Note: Descriptions are shown in the official language in which they were submitted.
CA 02509308 2008-05-01
WATER TREATMENT METHOD FOR HEAVY OIL PRODUCTION
RELATED PATENT APPLICATIONS
[0001] This application is related to Canadian Patent Application 2,307,819
issued April 19, 2005.
COPYRIGHT RIGHTS IN THE DRAWING
[0002] A portion of the disclosure of this patent document contains
material that is subject to copyright protection. The applicant has no
objection to the
facsimile reproduction by anyone of the patent document or the patent
disclosure, as it
appears in the Patent and Trademark Office patent file or records, but
otherwise reserves
all copyright rights whatsoever.
TECHNICAL FIELD
[0003] The invention disclosed and claimed herein relates to treatment of
water to be used for steam generation in operations which utilize steam to
recover oil
from geological formations. More specifically, this invention relates to
novel, improved
techniques for efficiently and reliably generating from oil field produced
waters, in high
pressure steam generators, the necessary steam for down-hole use in heavy oil
recovery
operations.
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CA 02509308 2005-06-08
BACKGROUND
[0004] Steam generation is necessary in heavy oil recovery operations.
This is because in order to recover heavy oil from certain geologic
formations,
steam is required to increase the mobility of the sought after oil within the
formation. In prior art systems, oil producers have often utilized once-
through
type steam generators ("OTSG's). As generally utilized in the industry, once
through steam generators -OTSG's -usually have high blowdown rates, often in
the range of from about 20% to about 30% or thereabouts. Such a blowdown
rate leads to significant thermal and chemical treatment inefficiencies. Also,
once through steam generators are most commonly provided in a configuration
and with process parameters so that steam is generated from a feedwater in a
single-pass operation through boiler tubes that are heated by gas or oil
burners.
Typically, such once through steam generators operate at from about 1000
pounds per square inch gauge (psig) to about 1600 psig or so. In some cases,
once through steam generators are operated at up to as much as about 1800
psig. Such OTSG's often operate with a feedwater that has from about 2000
mg/L to about 8000 mg/L of total dissolved solids. As noted in FIG. 1, which
depicts the process flow sheet of a typical prior art water treatment system
10,
such a once through steam generator 12 provides a low quality or wet steam,
wherein about eighty percent (80%) quality steam is produced. In other words,
the 80% quality steam 14 is about 80% vapor, and about 20% liquid, by weight
percent. The steam portion, or high pressure steam produced in the steam
generators is injected via steam injection wells 16 to fluidize as indicated
by
reference arrows 18, along or in combination with other injectants, the heavy
oil
formation 20, such as oils in tar sands formations. The injected steam 14
eventually condenses and an oil/water mixture 22 results, and which mixture
migrates through the formation 20 as indicated by reference arrows 24. The
oil/water mixture 22 is gathered as indicated by reference arrows 26 by
oil/water
gathering wells 30, through which the oil/water mixture is pumped to the
surface.
Then, the sought-after oil is sent to an oillwater separator 32 in which the
oil
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CA 02509308 2008-05-01
product 34 separated from the water 35 and recovered for sale. The produced
water stream 36, after separation from the oil, is further de-oiled in a de-
oiling
process step 40, normally by addition of a de-oiling polymer 42 or by other
appropriate processes. Such a de-oiling process usually results in generation
of
an undesirable waste oil/solids sludge 44. However, the de-oiled produced
water
stream 46 is then further treated for reuse.
[0005] The design and operation of the water treatment plant which treats
the de-oiled produced water stream 46, i.e., downstream of the de-oiling unit
40
and upstream of injection well 16 inlet 48, is the key to the improvement(s)
described herein.
[0006] Most commonly in prior art plants such as plant 10, the water is
sent to the "once-through" steam generators 12 for creation of more steam 14
for
oil recovery operations. The treated produced water stream 12F which is the
feed stream for the once through steam generator, at time of feed to the steam
generator 12, is typically required to have less than about 8000 parts per
million
("PPM") of total dissolved solids ("TDS"). Less frequently, the treated
produced
water stream. 12F may have up to about 12000 parts per million (as CaCO3
equivalent) of total dissolved solids, as noted in FIG. 8. Further, it is
often
necessary to meet other specific water treatment parameters before the water
can be reused in such once-through steam generators 12 for the generation of
high pressure steam.
[0007] In most prior art water treatment schemes, the de-oiled recovered
water 46 must be treated in a costly water treatment plant sub-system 10
before
it can be sent to the steam generators 12. Treatment of water before feed to
the
once-through steam generators 12 is often initially accomplished by using a
warm lime softener 50, which removes hardness, and which also removes some
silica from the de-oiled produced water feedstream 46. Various softening
chemicals 52 are usually necessary, such as lime, flocculating polymer, and
perhaps soda ash. Underflow 56 produces a waste sludge 58 which must be
further handled and disposed. Then, an "after-filter" 60 is often utilized on
the
clarate stream 59 to prevent carry-over of any precipitate or other suspended
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CA 02509308 2008-05-01
solids, which substances are thus accumulated in a filtrate waste stream 62.
For
polishing, an ion exchange step 64, normally including a hardness removal step
such as a weak acid cation (WAC) ion-exchange system that can be utilized to
simultaneously remove hardness and the alkalinity associated with the
hardness,
is utilized. The ion exchange systems 64 require regeneration chemicals 66 as
is
well understood by those of ordinary skill in the art and to which this
disclosure is
directed. As an example, however, a WAC ion exchange system is usually
regenerated with hydrochloric acid and caustic, resulting in the creation of a
regeneration waste stream 68. Overall, such prior art water treatment plants
are
relatively simple, but, result in a multitude of liquid waste streams or solid
waste
sludges that must be further handled, with significant additional expense.
[0008] In one relatively new heavy oil recovery process, known as the
steam assisted gravity drainage heavy oil recovery process (the "SAGD"
process), it is preferred that one hundred percent (100%) quality steam be
provided for injection into wells (i.e., no liquid water is to be provided
with the
steam to be injected into the formation). Such a typical prior art system 11
is
depicted in FIG. 2. However, given conventional prior art water treatment
techniques as just discussed in connection with FIG. 1, the 100% steam quality
requirement presents a problem for the use of once through steam generators 12
in such a process. That is because in order to produce 100% quality steam 70'
using a once-through type steam generator 12, a vapor-liquid separator 72 is
required to separate the liquid water from the steam. Then, the liquid
blowdown
73 recovered from the separator is typically flashed several times in a series
of
flash tanks F1, F2, etc. through FN (where N is a positive integer equal to
the
number of flash tanks) to successively recover as series of lower pressure
steam
flows S1, S2, etc. which may sometimes be utilized for other plant heating
purposes. After the last flashing stage FN, a residual hot water final
blowdown
stream 74 must then be handled, by recycle and/or disposal. The 100% quality
steam is then sent down the injection well 16 and injected into the desired
formation 20. Fundamentally, though, conventional treatment processes for
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CA 02509308 2005-06-08
produced water used to generate steam in a once-through steam generator
produces a boiler blowdown which is roughly twenty percent (20%) of the
feedwater volume. This results in a waste brine stream that is about fivefold
the
concentration of the steam generator feedwater. Such waste brine stream must
be disposed of by deep well injection, or if there is limited or no deep well
capacity, by further concentrating the waste brine in a crystallizer or
similar
system which produces a dry solid for disposal.
[0009] As depicted in FIG. 3, another method which has been proposed
for generating the required 100% quality steam for use in the steam assisted
gravity drainage process involves the use of boilers 80, which may be
packaged,
factory built boilers of various types or field assembled boilers with mud and
steam drums and water wall piping. Various methods can be used for producing
water of a sufficient quality to be utilized as feedwater 80F to a boiler 80.
One
method which has been developed for use in heavy oil recovery operations
involves de-oiling 40 of the produced water 36, followed by a series of
physical-
chemical treatment steps. Such treatment steps normally include a series of
unit
operations as warm lime softening 54, followed by filtration 60 for removal of
residual particulates, then an organic trap 84 (normally non-ionic ion
exchange
resin) for removal of residual organics. The organic trap 84 may require a
regenerant chemical supply 85, and, in any case, produces a waste 86, such as
a regenerant waste. Then, a pre-coat filter 88 can be used, which has a
precoat
filtrate waste 89. In one alternate embodiment, an ultrafiltration ("UF") unit
90 can
be utilized, which unit produces a reject waste stream 91. Then, effluent from
the
UF unit 90 or precoat filter 88 can be sent to a reverse osmosis ("RO") system
92,
which in addition to the desired permeate 94, produces a reject liquid stream
96
that must be appropriately handled. Permeate 94 from the RO system 92, can be
sent to an ion exchange unit 100, typically but not necessarily a mixed bed
demineralization unit, which of course requires regeneration chemicals 102 and
which consequently produces a regeneration waste 104. And finally, the boiler
80
produces a blowdown 110 which must be accommodated for reuse or disposal.
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[0010] The prior art process designs, such as depicted in FIG. 3, for
utilizing packaged boilers in heavy oil recovery operations, have a high
initial
capital cost. Also, such a series of unit process steps involves significant
ongoing
chemical costs. Moreover, there are many waste streams to discharge, involving
a high and ongoing sludge disposal cost. Further, where membrane systems
such as ultrafiltration 90 or reverse osmosis 92 are utilized, relatively
frequent
replacement of membranes 106 or 108, respectively, may be expected, with
accompanying on-going periodic replacement costs. Also, such a process
scheme can be labor intensive to operate and to maintain.
[0011] In summary, the currently known and utilized methods for treating
heavy oil field produced waters in order to generate high quality steam for
down-
hole use are not entirely satisfactory because:
such physical-chemical treatment process schemes are usually
quite extensive, are relatively difficult to maintain, and require significant
operator attention;
such physical-chemical treatment processes require many chemical
additives which must be obtained at considerable expense, and many of
which require special attention for safe handling;
such physical-chemical treatment processes produce substantial
quantities of undesirable sludges and other waste streams, the disposal of
which is increasingly difficult, due to stringent environmental and
regulatory requirements.
[0012] It is clear that the development of a simpler, more cost effective
approach to produced water treatment would be desirable in the process of
producing steam in heavy oil production operations. Thus, it can be
appreciated
that it would be advantageous to provide a new produced water treatment
process which minimizes the production of undesirable waste streams, while
minimizing the overall costs of owning and operating a heavy oil recovery
plant.
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CA 02509308 2010-10-14
SUMMARY OF THE INVENTION
[0013] The new water treatment process(es) disclosed herein, and various
embodiments thereof, can be applied to heavy oil production operations. Such
embodiments are particularly advantageous in they minimize the generation of
waste products, and are otherwise superior to water treatment processes
heretofore used or proposed in the recovery of bitumen from tar sands or other
heavy oil recovery operations.
[0014] From the foregoing, it will be apparent to the reader that one of the
important and primary aspects of the invention seeks to provide a novel
process,
including several variations thereof, for the treatment of produced waters, so
that
such waters can be re-used in producing steam for use in heavy oil recovery
operations.
[0015] Another important aspect is to simplify process plant flow sheets,
i.e., minimize the number of unit processes required in a water treatment
train,
which importantly simplifies operations and improves quality control in the
manufacture of high purity water for down-hole applications.
[0016] Other important but more specific aspect reside in the provision
of various embodiments for an improved water treatment process for production
of high purity water for down-hole use in heavy oil recovery, which
embodiments
may:
in one embodiment, eliminate the requirement for flash separation of the
high pressure steam to be utilized downhole from residual hot pressurized
liquids;
eliminate the generation of softener sludges;
minimize the production of undesirable liquid or solid waste streams;
minimize operation and maintenance labor requirements;
minimize maintenance materiel requirements;
minimize chemical additives and associated handling requirements;
increase reliability of the OTSG's, when used in the process;
decouple the de-oiling operations from steam production operations; and
reduce the initial capital cost of water treatment equipment.
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CA 02509308 2010-10-14
[0016A] The invention, in a broad aspect, seeks to provide a process
for producing steam for downhole injection in the recovery of heavy oil. The
process comprises providing an oil/water mixture gathered from an oil/water
collection well, separating oil from the oil/water mixture to provide an oil
product and a produced water product containing oil therein, and de-oiling the
oil containing produced water product to at least partially provide an
evaporator
feedwater stream, the evaporator feedwater stream comprising water, dissolved
gases, and dissolved solutes, and the dissolved solutes comprising silica.
There is provided an evaporator having a plurality of heat transfer elements,
a
liquid containing sump reservoir, and a recirculating pump to recycle a
concentrated brine from the sump reservoir to the plurality of heat transfer
elements. The evaporator feedwater stream is injected into the evaporator and
evaporates a portion of the feedwater stream to produce the concentrated
brine,
recirculating the concentrated brine in the evaporator. A selected base is
added
to the concentrated brine to maintain a preselected pH level to enhance
solubility of the silica, by adding the selected base to one or more of (i)
the
evaporator feedwater stream, or (ii) the liquid containing sump reservoir, or
(iii) the concentrated brine recirculated from the sump reservoir to the
plurality
of heat transfer elements. The concentrated brine is distributed on a first
surface of at least one of the plurality of heat transfer elements to generate
a
steam vapor, which steam vapor is compressed to a second surface of at least
one of the plurality of heat transfer elements to condense the compressed
steam
vapor and to form a distillate which is collected. At least some of the
concentrated brine is discharged as an evaporator blowdown stream. The
distillate is introduced into a steam generator, to produce (i) high pressure
steam, and (ii) a boiler blowdown stream, the boiler blowdown stream
comprising water and residual dissolved solids. The high pressure steam is
injected in injection wells to fluidize oil present in a selected geological
formation, to produce an oil and water mixture, and to gather the oil/water
mixture.
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CA 02509308 2010-10-14
[0017] (Other important aspects, features, and additional advantages of
the various embodiments of the novel process disclosed herein will become
apparent to the reader from the foregoing and from the appended claims and the
ensuing detailed description, as the discussion below proceeds in conjunction
with examination of the accompanying drawing.
BRIEF DESCRIPTION OF THE DRAWING
(0018] In order to enable the reader to attain a more complete
appreciation of the novel water treatment process disclosed and claimed
herein,
and the various embodiments thereof, and of the novel features and the
advantages thereof over prior art processes, attention is directed to the
following
detailed description when considered in connection with the accompanying
figures of the drawing, wherein:
[00191 FIG. I shows one typical prior art process, namely a generalized
process flow diagram for a physical-chemical water treatment process
configured
for use in heavy oil recovery operations.
[0020] FIG. 2 shows another prior art process, namely a generalized
process flow diagram far a physical-chemical water treatment process as used
in
a steam assisted gravity drainage (SAGD) type heavy oil operation.
[0021] FIG. 3 shows yet another prior art physical-chemical treatment
process scheme, also as it might be applied for use in steam assisted gravity
drainage (SAGD) type heavy oil recovery operations.
[0022] FIG. 4 shows one embodiment of an evaporation based water
treatment process, illustrating the use of the evaporation based process in
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CA 02509308 2005-06-08
combination with the use of packaged boilers for steam production, as applied
to
heavy oil recovery operations.
[0023] FIG. 5 shows another embodiment for an evaporation based water
treatment process for heavy oil production, illustrating the use of the
process in
combination with the use of once-through steam generators for steam
production,
as applied to heavy oil recovery operations, which process is characterized by
feed of evaporator distillate to once-through steam generators without the
necessity of further pretreatment.
[0024] FIG. 6 shows a common variation for the orientation of injection
and gathering wells as utilized in heavy oil recovery, specifically showing
the use
of horizontal steam injection wells and of horizontal oil/water gathering
wells, as
often employed in a steam assisted gravity drainage heavy oil gathering
project.
[0025] FIG. 7 shows the typical feedwater quality requirements for steam
generators which produce steam in the 1000 pounds per square inch gauge
range, or thereabouts, for conventional steam boiler installations.
[0026] FIG. 8 shows the typical feedwater quality requirements for steam
generators which produce steam in the 1000 pounds per square inch gauge
range, or thereabouts, for once-through type steam generator installations.
[0027] FIG. 9 provides a simplified view of a vertical tube falling film
evaporator operating a high pH in the treatment of produced water from heavy
oil
operations, for production of distillate for reuse in once through steam
generators
or in conventional steam boilers.
[0028] FIG. 10 shows further details of the use of evaporators at high pH,
illustrated by use of falling film evaporators, and indicates selected
injection
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points for caustic injection to raise the pH of the concentrated brine to
maintain
solubility of silica in the concentrated brine.
[0029] FIG. 11 illustrates the solubility of silica in water as a function of
pH
at 25 C when such silica species are in equilibrium with amorphous silica, as
well
as the nature of such soluble silica species (molecule or ion) at various
concentration and pH ranges.
[0030] FIG. 12 diagrammatically illustrates functional internal details of the
operation of a falling film evaporator, which evaporator type would be useful
in
the evaporation of produced waters from heavy oil production; details
illustrated
include the production of steam from a falling brine film, by a heat exchange
relationship from condensation of steam on a heat exchange tube, and the
downward flow of such steam condensate (distillate) by gravity for the
collection
of such condensate (distillate) above the bottom tube sheet of the evaporator.
[0031] The foregoing figures, being merely exemplary, contain various
elements that may be present or omitted from actual process implementations
depending upon the circumstances. An attempt has been made to draw the
figures in a way that illustrates at least those elements that are significant
for an
understanding of the various embodiments and aspects of the invention.
However, various other elements of the unique process methods, and the
combination of apparatus for carrying out the methods, are also shown and
briefly described to enable the reader to understand how various features,
including optional or alternate features, may be utilized in order to provide
an
efficient, low cost process design which can be implemented in a desired
throughput size and physical configuration for providing optimum water
treatment
plant design and operation.
CA 02509308 2005-06-08
DESCRIPTION
[0032] Many steam assisted heavy oil recovery schemes, such as a steam
assisted gravity drainage (SAGD) heavy oil recovery process injection and
recovery well arrangements of the type depicted in FIG. 6, most efficiently
utilize
a 100% quality steam supply 70. It would therefore be desirable to produce
such
a steam supply by an efficient process scheme such as I have found may be
provided by evaporation based heavy oil produced water treatment method(s).
Various embodiments and details of such evaporation based produced water
treatment method(s) are depicted in FIGS. 4, 5, 6, 9, 10 and 12.
[0033] As depicted in FIG. 6, in a SAGD process, horizontal injection
wells 16' and horizontal oil/water gathering wells 30' are advantageously
utilized
spaced apart within an oil bearing formation 20. As particularly illustrated
in
FIGS. 4 and 5, a process for the use of an evaporation based water treatment
system 120 has been developed to treat produced water, in order to produce
high quality steam for use in further heavy oil recovery. Conceptually, such.
an
evaporative water treatment process may, in one embodiment, be situated
process wise - that is, water flow wise - between the point of receipt of a de-
oiled
produced water stream 46 and the point of steam injection at well head 48 of
injection well 16. The process, in combination with the steam injection well
16,
oil recovery well 30, and related oil water separation equipment 32 and de-
oiling
equipment 40, and boilers 80 as shown in FIG. 4, or alternately, once through
steam generators 12 as shown in FIG. 5, can substantially reduce capital costs
and can minimize ongoing operation and maintenance costs of heavy oil
recovery installations. Boilers 80 may be packaged, factory built boilers of
various types or field assembled boilers with mud and steam drums and water
wall piping, or more generally, conventional steam boilers. In some locales,
such
as northern Canada, the possibility of elimination of the need for handling of
waste sludges and other waste streams made possible by the evaporation based
water treatment system 120 may be especially important, since it may be
difficult
to work with such waste materials during the extremely cold winter months.
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[0034] It has been observed that it may be desirable in some instances to
use a packaged boiler 80 to produce the required steam 70, rather than to
utilize
a traditional once-through type steam generator 12 to produce 80% quality
steam
14 and then utilize separator(s) 130 to separate steam 132 from liquid 134. It
is
noteworthy in such an economic process evaluation that packaged boilers 80 are
often less expensive on a capital cost basis and on an operating cost basis
than
once-through type oil-field steam generators 12. Also, package boilers can be
utilized to produce pure steam 70, and thus produce only a minimal liquid
blowdown stream 110. Also, as shown in FIGS. 4 and 5, boiler blowdown
stream can be either sent to the evaporator feed tank 210, or injected into
the
sump reservoir 152 of evaporator 140, such as via line 111, or into the
recirculating brine via line 111'. One type of packaged boiler suitable for
use in
the process described herein is a water tube boiler having a lower mud drum
and
an upper steam drum and water cooled sidewalls substantially extending
therebetween in a manner which encloses a combustion chamber. However,
most such packaged boilers require a much higher quality feed water 80F than
is
the case with requirements for feedwater 12F for a once-through type steam
generator. As a result, in one embodiment, the process disclosed herein
includes an evaporation unit 140 based approach to packaged boiler 80
feedwater 80F pretreatment. In other words, the de-oiled produced water 46
generated can be advantageously treated by an evaporative process operating at
elevated pH, and provides a significantly improved method for produced water
treatment in heavy oil production.
[0035] An oil/water mixture 22 is pumped up through oil gathering wells 30.
The oil water mixture 22 is sent to a series of oiltwater separators 32. An
oil
product 34 is gathered for further conditioning, transport, and sale. The
produced water 36 which has been separated from the oil/water mixture 22 is
then sent to a produced water de-oiling step 40, which may be accomplished in
dissolved air flotation units with the assistance of the addition of a de-
oiling
polymer 42, or by other appropriate unit processes.
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CA 02509308 2008-05-01
[0036] In the water treatment method disclosed herein, the de-oiled
produced water 46 is treated and conditioned for feed to one or more
mechanical
vapor recompression evaporator units 140 (normally, multiple redundant units)
to
concentrate the incoming produced water stream 46. The necessary treatment
and conditioning prior to the evaporator unit 140 can be efficiently
accomplished,
but may vary somewhat based on feedwater chemistry - i.e. the identity and
distribution of various dissolved and suspended solids - and on the degree of
concentration selected for accomplishment in evaporator units 140.
[0037] In the usual case, it may be necessary or appropriate to add a
selected base such as caustic 232 via line 146 to the evaporator feed tank
210,
or by line 147 to a point upstream of the feedwater heat exchanger 148, such
as
before the suction of pump 149 as seen in FIG. 10, in order to avoid silica
scaling
in the feedwater deaerator 150 and the feed heat exchanger 148. Moreover,
feed of a selected base such as caustic 232 to the evaporator feed tank 210
via
line 146 or to another point upstream of the feedwater heat exchanger 148 via
line 147can be utilized to raise the pH of the concentrated brine
recirculating in
the evaporator 140. Also, raising the pH of the concentrated brine
recirculating in
the evaporator can be accomplished by direct injection of a selected base such
as caustic 232 into the sump 141, as indicated by line 157, or by feed of a
selected base such as caustic 232 into the suction of recirculation pump 153,
as
indicated by line 159. Moreover, in one embodiment, it may be advantageous to
use addition of a selected base such as caustic 232 to the feed tank 210 such
as
by line 146 as the primary method for raising the pH of the recirculating
brine in
the sump of the evaporator 140 to a desired pH level, and to use addition of a
selected base such as caustic 232 to the evaporator sump 141, such as by line
157 or line 159, as a pH trim control to fine tune the pH of the brine 152 in
sump
140 and thus of recirculating brine 162. Also, the selected base such as
caustic
232 can be added at an appropriate point upstream of the feed tank 210 when
desired such as via line 146'.
[0038] At feedwater heat exchanger, the feedwater pump 149 is used to
provide sufficient pressure to send feedwater from the evaporator feed tank
210
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CA 02509308 2005-06-08
through the feedwater heat exchanger 148, prior to the deaerator 150. In the
opposite direction, the distillate pump 143 moves distillate 180 through the
feedwater heat exchanger 148, so that the hot distillate is used to heat the
feedwater stream directed toward the deaerator 150.
[0039] The conditioned feedwater 151 is sent as feedwater to evaporator
140. The conditioned feedwater 151 may be directed to the inlet of
recirculation
pump 153, or alternately, directed to the sump 141 of evaporator 140 as
indicated by broken line 151' in FIG. 10. Concentrated brine 152 in the
evaporator 140 is recirculated via pump 153, so only a small portion of the
recirculating concentrated brine is removed on any one pass through the
evaporator 140. In the evaporator 140, the solutes in the feedwater 46 are
concentrated via removal of water from the feedwater 46. As depicted in FIGS.
10 and 12, an evaporator 140 is in one embodiment provided in a falling film
configuration wherein a thin brine film 154 is provided by distributors 155
and
then falls inside of a heat transfer element, e.g. tube 156. A small portion
of the
water in the thin brine film 154 is extracted in the form of steam 160, via
heat
given up from heated, compressed steam 162 which is condensing on the
outside of heat transfer tubes 156. Thus, the water is removed in the form of
steam 160, and that steam is compressed through the compressor 164, and the
compressed steam 162 is condensed at a heat exchange tube 156 in order to
produce yet more steam 160 to continue the evaporation process. The
condensing steam on the outer wall 168 of heat transfer tubes 156, which those
of ordinary skill in the evaporation arts and to which this disclosure is
directed
may variously refer to as either condensate or distillate 180, is in
relatively pure
form, low in total dissolved solids. In one embodiment, such distillate
contains
less than 10 parts per million of total dissolved solids of non-volatile
components.
Since, as depicted in the embodiments shown in FIGS. 4, 5, 9, and 10, a single
stage of evaporation is provided, such distillate 180 may be considered to
have
been boiled, or distilled, once, and thus condensed but once.
[0040] It is to be understood that the falling film evaporator 140 design is
provided only for purposes of illustration and thus enabling the reader to
14
CA 02509308 2008-05-01
understand the water treatment process(es) taught herein, and is not intended
to
limit the process to the use of such evaporator design, as those in the art
will
recognize that other designs, such as, for example, a forced circulation
evaporator, or a rising film evaporator, may be alternately utilized with the
accompanying benefits and/or drawbacks as inherent in such alternative
evaporator designs.
[0041] In any event, in a falling film evaporator embodiment, the distillate
180 descends by gravity along tubes 156 and accumulates above bottom tube
sheet 172, from where it is collected via condensate line 174. A small portion
of
steam in equilibrium with distillate 180 may be sent via line 173 to the
earlier
discussed deaerator 150 for use in mass transfer, i.e, heating and steam
stripping descending liquids in a packed tower to remove non-condensable
gases 148 such as carbon dioxide. However, the bulk of the distillate 180 is
removed as a liquid via line 180', and may optionally be sent for further
treatment
in a distillate treatment plant, for example such as depicted in detail in
FIG. 4, or
as merely depicted in functional form as plant 181 in FIG. 5, to ultimately
produce
a suitable feedwater, such as feedwater 80F' in the case where packaged
boilers
80 are utilized as depicted in FIG. 4. As shown in the embodiment set forth in
FIG. 5, the distillate treatment plant 181 is optional, especially in the case
of the
use of once through steam generators, and in such instance the distillate 180
may often be sent directly to once-through steam generators as feedwater 12F'
(as distinguished from the higher quality from feedwater 12F discussed
hereinabove with respect to prior art processes) for generation of 80% quality
steam 14. Also, as shown in FIG. 4, a distillate treatment plant 181 may also
be
optional in some cases, depending on feedwater chemistry, and in such cases,
distillate 180 may be fed directly to boiler 80 as indicated by broken line
81.
[0042] In an embodiment where boilers 80 are used rather than once
through steam generators 12, however, it may be necessary or desirable to
remove the residual organics and other residual dissolved solids from the
distillate 180 before feed of distillate 180 to the boilers 80. For example,
as
illustrated in FIG_ 4, in some cases, it may be necessary to remove residual
ions
CA 02509308 2005-06-08
from the relatively pure distillate 180 produced by the evaporator 140. In
most
cases the residual dissolved solids in the distillate involve salts other than
hardness. In one embodiment, removal of residual dissolved solids can be
accomplished by passing the evaporator distillate 180, after heat exchanger
200,
through an ion exchange system 202. Such ion-exchange systems may be of
mixed bed type or include an organic trap, and directed to remove the salts
andlor organics of concern in a particular water being treated. In any event,
regenerant chemicals 204 will ultimately be required, which regeneration
results
in a regeneration waste 206 that must be further treated. Fortunately, in the
process scheme described herein, the regeneration waste 206 can be sent back
to the evaporator feed tank 210 for a further cycle of treatment through the
evaporator 140.
[0043] In another embodiment, removal of residual dissolved solids can be
accomplished by passing the evaporator distillate 180 through a heat exchanger
200' and then through electrodeionization (EDI) system 220. The EDI reject 222
is also capable of being recycled to evaporator feed tank 210 for a further
cycle
of treatment through the evaporator 140.
[0044] The just described novel combination of process treatment steps
produces feedwater of sufficient quality, and in economic quantity, for use in
packaged boilers 80 in heavy oil recovery operations. Advantageously, when
provided as depicted in FIG. 4 a single liquid waste stream is generated,
namely
evaporator blowdown 230, which contains the concentrated solutes originally
present in feedwater 46, along with additional contaminants from chemical
additives (such as sodium hydroxide or caustic 232, when utilized to elevate
the
pH of recirculating brine 152, or regeneration chemicals 204). Also, in many
cases, even the evaporator blowdown 230 can be disposed in an
environmentally acceptable manner, which, depending upon locale, might involve
injection in deep wells 240. Alternately, evaporation to complete dryness in a
zero discharge system 242, such as a crystallizer or drum dryer, to produce
dry
solids 244 for disposal, may be advantageous in certain locales.
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[0045] Various embodiments for new process method(s), as set forth in
FIGS. 4 and 5 for example, are useful in heavy oil production since they
generally offer one or more of the following advantages: (1) eliminate many
physical-chemical treatment steps commonly utilized previously in handing
produced water (for example, lime softening, filtrating, ion exchange systems,
and certain de-oiling steps are eliminated); (2) result in lower capital
equipment
costs, since the evaporative approach to produced water treatment results in a
zero liquid discharge system footprint size that is about 80% smaller than
that
required if a prior art physical-chemical treatment scheme is utilized, as
well as
eliminating vapor/liquid separators and reducing the size of the boiler feed
system by roughly 20%; (3) result in lower operating costs for steam
generation;
(4) eliminate the production of softener sludge, thus eliminating the need for
the
disposal of the same; (5) eliminate other waste streams, thus minimizing the
number of waste streams requiring disposal; (6) minimize the materiel and
labor
required for maintenance; (7) reduce the size of water do-oiling equipment in
most operations; and (8) decouple the de-oiling operations from the steam
generation operations.
[0046] One of the significant economic advantages of using a vertical tube,
falling film evaporator such as of the type described herein is that the on-
line
reliability and redundancy available when multiple evaporators are utilized in
the
treatment of produced water. An evaporative based produced water treatment
system can result in an increase of from about 2% to about 3% or more in
overall
heavy oil recovery plant availability, as compared to a produced water
treatment
system utilizing a conventional prior art lime and clarifier treatment process
approach. Such an increase in on-line availability relates directly to
increased oil
production and thus provides a large economic advantage over the life of the
heavy oil recovery plant.
[0047] In the process disclosed herein, the evaporator 140 is designed to
produce high quality distillate (typically 2-5 ppm non-volatile TDS) which,
after
temperature adjustment to acceptable levels in heat exchangers 200 or 200'
(typically by cooling to about 45 C., or lower) can be fed directly into
polishing
17
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equipment (EDI system 220, ion exchange system 202, or reverse osmosis
system 224) for final removal of dissolved solids. The water product produced
by
the polish equipment just mentioned is most advantageously used as feedwater
for the packaged boiler 80. That is because in the typical once-though steam
generator 12 used in oil field operations, it is normally unnecessary to incur
the
additional expense of final polishing by removal of residual total dissolved
solids
from the evaporator distillate stream 180. In some applications, final
polishing is
not necessary when using conventional boilers 80. This can be further
understood by reference to FIG. 6, where a typical boiler feed water chemistry
specification is presented for (a) packaged boilers, and (b) once-through
steam
generators. It may be appropriate in some embodiments from a heat balance
standpoint that the de-oiled produced waters 46 fed to the evaporator for
treatment be heated by heat exchange with the distillate stream 180. However,
if
the distillate stream is sent directly to once-through steam generators 12,
then no
cooling of the distillate stream 180 may be appropriate. Also, in the case of
once-
through steam generators 12, it may be necessary or appropriate to utilize a
plurality of flash tanks F1, etc., in the manner described above with
reference to
FIG. 2.
[0048] Also, as briefly noted above, but significantly bears repeating, in
those cases where the EDI system 220 is utilized for polishing, the membrane
reject stream includes an EDI reject stream 222 that is recycled to be mixed
with
the de-oiled produced water 46 in the evaporator feed tank 210 system, for
reprocessing through the evaporator 140. Similarly, when reverse osmosis is
utilized the a membrane reject stream includes the RO reject stream which is
recycled to be mixed with the de-oiled produced water 46 in the evaporator
feed
tank 210 system, for reprocessing through the evaporator 140. Likewise, when
ion-exchange system 202 is utilized, the regenerant waste stream 206 is
recycled to be mixed with the de-oiled produced water 46 in the evaporator
feed
tank system, for reprocessing through the evaporator 140.
[0049] Again, it should be emphasized that the blowdown 230 from the
evaporator 140 is often suitable for disposal by deep well 240 injection.
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Alternately, the blowdown stream can be further concentrated and/or
crystallized
using a crystallizing evaporator, or a crystallizer, in order to provide a
zero liquid
discharge 242 type operation. This is an important advantage, since zero
liquid
discharge operations may be required if the geological formation is too tight
to
allow water disposal by deep well injection, or if regulatory requirements do
not
permit deep well injection.
[0050] Since many produced waters encountered in heavy oil production
are high in silica, with typical values ranging up to about 200 mg/I as Si02,
or
higher. In order to minimize the capital cost of an evaporator, and
particularly, a
mechanical vapor recompression (MVR) evaporation system 140, and while
simultaneously providing a process design which prevents the scaling of the
inner surfaces 260 of the heat transfer tubes 156 with the ever-present
silica,
operation of the evaporator 140 at high pH, i.e., in preferably excess of
about
10.5 is undertaken. More preferably, operation in the range from about 11 to
about 12, or even a higher pH in appropriate cases, can be used to keep the
silica in aqueous solution. This is important, since silica solubility must be
accounted for in the design and operation of the evaporator 140, in order to
prevent silica scaling of the heat transfer surfaces 260. The solubility
characteristics of silica are shown in FIG. 11. Since the high pH operation
assures increased silica solubility, a concentration factor (Le, ratio of rate
of feed
151 to rate of blowdown 230) for the evaporator 140 can be selected so that
silica solubility is not exceeded. Operation at high pH also allows the use of
low
cost heat transfer tubes 156 and other brine wetted surfaces such as sump
walls
270 of sump 141, thus minimizing the capital cost of the system.
[00511 Since the calcium hardness and sulfate concentrations of many
produced waters is low (typically 20-50 ppm Ca as CaCO3), it is possible in
many cases to operate the evaporators 140 with economically efficient
concentration factors, while remaining below the solubility limit of calcium
sulfate,
assuming proper attention to feedwater quality and to pre-treatment processes.
[0052] It is to be appreciated that the water treatment process described
herein for preparing boiler feedwater in heavy oil recovery operations is an
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CA 02509308 2005-06-08
appreciable improvement in the state of the art of water treatment for oil
recovery
operations. The process eliminates numerous of the heretofore encountered
waste streams, while processing water in reliable mechanical evaporators, and
in
one embodiment, in mechanical vapor recompression ("MVR") evaporators.
Polishing, if necessary, can be accomplished in ion exchange,
electrodeionization, or reverse osmosis equipment. The process thus improves
on currently used treatment methods by eliminating most treatment or
regeneration chemicals, eliminating many waste streams, eliminating some types
of equipment. Thus, the complexity associated with a high number of treatment
steps involving different unit operations is avoided.
[0053] In the improved water treatment method, the control over waste
streams is focused on a the evaporator blowdown, which can be conveniently
treated by deep well 240 injection, or in a zero discharge system 242 such as
a
crystallizer and/or spray dryer, to reduce all remaining liquids to dryness
and
producing a dry solid 244. This contrasts sharply with the prior art
processes, in
which sludge from a lime softener is generated, and in which waste solids are
gathered at a filter unit, and in which liquid wastes are generated at an ion
exchange system and in the steam generators. Moreover, this waste water
treatment process also reduces the chemical handling requirements associated
with water treatment operations.
[0054] It should also be noted that the process described herein can be
utilized with once through steam generators, since due to the relatively high
quality feedwater-treated produced water-provided to such once through
steam generators, the overall blowdown rate of as low as about 5% or less may
be achievable in the once through steam generator. Alternately, as shown in
FIG.
5, at least a portion of the liquid blowdown 134 from the once through steam
generator 12 can be recycled to the steam generator 12, such as indicated by
broken line 135 to feed stream 12F1.
[0055] In yet another embodiment, to further save capital and operating
expense, industrial boilers of conventional design may be utilized since the
distillate-treated produced water-may be of sufficiently good quality to be an
CA 02509308 2005-06-08
acceptable feedwater to the boiler, even if it requires some polishing. It is
important to observe that use of such boilers reduces the boiler feed system
and
evaporative produced water treatment system size by twenty percent (20%),
eliminates vapor/liquid separation equipment as noted above, and reduces the
boiler blowdown flow rate by about ninety percent (90%).
[0056] In short, evaporative treatment of produced waters using a falling
film, vertical tube evaporator is technically and economically superior to
prior art
water treatment processes for heavy oil production. It is possible to recover
ninety five percent (95%) or more, and even up to ninety eight percent (98%)
or
more, of the produced water as high quality distillate 180 for use as high
quality
boiler feedwater (resulting in only a 2% boiler blowdown stream which can be
recycled to the feed for evaporator 140). Such a high quality distillate
stream
may be utilized in SAGD and non-SAGD heavy oil recovery operations. Such a
high quality distillate stream may have less than 10 mg/L of non-volatile
inorganic
TDS and is useful for feed either to OTSGs or to conventional boilers.
[0057] The overall life cycle costs for the novel treatment process
described herein are significantly less than for a traditional lime softening
and ion
exchange treatment system approach. And, an increase of about 2% to 3% in
overall heavy oil recovery plant availability is achieved utilizing the
treatment
process described herein, which directly results in increased oil production
from
the facility. Since boiler blowdown is significantly reduced, by as much as
90%
or more, the boiler feed system may be reduced in size by as much as fifteen
percent (15%) or more. Finally, the reduced blowdown size results in a reduced
crystallizer size when zero liquid discharge is achieved by treating blowdown
streams to dryness.
[0058) Although only several exemplary embodiments of this invention
have been described in detail, it will be readily apparent to those skilled in
the art
that the novel produced waste treatment process, and the apparatus for
implementing the process, may be modified from the exact embodiments
provided herein, without materially departing from the novel teachings and
advantages provided by this invention, and may be embodied in other specific
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forms without departing from the spirit or essential characteristics thereof.
Therefore, the disclosures presented herein are to be considered in all
respects
as illustrative and not restrictive. It will thus be seen that the objects set
forth
above, including those made apparent from the preceding description, are
efficiently attained. Many other embodiments are also feasible to attain
advantageous results utilizing the principles disclosed herein. Therefore, it
will
be understood that the foregoing description of representative embodiments of
the invention have been presented only for purposes of illustration and for
providing an understanding of the invention, and it is not intended to be
exhaustive or restrictive, or to limit the invention only to the precise forms
disclosed.
[0059] All of the features disclosed in this specification (including any
accompanying claims, and the drawing) may be combined in any combination,
except combinations where at least some of the features are mutually
exclusive.
Alternative features serving the same or similar purpose may replace each
feature disclosed in this specification (including any accompanying claims,
and
the drawing), unless expressly stated otherwise. Thus, each feature disclosed
is
only one example of a generic series of equivalent or similar features.
Further,
while certain process steps are described for the purpose of enabling the
reader
to make and use certain water treatment processes shown, such suggestions
shall not serve in any way to limit the claims to the exact variation
disclosed, and
it is to be understood that other variations, including various treatment
additives
or alkalinity removal techniques, may be utilized in the practice of my
method.
[0060] The intention is to cover all modifications, equivalents, and
alternatives falling within the scope and spirit of the invention, as
expressed
herein above and in any appended claims. The scope of the invention, as
described herein and as indicated by any appended claims, is thus intended to
include variations from the embodiments provided which are nevertheless
described by the broad meaning and range properly afforded to the language of
the claims, as explained by and in light of the terms included herein, or the
legal
equivalents thereof.
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