements for SAGD are immense. In addition to
the cost of steam generation, each barrel of oil produced in SAGD
is coproduced with 2-5 barrels of water, which then must be
separated from the oil, and treated and/or reused. Water treatment
facilities further contribute to cost.
[0006] In general, the SAGD process requires high quality, high
temperature and high pressure steam. For example, the SAGD process
may call for 100% quality, 7,000-11,000 kilopascals (kPa) and
285-318.degree. C. temperature steam. In a typical steam/water
circuit, boiler feedwater is supplied by a high pressure feed-water
pump at an elevated pressure around 13,000 kPa or less, which
results in corresponding steam pressure that gradually decreases
along the pipeline to aforementioned injection pressures due to the
loss in transportation and eventually by choking to the desired
injection pressure.
[0007] A once-through steam generator (OTSG), for example,
generates around 75% to 80% quality steam, which then goes through
a series of liquid-steam separators (also called "flash drum") to
increase the low steam quality of OTSG. OTSG is a large, continuous
tube type steam generator in which the steam is produced at the
outlet of the continuous tube. Feedwater is supplied at one end of
the tube having low temperature, and then undergoes heating and
boiling as it travels in a single pass along the tube.
[0008] Typically, an OTSG comprises a convection section (also
called economizer section) and a radiant section. In the convection
section, the feedwater is preheated by heat exchange with a hot
combustion gas, usually flue gas. In the radiant section, the
feedwater/wet steam is heated by the heat radiated from the
furnace, resulting in about 80% quality steam, i.e. the weight
ratio of water to steam at the outlet of the generator is about
1:4.
[0009] A source of large amounts of fresh or brackish water and
large water recycling facilities are required in order to create
the steam for the SAGD process. Boiler feed-water (BFW) quality is
critical because dissolved solids develop scales that are the major
cause of boiler failure and efficiency losses. Therefore, the total
dissolved solids (TDS) for BFW needs to be controlled under a
certain level to prevent or alleviate the scaling issue.
[0010] Fouling is the contamination of the heating surface, and the
build-up of contaminant eventually decreases the heat-flux and thus
the heating efficiency. Therefore, the boiler has to be shut down
several times a year to remove the fouling layer and/or repair the
tubing. In addition to the repair cost, the downtime increases the
cost of the SAGD operation.
[0011] Therefore, a need exists for an improved heating/pre-heating
scheme that can minimize the fouling issues and reduce the downtime
and cost for SAGD operations.
SUMMARY
[0012] The present disclosure provides a method of eliminating or
minimizing the fouling caused by nucleate boiling and/or transition
boiling of the feedwater in a steam generator. The current
invention significantly reduces or even eliminates fouling by
heating the boiler feedwater at pressures significantly higher than
the output steam delivery pressure, thereby maintaining the
feedwater in liquid phase before flashing it off to generate steam.
The method, therefore, minimizes or eliminates the fouling caused
by nucleate boiling in the boiler. This approach can minimize the
downtime of the boiler for repairing or removing the fouling,
thereby, increasing the operating time.
[0013] The disclosure provides a method of preheating feedwater of
a steam generator, comprising a) providing feedwater through a
steam generator system under sufficient pressure to heat said
feedwater and prevent the formation of steam; b) passing said
heated feedwater out of said steam generator system to a pressure
reducer where said heated feedwater is flashed to steam; and c)
conveying said steam into a wellbore for mobilizing oil.
[0014] Any suitable valve can be used to flash the heated feedwater
to steam, including a throttling valve, fine nozzle or orifice, and
the like. Armstrong-Yoshitake Inc., for example, makes the GP-2000R
valve, which is a high performance externally piloted throttling
back pressure valve for large capacity applications, and they make
many additional pressure reducing valves.
[0015] As discussed further herein, the pressure-reducing step can
be performed before or after the feedwater enters the radiant
section, depending on the system configuration, where fouling tends
to build up, and other considerations. For feedwater supplied to
the boiler's economizer, the pressure-reducing step can be
performed before or after the heating step at the radiant section.
For feedwater supplied directly to the furnace heater tubes or heat
exchanger tubes, the pressure-reducing step is performed after such
heating.
[0016] The disclosure also provides a steam generator system,
comprising the following components in fluid communication: a) a
pressurizing element for increasing the pressure of said feedwater;
b) an economizer for preheating the feedwater; c) a radiant section
for further heating said feedwater from said economizer, and d) a
pressure-reducing element for reducing the system pressure
positioned as desired to at least minimize fouling. As noted above,
the pressure reducing element can be after the preheating, or after
the further heating steps. The system can also be combined with a
flash vessel or other separator, for separating steam from
condensate, and the flash steam can be injected downhole, and
condensate rerouted, for example to the economizer section, either
as feedwater, or as a heat source for the economizer.
[0017] In another embodiment, an improved method of producing oil
includes heating feedwater sufficiently to make steam to inject
into a wellbore and mobilize oil for production, the improvement
comprising heating feedwater under a pressure sufficient to prevent
nucleate boiling, rapidly reducing said pressure to make flash
steam and injecting said flash steam into a wellbore, thus
mobilizing oil for production.
[0018] As used herein, "heat flux" is the rate of heat energy
transfer through a given surface, in other words, the heat rate per
unit area, whereas "critical heat flux" describes the thermal limit
of a phenomenon where a phase change occurs during heating, which
suddenly decreases the efficiency of heat transfer, thus causing
localized overheating of the heating surface.
[0019] As used herein, "flash steam" is the name given to the steam
formed from hot condensate when the pressure is reduced. Flash
steam is no different from normal steam, it is just a convenient
name used to explain how the steam is formed. Normal or "live"
steam is produced while heating at a boiler, steam generator, or
waste heat recovery generator--whereas flash steam occurs when high
pressure/high temperature condensate is exposed to a large pressure
drop, such as when exiting a steam trap.
[0020] High temperature condensate contains high energy that cannot
remain in liquid form at a lower pressure because there is more
energy than that required to achieve saturated water at the lower
pressure. The result is that some of the excess energy causes a
percentage of the condensate to flash.
[0021] The percentage of flash steam generated (flash steam ratio)
can be calculated from:
Flash%=(Hf1-Hf2)/Hfg2*100
where hf1=Specific Enthalpy of Saturated Water at Inlet,
hf2=Specific Enthalpy of Saturated Water at Outlet and hfg2=Latent
Heat of Saturated Steam at Outlet. This assumes no energy transfer
to the surroundings (adiabatic).
[0022] As used herein, "economizer" means the devices for reducing
energy consumption in a steam-generating operation by preheating
feedwater. Typically, an economizer is in the form of a heat
exchanger where the thermal energy is transferred from a high
temperature fluid (e.g., steam condensate, flue gas or other waste
heat source) to the feedwater such that less energy is required to
vaporize it. Economizers are mechanical devices intended to reduce
energy consumption or to perform another useful function such as
preheating a fluid. They are fitted to a boiler and save energy by
using e.g., the exhaust gases from the boiler or other hot plant
fluids to preheat the cold feedwater. It has been reported that
approximately 35 to 50% of the total absorbed heat in OTSG is
transferred in the economizer.
[0023] As used herein, a "flash steam recovery vessel" or "flash
vessel" is that vessel used to separate flash steam from
condensate. After condensate and flash steam enter the flash
vessel, the condensate falls by gravity to the base of the vessel,
from where it is drained, via a float trap, usually to a vented
receiver from where it can be pumped. The flash steam in the vessel
is piped from the top of the vessel to any appropriate pressure
steam equipment or injected directly into the wellbore.
[0024] As used herein, "radiant section" means the section in a
steam generator where the heating of feedwater is primarily
achieved by radiant heat transfer.
[0025] The use of the word "a" or "an" when used in conjunction
with the term "comprising" in the claims or the specification means
one or more than one, unless the context dictates otherwise.
[0026] The term "about" means the stated value plus or minus the
margin of error of measurement or plus or minus 10% if no method of
measurement is indicated.
[0027] The use of the term "or" in the claims is used to mean
"and/or" unless explicitly indicated to refer to alternatives only
or if the alternatives are mutually exclusive.
[0028] The terms "comprise", "have", "include" and "contain" (and
their variants) are open-ended linking verbs and allow the addition
of other elements when used in a claim.
[0029] The phrase "consisting of" is closed, and excludes all
additional elements.
[0030] The phrase "consisting essentially of" excludes additional
material elements, but allows the inclusions of non-material
elements that do not substantially change the nature of the
invention.
[0031] The following abbreviations are used herein:
TABLE-US-00001 ABBREVIATION TERM ATM Atmosphere CPF Central
processing facility OTSG Once-through steam generator SAGD
Steam-assisted gravity drainage Ts Saturation temperature
BRIEF DESCRIPTION OF THE DRAWINGS
[0032] FIG. 1 illustrates one embodiment of the disclosure, where
the pressurization of feedwater occurs prior to entering the
economizer, and wherein the pressure-reducing step occurs within
the economizer (but after preheating).
[0033] FIG. 2 illustrates another embodiment of the disclosure,
where the feedwater is pressurized prior to entering the
economizer, and wherein the depressurization step occurs outside
the economizer.
[0034] FIG. 3 illustrates another embodiment of the disclosure,
where the pressurization of feedwater occurs prior to entering the
economizer, and the flash steam occurs after both the preheating
and the further heating in the radiant section.
[0035] FIG. 4 illustrates a variation wherein a different heating
system is used, but feedwater is still pressurized to prevent
nucleate boiling, and the very hot water is flashed to steam after
exiting the heating unit.
[0036] FIGS. 5 and 6 are pressure-enthalpy diagrams of embodiments
illustrated in FIGS. 1-4.
DETAILED DESCRIPTION
[0037] The disclosure provides a novel method for generating steam
with minimized or eliminated fouling caused by nucleate boiling
and/or transition boiling. The disclosure also provides a novel
system for implementing the method. It is believed that by using
the method and system of the methods described herein, fouling in
the steam generator due to nucleate boiling can be greatly reduced
or eliminated, thereby reducing the operational cost and downtime
for repairing and maintaining the steam generator.
[0038] Methods and systems generate steam for thermal oil recovery,
such as a steam assisted gravity drainage (SAGD) operation.
Feedwater is first pressurized to a pressure above that desired for
steam injection in the SAGD operation before being heated to avoid
at least some nucleate boiling. After being throttled, the local
boiling regime is beyond the nucleate boiling regime due to the
local pressure drop and the enhanced mixing caused by the
throttling process. Two-phase liquid may continue through the
boiler generating higher quality steam.
[0039] Nucleate boiling is considered one of the main reasons for
fouling in heat transfer tubes. Nucleate boiling is characterized
by the growth of bubbles on a heated surface, which rise from
discrete points on a surface, whose temperature is only slightly
above the liquid temperature. As the bubble enters the bulk flow,
the bubble condenses back to liquid. In general, the number of
nucleation sites are increased by an increasing surface temperature
and by irregular surfaces of the boiling vessel.
[0040] Nucleate boiling takes place when the surface temperature is
hotter than the saturated fluid temperature by a certain amount but
where the heat flux is below the critical heat flux. The critical
heat flux defines a maximum heat flux between nucleate boiling and
transition boiling. For example, nucleate boiling for water may
occur when the surface temperature is higher than the saturation
temperature (T.sub.s) by between 4.degree. C. to 30.degree. C.
[0041] In general, an improved method of generating steam for SAGD
and other heavy oil production uses is provided, wherein feedwater
is pressurized, to e.g., about 14,000 kilopascals (kPa), thus
minimizing or eliminating nucleate boiling. The pressurized water
is heated while maintaining a liquid phase, and then later flashed
to steam, which can be used downhole. According to the phase
diagram of water, the pressurized feedwater can be heated in liquid
phase without vaporizing, and the absorbed enthalpy will transform
the water into steam once the heated feedwater is flashed off. The
flash-off process involves little or no boiling, thereby reducing
the fouling. The method can be combined with an economizer or heat
exchange system and the feedwater can be pressurized either before
the heat exchanger or afterwards, as convenient or as dictated by
the existing tendencies towards fouling.
[0042] A system for generating steam is also provided, comprising a
pressurizing element for increasing the pressure of the system, an
optional economizer for preheating feedwater, a radiant section for
heating the feedwater from the economizer, and a pressure-reducing
element for reducing the system pressure and producing flash steam.
In this system, the pressurizing element increases the pressure of
the system before the feedwater is supplied to the economizer
and/or radiant section. Of course, all of the elements are in
fluidic connection, such that pressure and fluid can travel from
one part of the system to another.
[0043] A flash vessel can be used to separate flash steam from
condensate, which can be routed back to e.g., the economizer for
preheating feedwater.
[0044] In preferred embodiments, the pressurizing element is
upstream of the economizer, but not necessarily so. The economizer,
if used, is preferably upstream of the radiant section, which is
upstream of the pressure reducing valve, which is upstream of the
flash vessel, and the condensate from the flash vessel preferably
routes back to the pressurizing element or economizer.
Alternatively, the pressure reducing valve can be upstream of the
radiant section if there is limited or no tendency to fouling in
the radiant section.
[0045] The pressurizing element can be any device, such as a pump,
known in the field to increase system pressure. The
pressure-reducing element can be any device known in the field to
reduce system pressure. Non-limiting examples include orifices,
valves, or a combination thereof.
[0046] The system pressure is increased to the extent that the
feedwater can remain in liquid phase without boiling when being
heated. Also, the system pressure may only be increased to the
extent commercially feasible without incurring additional cost to
replace the piping to withstand higher pressure. In one embodiment,
the system pressure is increased to at least 20,000 kPa, at least
14,000 kPa or at least 10,000 kPa. Use of solvents and/or
generation of the steam at a pad for injection instead of a central
processing facility may enable lower pressure steam needs such that
the increase of the system pressure may be to only at least 5000
kPa or at least 3500 kPa.
[0047] The placement of the pressure-reducing element in the system
may vary, depending on where the fouling is to be reduced. For
example, if fouling is serious in the economizer, but not in other
parts of the OTSG, then the pressure-reducing element can be
installed right after where the feedwater exits the economizer. If,
on the other hand, the fouling is to be reduced throughout the
OTSG, the pressure-reducing element can be installed after where
the feedwater exits the radiant section, thereby maintaining the
feedwater in liquid phase to avoid nucleate boiling throughout. In
some embodiments, the pressure-reducing element accommodates
pigging and may be located to enable and/or not interfere with
other maintenance and operational needs.
[0048] The system pressure is reduced to the extent that the
feedwater can rapidly convert from liquid to steam after being
preheated or heated. This rapid phase conversion involves little or
no boiling, and therefore can minimize the fouling associated with
nucleate boiling. In one embodiment, the system pressure is reduced
by at least 6500 kPa or at least 3500 kPa due to the
pressure-reducing element.
[0049] FIG. 1 illustrates an embodiment of the present disclosure.
Shown therein is a configuration of a steam generator, wherein
feedwater is supplied first to an economizer section 101 for
pre-heating, followed by a radiant section 102 to generate the
steam of specified quality. A separator 103 then separates the
steam 104 from the un-vaporized water 105, where the steam is
supplied to SAGD operations and the hot water is discharged or
recycled for further use.
[0050] Prior to being supplied to the economizer 102, the feedwater
is first pressurized by a pressurizing means 107. The pressurizing
means can be any mechanism that effectively pressurizes the system
such that the boiling point of water is elevated. Here the
pressurizing means 107 is a pump for supplying water under
pressurized condition.
[0051] The pressurized feedwater 108 is then supplied to the
economizer section 101, where the feedwater can be pre-heated by
the exhausted gases of the boiler, such as CO.sub.2, through heat
exchange. Because of the elevated pressure of the system, the
feedwater will not be vaporized at this point, but instead remains
in liquid phase. One or more pressure reducers 106 can be installed
in the pipeline so as to reduce the system pressure before the
water enters the radiant section 102, where the feedwater is
further heated to produce the remaining steam. The pressure reducer
can be any known mechanism that reduces the pressure, for example,
orifices inserts, valves, etc.
[0052] FIG. 5 shows a pressure-enthalpy steam diagram for water.
The upper left region is the liquid region where all water remains
liquid. The upper right is the supercritical region where distinct
liquid and gas phases do not exist. As the critical point is
approached, the properties of the gas and liquid phases approach
one another, resulting in only one phase at the critical point: a
homogeneous supercritical fluid. The lower left is the saturated
region, where liquid and vapor water co-exists. The lower right is
the vapor region where all water becomes vaporized. In a
traditional steam generator, the water is heated along the line
from point 1 to 2, where nucleate boiling occurs around the
intersection between the 1-2 line and the boundary between liquid
and saturated water (point 6). As discussed above, nucleate boiling
is one major reason resulting in fouling, and is preferably
avoided.
[0053] The methods described herein with reference to FIG. 1, in
contrast, take a different route. From point 1 in FIG. 5, the
system is first pressurized to point 3, where the pressure is
significantly higher than one atmosphere, and preferably higher
than the highest pressure of the saturated region to the extent
commercially feasible, and more preferably at least 14,000 kPa. The
feedwater under pressurized condition is then pre-heated at this
pressure along the line between points 3 and 4. The pressure of the
system is then reduced (e.g., to between 5000 kPa and 13,000 kPa)
after passing through the orifices, thus reaching point 5. At this
stage, there could be a mixture of steam and liquid water, which is
then heated at the radiant section 102 as depicted by the line from
point 5 to point 2. As such, the majority of nucleation boiling,
particularly corresponding to the path from point 1 to point 5, is
avoided.
[0054] In the conventional non-pressurized condition, continuous
heat is provided to the heat exchange tubes, where slow (e.g.,
residence times around one minute) and continuous boiling takes
place, and this is believed to be the main reason of fouling. In
this example, because the enthalpy of the feedwater is sufficiently
high and would have vaporized the feedwater under lower pressure,
the reduction of the system pressure at the pressure reducers 106
rapidly converts the feedwater from liquid to vapor, and therefore
involves little or no boiling of the water. This vaporization
approach in turn reduces the chance of fouling.
[0055] FIG. 2 shows a variation of the embodiment illustrated in
FIG. 1. The basic configuration of the steam generator still
includes the economizer section 201, the radiant section 202 and
the separator 203. The feedwater is still supplied with the system
pressurized by a pump 207 prior to entering the economizer 201. The
only difference between FIG. 1 and FIG. 2 is that in this
embodiment, the pre-heated water circulates outside the economizer
section 201 and the pressure is reduced at this point. This
embodiment shows that the inventive method can be applied to
different OTSG configurations.
[0056] FIG. 3 illustrates another embodiment that still includes an
economizer section 302 and a radiant section 303. The pressurized
feedwater 308 is supplied from a pump 307 to the economizer 302 for
pre-heating, followed by further heating in the radiant section
303. The heated water then goes through a pressure reducer 310
where flash steam is produced before entering the separator 304.
The separator 304 then separates the high pressure steam 305 from
the residual water 306 (if any), which will e.g., be directed to
heat exchanger 309 to further pre-heat the pressurized feedwater
308 before being discharged or recycled.
[0057] FIG. 6 shows a pressure-enthalpy diagram for this embodiment
depicted in FIG. 3. Similar to FIG. 5, the path from point 1 to 3
corresponds to the pressurizing step by the pump 307 in FIG. 3.
However, as seen in FIG. 3, the pressure-reducing step is not
performed until after all the heating steps, including preheating
at the economizer and heating at the furnace, are completed, hence
the end point 2 in this figure coincides with the reduced-pressure
point 5 in FIG. 5. It is to be noted that the pressurizing-heating
path of points 1-3-4-5-2 effectively bypasses the boiling
nucleation, thereby reducing the fouling.
[0058] FIG. 4 is another embodiment of the method utilizing a heat
exchanger 501 with high pressure water heating tubes. In this
embodiment, the heat exchanger 501 for heating water has a hot
fluid inlet 502 for introducing a high temperature fluid,
preferably having high thermal capacity, and a hot fluid exit 503
for discharging the hot fluid after heat exchange. The high thermal
capacity also helps the efficiency of the heat exchange. The system
is first pressured by pump 507, and feedwater 508 is then supplied
to the heat exchanger 501 for heating to generate a very hot water.
The hot fluid preferably has a temperature significantly higher
than the feedwater 508 such that the transferred heat would be
sufficient to vaporize the feedwater 508 without the additional
pressure that maintains the feedwater 508 in liquid phase. A
pressure reducer, in this case a valve 510, reduces the pressure of
the water for vaporization, which then enters the separator 504.
The high pressure steam 505 is then supplied to SAGD operations,
whereas the water 506 can be introduced to another heat recovery
device 509 to pre-heat the pressurized feedwater 508.
[0059] Although the steam is generated by a different mechanism,
operation of the heat exchanger 501 in FIG. 4 follows a similar
path in the pressure-enthalpy diagram illustrated in FIG. 6,
thereby avoiding boiling nucleation and minimizing fouling in the
water pipelines.
[0060] For the embodiments illustrated in FIG. 3-4, the heated high
pressure feedwater flashes off at either the central processing
facility or well pad operating pressure, depending on the steam
generation location. As implemented, in addition to the costs
saved, this method can reduce the pipe rack capital of the surface
facility because now only water lines need to go to the well pads
instead of steam lines since steam may be produced on site (i.e. at
the well pads) and not before.
[0061] Based on the above illustrations, it is clearly shown that
the methods and systems herein described pressurize the feedwater
before it enters the heating mechanism and thereby avoids the
nucleate boiling phase that directly contributes to fouling.
Downtime for pigging/repairing the boiler and pipes can be greatly
reduced, therefore cutting down the operation cost.
[0062] The following documents are incorporated by reference in
their entirety:
[0063] Gwak et al., A Review of Steam Generation for In-Situ Oil
Sands Projects, Geosystem Engineering, 13(3), 111-118 (September
2010).
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