Note: Descriptions are shown in the official language in which they were submitted.
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PRODUCED WATER STEAM GENERATION PROCESS
USING PRODUCED WATER BOILER WITH GAS TURBINE
Field of the Invention
The invention relates to thermal recovery methods and systems for heavy
hydrocarbon deposits,
and specifically to such methods and systems requiring steam injection to
mobilize the deposits.
Background of the Invention
In the field of subsurface hydrocarbon production, it is known to employ
various stimulation
procedures and techniques to enhance production. For example, in the case of
heavy oil and
bitumen housed in subsurface reservoirs, conventional drive mechanisms may be
inadequate to
enable production to surface, and it is well known to therefore inject steam
or steam-solvent
mixtures to make the heavy hydrocarbon more amenable to movement within the
reservoir
permeability pathways, by heating the hydrocarbon and/or mixing it with
lighter hydrocarbons or
hot water.
In steam-assisted gravity drainage ("SAGD") and cyclic steam stimulation
("CSS") hydrocarbon
recovery operations, steam is generated at surface by steam generation units
and injected
downhole into a well, where it is subsequently introduced into an underground
hydrocarbon
formation in which the well lies, after which the steam warms bitumen and oil
within the
formation. Thus-warmed hydrocarbon within the formation is mobilized and moves
or is drawn
toward the well, where it is then collected and produced to surface. The
steam, when contacting
cooler subterranean bitumen and oil, typically condenses to water, releasing
latent heat of
condensation and thereby effectively transferring heat to the oil/bitumen.
Due to the foregoing condensation of injected steam to water, and also by
reason that
underground formations typically contain amounts of water in the form of brine
or the like, water
is typically produced to surface with the recovered hydrocarbon and the brine.
Because
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proximate surface sources of water for producing steam for injection dovvnhole
are often in very
short supply, or their use prevented or limited due to governmental
restrictions, it is very
desirable to use produced water to generate steam. Not only is such water
(although
contaminated) available at site, but by generating steam from produced water
the disposal costs
of such contaminated produced water is reduced.
Typically, water that is produced to surface with the collected hydrocarbon
arrives in the form of
free water, suspended water, water-in-oil emulsions and/or oil-in-water
reverse emulsions. The
produced water must go through a series of processing steps to be useful as
conventional boiler
feedwater, such as de-oiling and softening, or evaporation. Typical de-oiler
systems include a
free water knock out ("FWKO") vessel, followed by a skim tank, induced gas
floatation and
finally an oil removal filter. The de-oiler system is conventionally used at
surface to separate the
recovered hydrocarbons from the produced water, and the produced water is
thereafter recycled
to the steam generation unit for re-use in converting same to steam for
injection downhole;
typically, however, the produced water contains significant impurities in the
form of inorganic
compounds, such as silica, calcium and magnesium ions, which must be addressed
before the de-
oiled produced water can be introduced to steam generation units as feedstock.
Conventional drum boilers operating at circa 2% blowdown cannot typically be
used to generate
steam from the produced water without using evaporators to generate high
purity feedwater due
to the concentration of dissolved salts and impurities such as calcium,
silica, organics and the
like that cause precipitation and thereby scaling within boiler tubes during
the boiling of the
water, which thereby very quickly renders the boiler ineffective in
transferring heat to the water
to generate steam and can also rupture boiler tubes due to the generation of
hot spots.
Alternatively, special types of steam generators are commonly used, namely so-
called -once-
through steam generators" ("OTSG" or "OTSGs"), which can better handle higher
amounts of
impurities in the produced water feed stream and generate steam ranging from
65% to 90%
steam quality (65-90 parts steam vapor, 10-35 parts liquid water containing
the impurities).
Operating at this steam quality greatly reduces the tendency of the dissolved
salts to precipitate
and scale the tubes. Nevertheless, produced water pre-conditioning steps are
still necessary, such
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as the conventional warm lime softening ("WLS") or hot lime softening ("HLS")
process, which
injects lime to reduce water hardness and alkalinity and precipitates silica
and carbonate ions out
of the water, and in conjunction with a weak acid cation or strong acid cation
ion exchange
("WACS" or "SACS") process, removes the calcium and magnesium scale generating
ions to
.5 acceptable concentrations, thereby reducing build-up of scale in the
OTSG. The major bulk
chemicals used in these processes are lime (Ca(OH)2), magnesium oxide (MgO),
soda ash
(Na2CO3), caustic (NaOH), and hydrochloric acid (HCI). Minor amounts of
coagulant and
polymer are used to aid in solid separation.
It is known and generally acknowledged that conventional steam generation
technologies suffer
from numerous disadvantages. For example, OTSGs still require boiler fcedwater
pretreatment,
require large volumes of feedwater since much of the feed is rejected downhole
as blowdown to
manage impurities, and can suffer large amounts of downtime to clean boiler
tubes (pigging).
However, alternative drum boilers require the use of evaporation technology,
which adds
significant capital expense and requires high energy consumption.
Indirect fired steam generators have been introduced as a possible alteinative
to such
conventional technologies. Indirect boiling can be used with more contaminated
feed water, thus
reducing some equipment line-up complexity and capital cost. For example,
United States
Patent Application Publication No. US 2014/0165928 to Larkin et al. teaches a
steam generation
system in which solid particulate such as sand is first heated by heat
exchange with hot fluids,
and then the heated particulate is contacted with the water to be vaporized.
As a further
example, Hipvap Technologies of Alberta, Canada, is pilot testing an indirect
fired steam
generation system which uses a forced-circulation heat exchanger process to
recirculate a hot oil
fluid in a closed loop to generate steam from produced water.
However, indirect boiling technologies currently under consideration are often
complex and
likely relatively expensive to implement as they may contain double the heat
transfer surface area
than what is required to generate steam alone. It would therefore be desirable
to have access to a
steam generation system that is relatively simple and inexpensive, while still
possessing the
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advantages of indirect boiling in terms of equipment line-up simplification,
moderated, constant
and homogeneous heat flux on the tube walls and reduced operating costs.
Summary of the Invention
The present invention therefore seeks to provide a system for generating steam
from produced
water with a value-added and simplified line-up that avoids heat transfer
surface area duplication
and minimizes the need for boiler feedwater treating.
According to a first aspect of the present invention there is provided a steam
generation system
for use in thermal hydrocarbon recovery operations, the system comprising a
produced water
boiler for generating steam, the boiler tubes heated indirectly by means of
flue gas, such as a gas
turbine exhaust. The gas turbine produces electricity which can be used for
powering
submersible pumps, andl or the central processing facility ("CPF") of the
hydrocarbon recovery
operation, and/or for powering an electric heater on the surface or
subsurface, while exhaust heat
from the turbine is used to heat the produced water boiler. The heat supplied
by the gas turbine
vaporizes a portion of the produced water in the produced water boiler, thus
providing steam for
injection downhole to enable subsequent hydrocarbon production.
In some exemplary embodiments of the present invention, mechanical cleaning
means, such as
but not limited to online "pigging", is applied to clean build-up from the
produced water boiler
tube interior.
A detailed description of exemplary embodiments of the present invention is
given in the
following. It is to be understood, however, that the invention is not to be
construed as being
limited to these embodiments.
Brief Description of the Drawings
In the accompanying drawings, which illustrate exemplary embodiments of the
present
invention:
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Figure 1 a is a simplified schematic view of a conventional prior art process
for recycling
produced water for steam generation;
Figure lb is a simplified schematic view of a conventional prior art process
for recycling
produced water for steam generation;
Figure 2 is a simplified schematic view of a novel produced water recycling
process line-
up according to the present invention; and
1.0
Figure 3 is a schematic view of a first exemplary embodiment of a system and
process in
accordance with the present invention, wherein electricity produced by the gas
turbine is
used for powering the central processing facility or electric heaters or
submersible pumps,
etc.
Exemplary embodiments of the present invention will now be described with
reference to the
accompanying drawings.
Detailed Description of Exemplary Embodiments
As mentioned above, conventional produced water steam generation systems
involve a number
of subsystems to separate and purify the produced water to a state that is
acceptable as feedwater
for the steam generation equipment. As is shown in Figure I a, the subsystems
of a conventional
prior art system for processing water produced from a wellhead I include a
gas/oil/free
water/solids separation stage 2, a produced water removal stage 3, a produced
water treatment
impurity removal stage 4 and steam generation 5. The oil/produced water
separation stage 2 is
designed to remove most of the oil from the water, the oil then being stored
or pipelined
elsewhere for processing, while the produced water ¨ still somewhat
contaminated with minor
amounts of oil ¨ must undergo additional processing in the oil removal stage 3
(in which further
removed oil is recovered and processed). The produced water treatment impurity
removal stage
4, as discussed above, removes various impurities from the de-oiled water that
create heat
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transfer tube surface scaling and fouling to prepare it for introduction to
the steam generation
stage 5, in which steam is produced for downhole injection at the wellhead I
(with waste water
or blowdown being rejected from the system for impurities buildup management
and disposed of
as necessary).
Figure lb provides further details regarding a conventional prior art OTSG
system for recycling
produced water to generate steam. Again, material from the reservoir (along
with condensed
water from previously injected steam) is produced at the wellhead 1, and a
water-oil mixture is
sent to the FWKO unit 6 to separate the oil and produced water (this is the
oil/water separation
stage 2 shown in Figure la). The de-oiled produced water is then sent to one
or more skim tanks
7, and then to an induced gas flotation ("IGF") unit 8, the oil froth being
removed and then
cleaned water being sent to the next stage in the process. To remove various
inorganic
impurities, the cleaned water stream is pumped to a WLS unit 9, through a
series of afterfilters
10, and then to an ion exchange unit 11 such as WACS. At this stage the
treated water is of
sufficient purity to be fed into an OTSG 12, producing waste/blowdown and
steam for downhole
injection.
The above description is simplified and does not include all of the equipment
or additives that
might be required under the conventional OTSG system, and thus it would be
clear to those
skilled in the art that such a system is complex, capital-intensive and
relatively expensive to
operate.
Turning now to Figures 2 to 3, exemplary embodiments of the present invention
are illustrated.
The exemplary embodiments are presented for the purpose of illustrating the
principles of the
present invention, and are not intended to be limiting in any way.
Figure 2 illustrates, in a simplified schematic view, the basic stages of
produced water processing
according to the present invention. Materials including gas, solids,
oil/bitumen and water are
produced at the wellhead 20, and subsequently separated into various desired
components at the
separator 22. Gas would normally be piped to the boiler burners, and may or
may not need to be
processed for sulphur removal in a sulphur recovery unit, while solids are
landfilled. The
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separated oil can be diluted with a diluent for pipelining if necessary, or it
may be subjected to
partial upgrading on-site in a manner known to those skilled in the art.
The separated water is then sent to an impurity containing produced water
boiler with on line
pigging unit 24, and again numerous types of produced water boiler
technologies may be useful
with the present invention so long as they enable a heat exchange to heat the
water pumped
through the impurity containing produced water boiler with on line pigging 24
to its boiling
point. The impurity containing produced water boiler with on line pigging 24
is heated by means
of the hot exhaust of a gas turbine 26, which turbine 26 can be powered by
either fresh gas or a
mixture of fresh gas and produced gas separated by the separator 22, as would
be clear to those
skilled in the art. Once the water is heated to boiling in the impurity
containing produced water
boiler with on line pigging 24, steam is generated for downhole injection at
the wellhead 20 and
the remaining concentrate can be disposed of as wasteiblowdown in a
conventional manner.
Turning now to Figure 3, a first embodiment of the present invention is
illustrated. As with
Figure 2, materials produced at the wellhead 20 are pumped to the separator 22
for separation
into the four main components. Figure 3 illustrates that separated gas may or
may not be re-used
as part of the feedstock for the gas turbine unit 26. Separated water is
pumped into the impurity
containing produced water boiler with on line pigging 24, which in this case
is a heat exchanger.
The gas turbine unit 26 comprises an upstream compressor 28 coupled to a
downstream turbine
30; although not shown in this simplified illustration, there is a combustion
chamber between the
compressor 28 and turbine 30, in a manner well known to those skilled in the
art. The gas is fed
into the compressor 28, which increases the pressure of the gas, and then into
the combustion
chamber, and then fuel is fed into the combustion chamber and ignited,
resulting in a high-
temperature gas flow that enters the turbine 30. The heated gas flow expands
in the turbine 30,
producing shaft work output. This work output is illustrated as creating
electricity, which would
be achieved by the shaft driving an electricity generator in a conventional
manner. Ihi this
embodiment, the created electricity would be used to power the submersible
pumps, the central
processing facility or "CRF" of the hydrocarbon recovery operation, or
electric heaters.
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The gas turbine 26 operation described above also produces exhaust heat. While
heat from a
turbine can be rejected as waste, or recovered through a heat recovery steam
generator, or
through a once through steam generator lacking on-line pigging and fed with
impurity removed
boiler feedwater, in the present case the exhaust heat is used to heat the
untreated produced water
boiler 24 that has online pigging. In a heat exchanger arrangement, the
exhaust heat would
indirectly heat the water that is pumped through the impurity containing
produced water boiler
with on line pigging 24, and at least some of the water would be converted to
steam as a result,
which can then be used for downhole injection as part of the thermal
hydrocarbon recovery
operation. Any water not converted to steam would be rejected from the
impurity containing
produced water boiler with on line pigging 24 as waste or blowdown and can be
disposed of by
conventional means. Online pigging or other conventional cleaning techniques
can be used to
clean the impurity containing produced water boiler with on line pigging 24
internals.
Unlike other prior art indirect boiling systems, which use energy simply to
power steam
is generation, the use of a turbine allows for both the production of heat
for the impurity containing
produced water boiler with on line pigging and useful electricity for the
hydrocarbon recovery
operation.
As can be readily seen, then, there are numerous advantages provided by the
present invention.
In addition to the simplified equipment line-up, which can in appropriate
circumstances
eliminate or reduce the need for de-oiling or water treatment stages such as
the IGF, ORF, warm
lime softener, after filters, and ion exchange, chemical additive requirements
can be reduced. In
addition to this potential reduction in capital and operating costs,
recovering what would have
been waste heat helps to reduce energy consumption when compared to indirect
fired steam
generation systems, and the turbine-generated electricity can be used to
either power the CPF or
even supplement the exhaust heat that is used to boil produced water in the
impurity containing
produced water boiler with on line pigging unit.
The foregoing is considered as illustrative only of the principles of the
invention. Thus, while
certain aspects and embodiments of the invention have been described, these
have been
presented by way of example only and are not intended to limit the scope of
the invention. The
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scope of the claims should not be limited by the exemplary embodiments set
forth in the
foregoing, but should be given the broadest interpretation consistent with the
specification as a
whole.
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