Abstract:
An evaporation based method of treating produced water from heavy oil production. A produced water from heavy oil recovery operations treated by first removing oil and grease to a desired level, preferably to about twenty parts per million, or less. If necessary, the pH is then adjusted, normally downward and by acid addition, to release at least some carbonate alkalinity as free carbon dioxide. Preferably, all non-hydroxide alkalinity is removed, or substantially so, by introducing the feedwater into a decarbonator. Feedwater is introduced into an evaporator, and the feedwater is evaporated to a selected concentration factor to produce (1) a distillate having a small amount of residual solutes, and (2) evaporator blowdown containing residual solids. Distillate may be directly used for steam generation in a once-through steam generator, or polished by ion exchange or electrodeionization prior to feed to a packaged boiler. In either case, 100% quality steam is produced, directly in indirectly, for downhole use.

Description:
RELATED PATENT APPLICATIONS  
       [0001]    This invention is a continuation-in-part of prior U.S. patent application Ser. No. 09/566,622, filed May 8, 2000, entitled WATER TREATMENT METHOD FOR HEAVY OIL PRODUCTION, which claimed priority from prior U.S. Provisional Patent Application Serial No. 60/133,172, filed on May 7, 1999, the disclosures of which are incorporated herein in their entirety by this reference, including the specification, drawing, and claims of each application. 
     
    
     
       TECHNICAL FIELD  
         [0002]    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 techniques for the preparation of high quality water for steam generators whose steam product is subjected to down-hole use in heavy oil recovery operations.  
         BACKGROUND  
         [0003]    Water treatment 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&#39;s). Such 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 heated by gas or oil burners. 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 at about 1000 pounds per square inch (psig), or up to as much as 1800 psig. The 80% quality steam  14  (which is 80% vapor, 20% liquid, by weight percent) 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. Steam  14  eventually condenses and an oil/water mixture  22  results that 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  and is pumped to the surface. Then, the sought-after oil is sent to an oil/water separator  32  in which the oil 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 , which de-oiling process usually results in waste oil/solids sludge  44 . The de-oiled produced water stream  46  is then further treated for reuse. The water treatment plant schemes which have heretofore been provided for the produced water stream  46 , i.e., downstream of the de-oiling unit  40  and upstream of injection well  16  inlet  48 , and the type of boilers which are necessary or desirable as a consequence thereof, is the focus the important improvements described in this disclosure.  
           [0004]    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. When used in once through steam generators  12 , the treated produced water stream 12F, 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”), and, less frequently, may have up to about 12000 parts per million (as CaCO3 equivalent) of total dissolved solids, as noted in FIG. 6B. Often, it is necessary to meet other specific parameters before the water can be reused in such once-through steam generators  12 .  
           [0005]    In most cases, the de-oiled recovered water  46  must be treated in a costly water treatment plant sub-system  101  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 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. The softener clarifier  54  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  from the softener clarifier  54 , to prevent carry-over from the softener clarifier  54  of any precipitate or other suspended 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 well understood by those of ordinary skill in the art and to which this disclosure is directed. However, regeneration of the ion-exchange system  64  results in the creation of a regeneration waste stream  68 . Overall, such water treatment plants are relatively simple, but, result in a multitude of waste streams that must be further handled, at additional expense.  
           [0006]    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 .  
           [0007]    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 packaged boilers  80 . Various methods are known for producing water of sufficient quality to be utilized as feedwater 80F to a packaged boiler. 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 packaged boiler  80  produces a blowdown  110  which must be accommodated for reuse or disposal.  
           [0008]    The prior art process designs, such as depicted in FIG. 3, for utilizing packaged boilers in heavy oil recovery operations can be expected to 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 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.  
           [0009]    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 are not entirely satisfactory because:  
           [0010]    physical chemical treatment process schemes are usually quite extensive, are relatively difficult to maintain, and require significant operator attention;  
           [0011]    they require many chemical additives which must be obtained at considerable expense, and many of which require special attention for safe handling;  
           [0012]    they produce substantial quantities of undesirable sludges and other waste streams, the disposal of which is increasingly difficult, due to stringent environmental and regulatory requirements.  
           [0013]    It is clear that the development of a simpler, more cost effective approach to produced water treatment would be desirable for heavy oil production operations. Thus, it can be appreciated that it would be advantageous to provide a new process which minimizes the production of undesirable waste streams, while minimizing the overall costs of owning and operating a heavy oil recovery plant.  
         SOME OBJECTS, ADVANTAGES, AND NOVEL FEATURES  
         [0014]    A new water treatment process 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.  
           [0015]    From the foregoing, it will be apparent to the reader that one of the important and primary objectives resides in the provision of 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.  
           [0016]    Another important objective 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.  
           [0017]    Other important but more specific objectives reside in the provision of various embodiments of an improved water treatment process for production of high purity water for down-hole use in heavy oil recovery, which:  
           [0018]    in one embodiment, eliminates the requirement for separation of the high pressure steam to be utilized downhole from residual hot liquids;  
           [0019]    eliminates the generation of softener sludges;  
           [0020]    in conjunction with the just mentioned objective, minimizes the production of undesirable waste streams;  
           [0021]    minimizes operation and maintenance labor requirements;  
           [0022]    minimizes maintenance materiel requirements;  
           [0023]    reduces the capital cost of water treatment equipment;  
           [0024]    minimizes chemical additives and associated handling requirements.  
           [0025]    Other important objectives, 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  
       [0026]    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:  
         [0027]    [0027]FIG. 1 shows a prior art process, namely a generalized process flow diagram for one typical physical-chemical water treatment process in heavy oil recovery operations.  
         [0028]    [0028]FIG. 2 shows a prior art process, namely a generalized process flow diagram for an industry state-of-the-art water treatment process in steam assisted gravity drainage (SAGD) type heavy oil operations.  
         [0029]    [0029]FIG. 3 shows a prior art physical-chemical treatment process scheme, as it might be applied for use in steam assisted gravity drainage (SAGD) type heavy oil recovery operations.  
         [0030]    [0030]FIG. 4 shows one embodiment of the novel water treatment process disclosed and claimed herein, illustrating the use of the process in combination with the use of packaged boilers for steam production, as applied to heavy oil recovery operations.  
         [0031]    [0031]FIG. 4A shows one common variation of well orientation utilized in heavy oil recovery, namely the use of horizontal steam injection wells and of horizontal oil/water gathering wells, such as commonly encountered in a steam assisted gravity drainage heavy oil gathering project.  
         [0032]    [0032]FIG. 5 shows another embodiment of the novel water treatment process disclosed and claimed herein, 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.  
         [0033]    [0033]FIG. 6A shows the typical feedwater quality requirements for steam generators which produce steam in the 1000 pounds per square inch gauge range, or thereabouts, for packaged boiler type installations.  
         [0034]    [0034]FIG. 6B 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.  
         [0035]    [0035]FIG. 7 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.  
         [0036]    [0036]FIG. 8 diagrammatically illustrates a seeded-slurry scale prevention mechanism useful in the evaporation of waters containing calcium sulfate and silica, as well as the condensation of an evaporated steam distillate at a heat exchange tube, and the downward flow of such condensate by gravity for the collection of such condensate above the bottom tubesheet of an evaporator.  
     
    
       [0037]    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.  
       DESCRIPTION  
       [0038]    Since many steam assisted heavy oil recovery schemes, such as a steam assisted gravity drainage (SAGD) heavy oil recovery process depicted in FIG. 4A, most efficiently utilize a 100% quality steam  70 , it would be desirable to produce such steam by an efficient process scheme such as the novel evaporation based heavy oil produced water treatment method depicted in FIG. 4. In the SAGD process horizontal injection wells  16 ′ and horizontal oil/water gathering wells  10 ′ are advantageously utilized spaced apart within an oil bearing formation  20 . To produce high quality steam for use in heavy oil gathering, and especially in such production schemes just noted with reference to FIG. 4A, an evaporation based water treatment system  120  has been developed. This process, basically situated in one embodiment, process wise, between the de-oiled stream  46  and the injection well  16  head  48 , and 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 packaged boilers  80  as shown in FIG. 4 (or alternately, once through steam generators  12  in the process depicted in FIG. 5), can substantially reduce capital costs and can minimize ongoing operation and maintenance costs of heavy oil recovery. In some locales, such as northern Canada, the elimination of handling of waste sludges and waste streams made possible by the evaporation based water treatment system  120  may be especially important, since it may be difficult to work such waste materials during the extremely cold winter months.  
         [0039]    It has been observed that it may be desirable in some instances to use a standard 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  130  to separate steam  132  and liquid  134 . It is noteworthy in such an 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 are commonly utilized to produce pure steam  70 , and thus produce only a minimal liquid blowdown stream  110 . Unfortunately, packaged boilers require a much higher quality feed water 80F than is the case with feedwater 12F for a once-through type steam generator. As a result, in one embodiment, the novel process disclosed herein includes an evaporation unit  140  based approach to packaged boiler  80  feedwater 80F pretreatment (i.e., pretreatment of the de-oiled produced water  46  generated following the de-oiling process  40  in line after the oil/water separation process  32 ) has now been developed as a novel, improved method for produced water treatment in heavy oil production.  
         [0040]    An oil/water mixture  22  is pumped up through oil gathering wells  30  and this mixture is sent to a series of oil/water 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.  
         [0041]    In the water treatment method disclosed herein, the de-oiled produced water  46  is treated and conditioned for feed to a mechanical vapor recompression evaporator unit  140  to concentrate the incoming produced water stream  46 . The necessary treatment and conditioning prior to the evaporator unit  140  can usually be efficiently accomplished by addition when necessary and or appropriate of acid  144 , such as sulfuric acid or hydrochloric acid, to lower the pH sufficiently so that bound carbonates are converted to free gaseous carbon dioxide, which is removed, along with other non-condensable gases  148  dissolved in the feedwater  46  such as oxygen and nitrogen, in an evaporator feedwater deaerator  150 . The conditioned feedwater  151  is sent as feed to evaporator  140 . 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 FIG. 8, evaporator  140  is in one embodiment provided in a falling film configuration wherein a thin brine film  154  falls inside of a heat transfer 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  170 , 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 FIGS. 4 and 5, only a single stage of evaporation is provided, such distillate  170  may be considered to have been boiled, or distilled, once, and thus condensed but once. Also, the falling film evaporator  140  design is provided only for purposes of enabling the reader to understand the water treatment process, and is not intended to limit the process to the use of same, as those in the art will recognized 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.  
         [0042]    In any event, the distillate  170  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 such distillate  170  may be sent via line  172  to the earlier discussed deaerator  150  for use in mass transfer, i.e, heating descending liquids in a packed tower to remove non-condensable gases  148  such as carbon dioxide. However, the bulk of the distillate is removed as a liquid in line  180 , and may be sent for further treatment to ultimately produce a feedwater 80F′, in the case where packaged boilers  80  are utilized as depicted in FIG. 4. Alternately, in the embodiment set forth in FIG. 5, distillate  180  may be sent directly to once-through steam generators as feedwater 12F′ (as distinguished by vastly higher quality from feedwater 12F discussed hereinabove with respect to prior art processes) for generation of 80% quality steam  14 .  
         [0043]    Before feed to the boilers, it may, in some embodiments, be necessary to remove the residual organics and other residual dissolved solids from the distillate  180 . For example, as seen in FIG. 4, in some cases, it may be necessary to remove residual ions 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 and directed to remove the salts of concern in a particular water being treated. In any event, regenerant chemicals  204  will ultimately be required, and regeneration results in a regeneration waste  206  that must be further treated. Fortunately, in the process scheme depicted, the regeneration waste  206  can be sent back to the evaporator feed tank  210  for a further cycle of treatment through the evaporator  140 . 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 caustic  232 , when utilized to elevate the pH of recirculating brine  152 , or regeneration chemicals  204 ). Also, even the evaporator blowdown  230  can be disposed in an environmentally acceptable manner, which, depending upon locale, might involve injection in deep wells  240 , or 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.  
         [0045]    The new process method, as variously set forth in FIGS. 4 and 5, is useful in heavy oil production since it (1) eliminates many physical-chemical treatment steps commonly utilized previously in handing produced water, (2) results in lower capital equipment costs, (3) results in lower operating costs for steam generation, (4) eliminates the production of softener sludge, thus eliminating the need for the disposal of the same, (5) eliminates other waste streams, thus minimizing the number of waste streams requiring disposal, (6) minimizes the materiel and labor required for maintenance, and (7) reduces the size of water de-oiling equipment in most operations.  
         [0046]    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 equipment (EDI system  220  or ion exchange system  202 ) 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 . 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.  
         [0047]    Also, as briefly noted above, but significantly bears repeating, in those cases where the EDI system  220  is utilized for polishing, the EDI reject stream  222  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 .  
         [0048]    Again, it should be emphasized that the blowdown  230  from the evaporator  140  is often suitable for disposal by deep well  240  injection. 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.  
         [0049]    Operating Modes for Evaporation  
         [0050]    Most produced waters encountered in heavy oil production are high in silica, with typical values ranging up to about 200 mg/l as SiO2, 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 higher 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. 6. Since the high pH operation assures increased silica solubility, a concentration factor (i.e, ratio of feed rate  151  to blowdown rate  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 , thus minimizing the capital cost of the system.  
         [0051]    Since the calcium hardness and sulfate concentrations of many produced waters is low (typically 20-50 ppm Ca as CaCO3), in many cases it is also possible to operate the evaporators  140  below the solubility limit of calcium sulfate, with proper attention to feedwater quality and to pre-treatment processes. However, if the produced water contains an appreciable amount of calcium and sulfate, the mechanical vapor recompression evaporator  140  can also be operated using a calcium sulfate seeded-slurry technique, even at the high pH of operation. That mode of operation can be made possible by the substantial elimination of carbonate alkalinity before the feedwater is introduced into the evaporator  140 . To allow the evaporator to be constructed with low cost materials of construction, the pH can be controlled between about 11 and about 12, while operating the evaporator  140  in the seeded-slurry mode.  
         [0052]    Operation of the MVR Evaporator in the Seeded-Slurry Mode  
         [0053]    Prior to the initial startup of the MVR evaporator in the seeded-slurry mode, the evaporator, which in such mode is provided in a falling-film, mechanical vapor recompression configuration, the fluid contents of the unit are “seeded” by the addition of calcium sulfate (gypsum). The circulating solids within the brine slurry serve as nucleation sites for subsequent precipitation of calcium sulfate  272 , as well as silica  274 . Such substances both are precipitated as an entering feedwater is concentrated. Importantly, the continued concentrating process produces additional quantities of the precipitated species, and thus creates a continuing source of new “seed” material as these particles are broken up by the mechanical agitation, particularly by the action of the recirculation pump  153 .  
         [0054]    In order to avoid silica and calcium sulfate scale buildup in the evaporator  140 , calcium sulfate seed crystals  272  are continuously circulated over the wetted surfaces, i.e., the falling film evaporator tubes  156 , as well as other wetted surfaces in the evaporator  140 . Through control of slurry concentration, seed characteristics, and system geometry, the evaporator can operate in the otherwise scale forming environment. The thermochemical operation within the evaporator  140  with regard to the scale prevention mechanism is depicted in FIG. 7. As the water is evaporated from the brine film  154  inside the tubes  156 , the remaining brine film becomes super saturated and calcium sulfate and silica start to precipitate. The precipitating material promotes crystal growth in the slurry rather than new nucleation that would deposit on the heat transfer surfaces; the silica crystals attach themselves to the calcium sulfate crystals. This scale prevention mechanism, called preferential precipitation, has a proven capability to promote clean heat transfer surfaces  260 . The details of one advantageous method for maintaining adequate seed crystals in preferentially precipitation systems is set forth in U.S. Pat. No. 4,618,429, issued Oct. 21, 1986 to Howard R. Herrigel, the disclosure of which is incorporated into this application in full by this reference.  
         [0055]    It is to be appreciated that the water treatment process described herein for preparing boiler feedwater in heavy oil recovery operations is an 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 and electrodeionization 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.  
         [0056]    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.  
         [0057]    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 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.  
         [0058]    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. 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.