Steam generation system with submerged superheater coil

A steam supply system includes a steam generator disposed to produce wet steam for introduction into a steam separator. The steam separator includes a saturated condensate outlet. A superheater receives dry saturated steam from the steam separator and produces superheated steam. An evaporator with an evaporator vessel having a saturated condensate inlet, a soluble solids slurry outlet and a dry steam outlet is in fluid communication with the saturated condensate outlet of the steam separator. Disposed within the evaporator vessel is a superheated steam heat exchanger having a superheated steam outlet and a superheated steam inlet which superheated steam inlet is in fluid communication with the superheater to receive superheated steam. The dry steam outlet of the evaporator is in fluid communication with a steam mixing vessel where the dry steam is mixed with superheated steam from the superheated steam outlet of the heat exchanger.

FIELD OF THE INVENTION

The present disclosure generally relates to production of steam for use in enhanced oil recovery operations, and more particularly, to the production of process steam having a high quality while minimizing scale formation during the production of the process steam.

BACKGROUND OF THE INVENTION

It is common in the oil and gas industry to inject steam into a wellbore as part of Enhanced Oil Recovery (EOR) operations. The steam for such EOR operations is usually produced by a once-through steam generator (OTSG), which, in its simplest form, is a continuous tube heat exchanger in which preheating and evaporation of feedwater takes place consecutively to produce a steam stream. Once the steam is produced by the OTSG, it may be prepared or enhanced prior to injection. For example, a steam separator may be utilized to separate the steam stream into gaseous water in the form of dry steam from liquid water in the form of condensate (or saturated water). Additionally, the temperature of the dray steam may be increased in a superheater to add heat, yielding superheated steam. As noted, an OTSG produces a steam stream having a gaseous water portion and a liquid water portion, where the term “steam quality” refers to the amount of liquid content in the produced steam such that the lower the water content, the higher the steam quality. In the steam separator, the gaseous water portion of the steam is separated from the liquid water portion of the steam, after which, the gaseous portion may be injected into a wellbore as part of EOR operations. In some cases, the gaseous portion of the steam may first be superheated before injection. Often, prior art OTSGs are limited to producing steam where the gaseous portion of the steam is limited to about 80% quality (weight %) steam or “wet” steam. 80% quality steam is comprised of 80 percent gas and 20 percent liquid water. The percent of water in steam will go down from 20% to only 5% when steam quality goes up from 80% to 95%. Although it is desirable for purposes of EOR applications to generate steam with a quality significantly higher than 80%, such as 95% quality steam or higher, such OTSGs are often limited to 80% because of the need to retain a certain amount of liquid in the steam for to avoid any precipitation of dissolved solids in water, it being understood solids in the steam can damage the OTSG as well as EOR equipment such as superheaters and injection nozzles utilized in the operations. Therefore, because of the need to retain liquid water, only about 80% of the feedwater is allowed to vaporize in the OTSG in order to maintain some of the feedwater as liquid to retain water soluble solids in solution. By maintaining a sufficient quantity of such feedwater as liquid in the OTSG, precipitation of solids, and thus the likelihood of scale formation, particularly in the heating tubes of the OTSG, is minimized. But because of the need to retain approximately 20% liquid to maintain dissolved solids in solution, it is difficult to achieve steam of a quality significantly higher than 80%, unless the incoming feedwater has zero or negligible amount of total dissolved solids thus obviating the need to retain a portion, i.e., 20%, of the feedwater as liquid. Feedwater with negligible amounts of total dissolved solids is not typically feasible because water treatment costs to remove solids in order to achieve a higher quality steam are expensive, especially to produce sufficiently pure feedwater that would be suitable to generate dry steam in radiant section of a conventional OTSG.

With that said, higher quality (95% nominal quality) steam, such as dry steam or superheated steam, has some definite advantages over 80% quality wet steam for purposes of EOR operations. Injecting higher quality steam in EOR operations can result in a more efficient process, and yield higher oil production volumes due to the higher heat injected into a wellbore for the same amount of steam. 95% quality steam has higher heat content compared to 80% quality steam on a unit mass basis. This is because of 18% more latent heat of vaporization present in 95% quality steam compared to 80% quality steam at 1,500 psig. Specifically, 95% quality steam has 83.4 Btu/lb of higher heat content than 80% quality steam. This translates into an increase of 8% higher heat content for 95% quality steam.

Moreover, the amount of feedwater that is required for the same heat output will decrease with dry or superheated steam. Because there is no liquid content associated with superheated steam, there will be a corresponding reduction in pumping and water treatment costs.

Additionally, when there is flow split in the piping system utilized to deliver the EOR steam to a formation, it is difficult to maintain the same steam quality through each branch of the piping system. Some piping branches will have higher quality steam (less water) and some will have lower quality (more water) steam. This results in an uneven flow distribution and significant variation in heat input among different formation injection points of the piping system. Dry or superheated steam will minimize this inherent problem associated with the distribution of wet steam in a split piping system. This distribution improvement is due to the lack of water in the steam.

Finally, regardless of the OTSG, there is a need to manage the liquid portion of the steam stream which exits the steam separator as saturated water. This saturated water, once it has been utilized to remove solids, must be disposed of. Because water disposal costs are often high and can decrease the cost effectiveness of EOR operations, one solution for disposal of this saturated water is to reintroduce it back into the steam stream that is to be injected into a wellbore. As noted above, steam from a steam separator is typically introduced into a superheater to produce superheated steam for injection into a wellbore. The effect of mixing at least a portion of the saturated water back into the superheated steam stream results in converting the superheated steam back into 95% quality steam. In the prior art, since it is in a liquid stated, the saturated water is often introduced directly into the flow path of the superheated steam stream in a desuperheater having nozzles through which the saturated water is sprayed. Typically, when the saturated water droplets come into contact with superheated steam (which is at a much higher temperature), the saturated water droplets evaporate instantaneously upon contact with superheated steam, resulting in precipitation of solids from saturated water. These solids often plug up the nozzles and cause scale formation in surrounding areas of desuperheater and piping.

DETAILED DESCRIPTION OF THE DISCLOSURE

Disclosed herein is a steam supply system for providing process steam of a desired quality for injection in a wellbore during enhanced oil recovery operations, wherein saturated condensate from a steam separator is evaporated using superheated steam to produce dry steam, which is then mixed with the superheated steam to produce process steam that is dry or superheated. More specifically, wet steam from a OTSG is separated in a steam separator into dry saturated steam and saturated condensate having totally dissolved solids (TDS) therein. The dry saturated steam is heated in a superheater to produce superheated steam. In order to recycle a portion of the saturated condensate while also removing the TDS, the saturated condensate collected in the steam separator and is sent to an evaporator. The evaporator includes a dry steam outlet in an upper portion of a fluid vessel and a slurry outlet in a lower portion of the fluid vessel. Saturated condensate is introduced into the vessel and forms a saturated condensate bath in the lower portion of the vessel. Submerged in the saturated condensate bath is one or more superheated steam heat exchange conduits that are in fluid communication with the superheater. The superheated steam heat exchange conduits are disposed to deliver heat from the superheated steam to the saturated condensate, resulting in evaporation of a portion of the saturated condensate. Evaporation of the saturated condensate produces dry steam that can then be mixed with superheated steam in a mixing vessel. In one or more embodiments, the mixing vessel is in fluid communication with the evaporator with respect to both the dry steam produced therein and the superheated steam used therein such that the superheated steam utilized in the evaporator for evaporation can then be mixed with the dry steam in the mixing vessel to produce the process steam.

With reference toFIG.1, a steam supply system10is shown and includes a steam source12, a steam separator16, a steam superheater32, an evaporator42and a steam mixing vessel68. Steam source12generates wet steam14. Steam source12is in fluid communication with steam separator16having a separation vessel20. Wet steam14is delivered to the steam separator16via a steam inlet18disposed in separation vessel20. While steam source12is not limited to a particular system, in one or more embodiments, steam source12may be an OTSG such is described below with reference toFIG.2. In other embodiments, steam source may be other types of steam generators, including but not limited to heat recovery steam generators (HRSGs), electric steam generators and the like. In any event, within separation vessel20of steam separator16, dry saturated steam22separates from saturated condensate24, with the saturated condensate24collecting in a lower portion26of separation vessel20, and the dry saturated steam22passing through a steam outlet28within separation vessel20. It will be appreciated that saturated condensate24includes retained water soluble solids in solution, and the saturated condensate24is utilized to remove these solids from the steam supply system10, and in particular, the steam separator16. As such, the saturated condensate24is removed from steam separator16through a condensate outlet30disposed within separation vessel20.

Steam separator16is in fluid communication with superheater32, allowing the dry saturated steam22exiting steam separator16to flow to superheater32. Specifically, the steam outlet28of separation vessel20is in fluid communication with a steam inlet34of superheater32. It will be appreciated that superheater32is not limited to any particular type of superheater, and may include radiant, convection, and separately fired superheaters. For purposes of the disclosure, while generally described inFIG.1, superheater32is shown as a convection superheater inFIG.2as described below. Notwithstanding the foregoing, in one or more embodiments, superheater32may include a heat exchanger interface36to increase the temperature of dry saturated steam22introduced to superheater32. For example, heat exchanger interface36may be superheater tubes exposed to convention and/or radiant heat transfer. In any event, dry saturated steam22entering superheater32is heated and exits superheater32via steam outlet38as superheated steam40.

Separation vessel20is in fluid communication with evaporator42, and in particular, condensate outlet30disposed within separation vessel20is in fluid communication with a saturated condensate inlet46of evaporator42to allow the saturated condensate24from steam separator16to flow to evaporator42. Evaporator42includes an elongated, horizontal vessel44having a saturated condensate inlet46, a superheated steam inlet48, a superheated steam outlet50, a dry steam outlet52and a slurry outlet54. Saturated condensate24is introduced into vessel44and collects as a saturated condensate bath24bin a lower portion44aof vessel44so that the surface24aof saturated condensate bath24bis spaced apart from dry steam outlet52disposed within an upper portion44bof vessel44. In one or more embodiments, upper portion44bis characterized by a height Hband lower portion44ais characterized by a height Harelative to elongated horizontal axis47extending along the surface24aof saturated condensate bath24b, where height Haand height Hbare substantially equivalent so that horizontal axis47is positioned along the centerline of the vessel44. The reason for keeping the condensate water surface at the centerline of vessel44is to obtain the maximum relieving surface area for the vessel of a given diameter.

Disposed within the interior45of vessel44are one or more superheated steam heat exchange conduits56, each with a first end56ain fluid communication with the superheated steam inlet48and a second end56bin fluid communication with the superheated steam outlet50. Superheated steam heat exchange conduits56are not limited to any particular arrangement or configuration. In some embodiments, steam heat exchange conduit56may be elongated and extend along a substantial portion of the length of vessel44in order to maximize surface contact with saturated condensate24disposed within vessel44. In some embodiments, steam heat exchange conduit56may be formed of two or more elongated, substantially linear pipes interconnected with one another. In some embodiments, steam heat exchange conduit56may have a plurality of coil loops formed along steam heat exchange conduit56. In some embodiments, steam heat exchange conduit56may a tube bank. In some embodiments, steam heat exchange conduit56may be other heat exchangers consistent with use as described herein. To maximize heat transfer and dry steam production, superheated steam heat exchange conduits56are submerged below the surface24aof saturated condensate24.

Saturated condensate24is introduced into vessel44via saturated condensate inlet46. In some embodiments, evaporator42may include a condensate pipe manifold58extending within vessel44with a condensate manifold inlet58ain fluid communication with saturated condensate inlet46of vessel44. In one or more embodiments, condensate pipe manifold58is elongated and extends along a portion of the length of vessel44. Condensate pipe manifold58may be horizontal and positioned in the lower portion44aof vessel44below steam heat exchange conduits56. Condensate pipe manifold58may include one or more nozzles60disposed along condensate pipe manifold58for injecting saturated condensate24into the interior45of vessel44. In some embodiments, condensate pipe manifold58may include a plurality of nozzles60in fluid communication with the interior45of vessel44. It will be appreciated that in some embodiments, to further maximize heat transfer and dry steam production, vessel44is elongated and horizontal to increase the area of surface24aof saturated condensate24within vessel44, and superheated steam heat exchange conduits56are likewise elongated and horizontal.

Superheated steam40passing through superheated steam heat exchange conduits56causes the saturated condensate24within vessel44to evaporate, yielding dry steam62which collects in the upper portion44bof vessel44before exiting vessel44through dry steam outlet52. In one or more embodiments, the dry steam62produced in evaporator42can be mixed in a steam mixing vessel68with superheated steam40, while in other embodiments, the dry steam62produced in evaporator42can be used for other purposes and need not be directed to steam mixing vessel68(and thereby eliminating the need for steam mixing vessel68in some embodiments). In one or more embodiments, superheated steam heat exchange conduits56may be coated with a high temperature superhydrophobic coating to reduce surface wettability thereby reducing potential for scale buildup on superheated steam heat exchange conduits56.

One benefit to the above-described system10is that the submerged superheated steam heat exchange conduits56can be quickly removed from vessel44and replaced without the need for interrupting operation of the OTSG12. To facilitate such a replacement of superheated steam heat exchange conduits56, three-way valve63may be utilized to divert superheated steam40from evaporator42. At other times, three-way valve63may be utilized to direct a first portion of superheated steam40to steam mixing vessel68and to direct a second portion of superheated steam40to evaporator42in order to supply the heat need for evaporation of saturated condensate24therein. Of course, three-way valve63also may be utilized to direct all of the superheated steam40to evaporator42.

Slurry outlet54is formed in the lower portion44ato remove as soluble solids slurry64those totally dissolved solids (TDS) that were in the saturated condensate24from separator16. The TDS in soluble solids slurry64is more concentrated than in saturated condensate24leaving separator16. In other words, saturated condensate24has a first concentration of soluble solids and soluble solids slurry64has a second concentration of soluble solids that is greater than the first concentration.

It should be noted that while evaporator42is most effective where vessel44is elongated and substantially horizontal, and likewise, superheated steam heat exchange conduit(s)56are elongated and substantially horizontal, other orientations are possible. For example, vessel44may be vertical and superheated steam heat exchange conduit(s)56may be vertical. Likewise, vessel44and superheated steam heat exchange conduit(s)56may have other shapes and/or orientations, provided that saturated condensate24can be evaporated utilizing superheated steam40to produced dry steam62that can be mixed with superheated steam40to produce process steam72. In one or more preferred embodiments, evaporation heat from superheated steam40is delivered via superheated steam heat exchange conduit(s)56that are submerged in the saturated condensate24.

The dry steam62from evaporator42may then mixed with superheated steam40in a steam mixing vessel68to produce process steam72which can be superheated or dry saturated steam. In other embodiments, the wet steam14has a % quality of approximately 80% and the process steam72has a % quality of approximately 95%. In one or more embodiments, at least a portion of the superheated steam40utilized in evaporator42may be utilized in steam mixing vessel68. In this regard, superheated steam40exiting the superheated steam heat exchange conduit(s)56is directed to steam mixing vessel68. In one or more embodiments, the dry steam outlet52of evaporator42is in fluid communication with a dry steam inlet66of steam mixing vessel68, and likewise, the superheated steam outlet50of evaporator42is in fluid communication with a superheated steam inlet70of steam mixing vessel68, thereby allowing the superheated steam from superheater32to be mixed with the dry steam of evaporator42before the process steam72exits steam mixing vessel68via an outlet74for downstream use. In one or more embodiments, once the superheated steam40is mixed with the dry steam62of evaporator42to create process steam72, the process steam72exits steam mixing vessel68via an outlet74for use with downstream equipment76. In one or more embodiments, downstream equipment may be a wellbore tool76utilized to inject the steam into an oil and gas wellbore (not shown). However, in other embodiments, downstream equipment76may be other devices, such as a steam turbine for electricity generation, a steam engine, a steam reformer for producing syngas, cracking units, distillation units. For purposes of the disclosure, the steam supply system10will be described in terms of steam utilized in EOR operations.

More specifically, mixing of dry steam62with superheated steam40within steam mixing vessel68occurs within a chamber69by spraying the dry steam62into the superheated steam40stream via one or more nozzles78arranged within steam mixing vessel68. Nozzles78are not limited to a particular device, but may be any device with an opening or aperture to allow dry steam62to be introduced into chamber69for mixing with superheated stream40. Because dry steam62has been produced by evaporation of saturated condensate24in evaporator42, dry steam62is primarily devoid of solids when dry steam62is introduce into chamber69. As used herein, steam mixing vessel68, which can also be referred to as a desuperheater, may be a chamber, enclosure, valve or other device that allows the dry steam62to be introduced into a superheated steam40stream. It will be appreciated that while outlet74is in fluid communication with downstream equipment76, various additional steam handling devices may be disposed therebetween to assist in handling of the process steam72, including but not limited to steam pumps (not shown). Moreover, where downstream equipment76is a wellbore tool, the wellbore tool may be a drill string, a production string or any other downhole tool that can be positioned within a wellbore (not shown) to release process steam72therein. As noted above, in some embodiments, the process steam72need not be injected into a wellbore at all, but may be used for other purposes consistent with the need for dry steam resulting from steam supply system10.

Turning toFIG.2, one embodiment of a steam source12is shown. In this embodiment, steam source12is depicted as a OTSG200having radiant section202, a convection section204and a flue gas stack206. Radiant section202is formed of an enclosure214having an interior216with a feedwater tube bank218disposed therein. Tube bank218includes a feedwater inlet220to provides feedwater222to tube bank218, and tube bank218includes a wet steam outlet224that is in fluid communication with steam inlet18of steam separator16(seeFIG.1) to deliver wet steam14to steam separator16. A fuel source232, provides fuel230, such as natural gas, to a burner234where air236is introduced into burner234by a blower238. Burner234is disposed to deliver heat to the interior216of radiant section202, converting the feedwater222within tube bank218into wet steam14.

In the illustrated embodiment, superheater32(see alsoFIG.1), is integrally formed as part of the convection section204of OTSG200. Specifically, convection section204includes an interior242through which flue gas244from radiant section202is directed before passing from OTSG200through a flue gas stack206. The heat exchange interface36of superheater32is disposed within convection section204to heat dry saturated steam22entering superheater32via steam inlet34. Specifically, in one or more embodiments, heat exchange interface36is a tube bank245disposed within the interior242of convection section204so that flue gas244passing therethrough can heat the dry saturated steam22within tube bank245before it passes through steam outlet38as superheated steam40.

Although not necessary, in one or more embodiments, feedwater222may be preheated in convection section204prior to introduction into feedwater tube bank218. In the illustrated embodiment, a preheater heat exchanger250may be disposed in the interior242of convection section204. Preheater heat exchanger250includes a feedwater inlet252and a feedwater outlet254, where feedwater outlet254is in fluid communication with feedwater inlet220of feedwater tube bank218. Preheater heat exchanger250may be a tube bank as shown. In any event, in addition to heating dry saturated steam22passing through superheater32, flue gas244flowing through convection section204to flue gas stack206also preheats feedwater222in tube bank250prior to introduction of the feedwater222into the radiant section202of OTSG200.

FIG.3illustrates a method100for steam supplying steam. In a step102of method100, wet steam14is separated into a dry saturated steam22and a saturated condensate24. The wet steam14may be produced from a steam source12, such as the OTSG200illustrated inFIG.2. In this regard, steam source12may convert feedwater222into wet steam14utilizing radiant heat. Moreover, in some embodiments, the feedwater222may be preheated utilizing flue gas244from OTSG200. In any event, the wet steam14generated by steam source12may be approximately 80% quality, but generally no more than 88% quality. Moreover, the wet steam14may include solids dissolved in the wet steam14. Thus, in step102, the dissolved solids remain in solution in the saturated condensate24so that the dissolved solids can be removed for disposal. Step102may occur in a steam separator16. In some embodiments, the temperature of the wet steam14may be approximately 597° F. at 1500 psig, while the temperature of the dry saturated steam22may be approximately 597° F. at 1500 psig. Finally, the temperature of the saturated condensate24may be approximately 597° F. at 1500 psig.

In step104, the dry saturated steam22is heated to produce superheated steam40. In this step104, the dry saturated steam22may be passed through a superheater32to produce the superheated steam40. In one or more embodiments, the heat is transferred by convection and radiation to the dry saturated steam22via flue gas resulting from the production of the wet steam14. In some embodiments, the temperature of the superheated steam40leaving superheater32may be approximately 800° F. at 1500 psig.

In step106, a portion of the saturated condensate24is evaporated utilizing superheated steam40to produce dry steam62. In one or more embodiments, superheated steam heat exchange conduits56are utilized to transfer heat to the saturated condensate24. In this regard, the superheated steam heat exchange conduits56through which superheated steam40passes may be submerged in a saturated condensate bath24bto facilitate evaporation. The saturated condensate24from steam separator16may be introduced into an elongated vessel44and collected in the lower portion44aof vessel44where the superheated steam heat exchange conduits56are positioned. It will be appreciated that this step also concentrates any dissolved solids in the liquid remaining after evaporation. This concentrated soluble solids slurry can then be disposed of, but with much less wastewater than prior art steam production systems. In one or more embodiments, during operation of system10, a small amount of solids soluble slurry64is continuously withdrawn from the lower portion44aof vessel44as saturated condensate24is introduced into vessel44. For example, in some embodiments, from the original 20% of water collected in the steam separator16, only about 2-5% of that need be discarded, while the remaining amount can be recycled into dry steam. In some embodiments, the temperature of the dry steam62may be approximately 595° F. at 1500 psig. In some embodiments, the temperature of the superheated steam40leaving evaporator42may be approximately 634° F. at 1500 psig.

In step108, the dry steam62is mixed with the superheated steam40to produce process steam72. In one or more embodiments, mixing occurs by introducing the dry steam62into a stream of superheated steam40. While in some embodiments, the dry steam62may be sprayed in the direction of flow of the superheated steam40, because solids have been removed from the dry steam62, the direction of the sprayed dry steam62relative to the direction of flow of the stream of superheated steam40is less significant, particularly in regards to concerns about the formation of scale resulting from the mixing. In any event, in one or more embodiments, at least a portion of the superheated steam40mixed with the dry steam62to produced process steam72is also utilized, prior to the step of mixing, to produce the dry steam62. Thus, in some embodiments, at least a portion of the superheated steam40used to produce process steam72is taken from an evaporator42. In some embodiments, all of the superheated steam40utilized in the step of mixing is first used for evaporation of saturated condensate24, while in other embodiments, at portion of the superheated steam40utilized in the step of mixing may also be taken directly from a superheater32. In some embodiments, the temperature of the process steam72may be approximately 624° F. at 1500 psig.

In step110, the process steam72is then introduced into downstream equipment for further use. Thus, in one or more embodiments, step110may include injecting the process stream into a wellbore for EOR operations. In one or more embodiments, step110may include passing the process steam72through a steam turbine for electricity generation. In one or more embodiments, step110may include utilizing the process steam72to drive a steam engine. In one or more embodiments, step110may include introducing the process steam72into a steam reformer to produce syngas. In one or more embodiments, step110may include utilizing the process steam72for the production of chemicals in cracking units or distillation units.

Although various embodiments have been shown and described, the disclosure is not limited to such embodiments and will be understood to include all modifications and variations as would be apparent to one skilled in the art. Therefore, it should be understood that the disclosure is not intended to be limited to the particular forms disclosed; rather, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the disclosure as defined by the appended claims.