Abstract:
A wellhead system for producing hydrocarbons from a subterranean formation that includes concentric tubulars that form an annulus. The annulus is vented by flowing fluid from the annulus through a bleed line having a valve that is selectively opened and closed. Upstream of the bleed line valve, the bleed line is routed adjacent a production flow line. The temperature of the fluid in the production flow line is greater than annulus temperature and warms the bleed line. Hydrate formation in the bleed line is thereby inhibited by the thermal energy it receives from the production flow line.

Description:
FIELD OF THE INVENTION 
     This invention relates in general to production of oil and gas wells, and in particular to a wellhead system having a flow line for venting an annulus in the wellhead system that is heated by a production flow line also within the wellhead system. 
     DESCRIPTION OF RELATED ART 
     Wellheads used in the production of hydrocarbons extracted from subterranean formations typically comprise a wellhead assembly attached at the upper end of a wellbore formed into a hydrocarbon producing formation. Wellhead assemblies usually provide support hangers for suspending production tubing and casing into the wellbore. The casing lines the wellbore, thereby isolating the wellbore from the surrounding formation. The tubing typically lies concentric within the casing and provides a conduit therein for producing the hydrocarbons entrained within the formation. 
     Wellhead assemblies also typically include a wellhead housing adjacent where the casing and tubing enter the wellbore, and a production tree atop the wellhead housing. The production tree is commonly used to control and distribute the fluids produced from the wellbore and selectively provide fluid communication or access to the tubing, casing, and/or annuluses between the tubing and casing. Valves assemblies are typically provided within wellhead production trees for controlling fluid flow across a wellhead, such as production flow from the borehole or circulating fluid flow in and out of a wellhead. 
     In  FIG. 1 , one example of a prior art wellhead assembly  10  is shown in a side sectional view. The wellhead assembly  10  is mounted on a wellbore  12  that intersects a subterranean formation  14 . As is typical, the wellhead assembly  10  includes a main bore  16  that registers with the wellbore  12  and extends vertically upwards through the wellhead assembly  10 . Swab valves  18  are generally set within the main bore  16  for isolating the main bore  16  and wellbore  12  from ambient conditions above the wellhead assembly  10 . Production from the wellbore  12  is generally accomplished via a production line  20  shown intersecting the main bore  16  and extending laterally through a production tree  21 . A production wing valve  22  is shown within the production line  20  for selectively regulating flow through the production flow line  20 . Often, wellhead assemblies  10  also include an annulus line  24  mounted on the production tree  21  and which usually includes an annulus wing valve  25  for controlling flow therein. 
     Generally, production tubing  26  is the inner most tubular within a wellhead assembly  10 , thus the inner surface of the production tubing  26  defines the production bore  16 . Circumscribing the production tubing  28  is casing  28  that generally extends down from the tree  21  and into the wellbore  28  to form a wellhead housing. An annulus  30  is defined between the tubing  26  and casing  28  which typically is in communication with the annulus line  24 . Often, for access or for venting of the annulus  30 , an annulus bleed line  32 , which is schematically illustrated in  FIG. 1 , has one end connected to the annulus  30  and often is routed to above sea surface in the case of sub sea wells. A bleed valve  34  is shown included within the bleed line  32  and is generally implemented for regulating flow through the bleed line  32 . 
     Fluids produced from within the wellbore  12  can include components that form hydrates when subjected to certain temperatures and pressures. When formed, hydrates are ice like solids made up of gases enclosed within a cage of hydrogen bonded water molecules. Because hydrate formation can occur when cooling a fluid having hydrate components, flow circuits that experience a sudden pressure drop, such as through a throttling valve, may induce hydrate formation. The ice like nature of hydrates typically impedes fluid flow through lines and valves of a flow circuit. Chemical injections can be useful for avoiding hydrate formation, but performing and maintaining the injections introduces added complexity to production of hydrocarbons. 
     SUMMARY OF THE INVENTION 
     Disclosed herein is an example of a wellhead assembly equipped with an annulus bleed line designed to prevent hydrate formation. In an example embodiment the wellhead assembly includes a wellhead housing mounted on a wellbore with a production tree connected on top of the wellhead housing. A production flow path is formed through the wellhead housing and production tree, where the production flow path is in fluid communication with the wellbore. Concentric tubulars are included with the wellhead assembly, both of which are registered with the wellbore. The concentric tubulars define an annulus therebetween. The annulus bleed line has an end in fluid communication with the annulus and has a portion routed so that it is in thermal communication with the production flow path. Thus when production fluid from the wellbore flows through the production flow path, and annulus fluid is in the bleed line, thermal energy from the production fluid transferred to bleed line heats the annulus fluid in the bleed line. Heating the annulus fluid that is in the bleed line prevents hydrate formation in the bleed line, even when the annulus fluid is throttled across a valve. In an example embodiment, the wellhead assembly further includes a cross over line for selectively providing fluid communication between the annulus and the production flow path; in this embodiment the bleed line has an end connected to the cross over line. In an example embodiment, the portion of the production tree having the production flow path defines a production wing block, and the bleed line is provided in the production wing block. In an example embodiment, a portion of the bleed line is adjacent a portion of the production flow path. In an example embodiment, a portion of the bleed line contacts a portion of the production flow path. In an example embodiment, a portion of the bleed line circumscribes a portion of the production flow path. In an example embodiment, the portion of the bleed line circumscribing the portion of the production flow path can be one or both of a helical line or a jacket. In an example embodiment the wellbore is subsea. 
     Also disclosed herein is a method of preventing hydrate formation in fluid produced from a wellbore. In an example embodiment, the method includes providing a wellhead assembly: where the wellhead assembly is made up of a wellhead housing mounted on the wellbore, a production tree connected on top of the wellhead housing, a production flow path formed through the wellhead housing and production tree and in fluid communication with the wellbore. An annulus is formed between concentric tubulars that are registered with the wellbore. Further included with the wellhead assembly is a bleed line having an end in fluid communication with the annulus and having a portion routed in thermal communication with the production flow path. The method further includes flowing production fluid from the wellbore through the production flow path, flowing annulus fluid from the annulus through the bleed line, and heating the annulus fluid by transferring heat from the production fluid to the annulus fluid so that the annulus fluid is at a temperature above that which hydrates are formed. In an example embodiment, after heating the annulus fluid, the annulus fluid is transferred to above sea surface while retaining sufficient thermal energy to remain above a hydrate forming temperature. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side partial sectional view of a prior art wellhead assembly. 
         FIG. 2  is a side sectional view of an example embodiment of a wellhead assembly in accordance with the present invention. 
         FIG. 3  is a side sectional view of an alternate embodiment of a wellhead assembly in accordance with the present invention. 
         FIG. 4  is a side sectional view of an alternate embodiment of a wellhead assembly in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The apparatus and method of the present disclosure will now be described more fully hereinafter with reference to the accompanying drawings in which embodiments are shown. This subject of the present disclosure may, however, be embodied in many different forms and should not be construed as limited to the illustrated embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout. For the convenience in referring to the accompanying figures, directional terms are used for reference and illustration only. For example, the directional terms such as “upper”, “lower”, “above”, “below”, and the like are being used to illustrate a relational location. 
     It is to be understood that the subject of the present disclosure is not limited to the exact details of construction, operation, exact materials, or embodiments shown and described, as modifications and equivalents will be apparent to one skilled in the art. In the drawings and specification, there have been disclosed illustrative embodiments of the subject disclosure and, although specific terms are employed, they are used in a generic and descriptive sense only and not for the purpose of limitation. Accordingly, the subject disclosure is therefore to be limited only by the scope of the appended claims. 
     Referring now to  FIG. 2 , one example of a wellbore assembly  40  is shown in a side sectional view; where the wellbore assembly  40  is shown mounted above a wellbore  42  that is formed within a subterranean formation  44 . In the example embodiment of  FIG. 2 , the wellhead assembly  40  includes a production tree  46  that mounts over a wellhead housing  48 . In the example of  FIG. 2 , the wellhead housing  48  includes an outer tubular  50  shown depending into the wellbore  42 . The outer tubular  50  can be a string of casing, such as conductor pipe or production casing. Also depending into the wellbore  42  is production tubing  52  illustrated as concentrically disposed within the casing  50  and registering with the wellbore  42 . A main bore  54  is shown above an upper end of the production tubing  52  that extends upward within the production tree  46  and includes a swab valve  56  in its upper portion. A production flow line  58  is shown formed laterally through the production tree  46  having an end in communication with the main bore  54 . The combination of the main bore  54  and production line  58  defines a production flow path for flowing production fluids from the wellbore  42  to a production facility (not shown). Set within the production line  58  is a wing valve  60  used for regulating flow through the production line  58 . 
     An annulus  62  is defined between the concentric production tubing  52  and casing  50 . An annulus passage  64  is illustrated in  FIG. 2  that extends within the production tree  46  and into communication with an annulus flow line  66 . The annulus flow line  66  may be used for providing access to the annulus  62  as well as introducing fluids from the surface into the annulus  62 , or for venting of fluids within the annulus  62  to surface or another designated location. An annulus wing valve  68  provided within the annulus flow line  66  can be used for selectively allowing flow through the annulus flow line  66 . 
     Still referring to  FIG. 2 , a crossover line  70  extends from the annulus flow line  66 , at a location upstream of the wing valve  68  and connects to the production flow line  58  upstream of the wing valve  60 . A crossover valve  72  is shown set within the crossover line  70  for controlling flow through the crossover line  70 . A bleed line  74  connects to the crossover line  70  in the portion between the crossover valve  72  and where the crossover line  70  contacts the annulus flow line  66 . A selectively opened and closed bleed valve  76  is provided within the bleed line  74  for venting fluid within the annulus  62  to a location away from the wellhead assembly  40 . Further illustrated in the embodiment of  FIG. 2  is how the bleed line  74  is disposed in thermal communication with the production line  58 . As the production fluid within the production flow line  58  is typically heated above that of the fluid in the annulus  62 , thermal energy (represented as Q) will be transferred from the production flow line  58  and into the bleed line  74 . As such, the fluid within the bleed line  74  may be maintained at a temperature sufficient such that hydrates will be prevented from forming within the fluid. Moreover, embodiments exist wherein the amount of heat Q transferred is sufficient to prevent hydrate formation even downstream of the valve  76  where the annulus fluid is let down to a much lower pressure. As is known, the throttling effect across a valve, especially in instances of relatively large pressure drop, can in turn produce a temperature reduction in which hydrate formation production is enhanced. Thus by heating the fluid in the bleed line  74 , especially prior to any pressure letdown such as provided in the bleed valve  76 , hydration formation may be prevented thereby enhancing fluid flow of the annulus fluid. 
       FIG. 3  illustrates in a side sectional view an example embodiment of a wellhead assembly  40 A that is configured for avoiding hydrate formation in the annulus fluid. More specifically, in the example of  FIG. 3 , fluid from the annulus  62  is routed through a bleed line  74 A that connects directly to the annulus  62  and is piped similar to the embodiment of  FIG. 2  so that thermal communication is maintained with the production flow line  58 . As such, the need for the crossover line or spool of  FIG. 2  is unnecessary for the venting of fluid within the annulus  62  to a surface location. 
       FIG. 4  illustrates yet another embodiment of a wellhead assembly  40 B that allows fluid from the annulus  62  to be conveyed above surface without formation of hydrates. More specifically, the embodiment of  FIG. 4  includes a bleed line  74 B that connects to the annulus  62  on one end and to a jacket  78  on an opposite end. The jacket  78  is shown circumscribing the production flow line  58  so that the fluid within the bleed line  74 B flows over and contacts with the outer surface of the production flow line  58  thereby receiving thermal energy from production fluid in the production flow line  58 . Dashed lines illustrate the portion of the production flow line  58  surrounded by the jacket  78 . On a discharge side of the jacket  78 , a bleed line  80  is provided for transferring the annulus fluid to a site away from the wellhead assembly  40 B. The bleed valve  76  is shown integrally provided within the bleed line  80 . 
     While the invention has been shown or described in only some of its forms, it should be apparent to those skilled in the art that it is not so limited, but is susceptible to various changes without departing from the scope of the invention.