Patent Abstract:
A wellbore tubular set concentrically between an inner an and outer annulus has a pressure relief valve that opens when pressure in the outer annulus exceeds pressure in the inner annulus by an amount that can damage the tubular. The relief valve closes and reseats when the pressure differential is reduced to below the damaging threshold. The relief valve can include a spring for reseating the valve. A pressure gauge can be included within the outer annulus for monitoring whether or not the relief valve is operating properly.

Full Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to and the benefit of co-pending U.S. Provisional Application Set. No. 61/261,882, filed Nov. 17, 2009, the full disclosure of which is hereby incorporated by reference herein. 
    
    
     1. FIELD OF THE INVENTION 
     This invention relates in general to production of oil and gas wells, and in particular to an automated vent system that prevents overpressure within an annulus in a wellhead assembly. 
     2. DESCRIPTION OF RELATED ART 
     Systems for producing oil and gas from subsea wellbores typically include a wellhead assembly that includes a wellhead housing attached at a wellbore opening, where the wellbore extends through one or more hydrocarbon producing formations. Casing and a tubing hanger are landed within the housing for supporting casing and production tubing inserted into the wellbore. The wellhead assembly may include strings of concentrically arranged casing, such as conductor pipe, surface casing, and an inner casing. Generally, the inner casing goes deeper than the conductor pipe and surface casing and lines the wellbore to isolate the wellbore from the surrounding formation. Tubing typically lies concentric within the inner casing and provides a conduit for producing the hydrocarbons entrained within the formation. Annuli are defined between each pair of adjacent concentric tubulars, where each annulus is sealed from pressure communication with any of the other annuli. If an annulus becomes unexpectedly pressurized, such as from a leak or thermal expansion of fluids contained and constrained within the annuli, a pressure differential will develop across a tubular wall adjacent the pressurized annulus. Thus a need exists to periodically monitor the pressure in certain tubular members in well installations, both on land and at sea. 
     Checking the pressure in the inner wellhead housing would indicate whether or not any casing leakage or thermal loading has occurred. Subsea wells do not monitor pressure because installing a pressure sensor requires drilling a hole through the sidewall of the inner wellhead housing, which is operationally non-preferred from a pressure integrity standpoint. Further, because of the harsh and corrosive environments often encountered in petroleum well installations, an installed pressure sensor may succumb to the damaging effects and no longer perform. 
     SUMMARY OF THE INVENTION 
     Disclosed herein is a wellhead assembly that includes a pressure vent device that vents between concentric annuli when the pressure differential reaches or exceeds a pre-designated value. In an example embodiment the wellhead assembly includes an inner annulus set in a wellbore that is surrounded by an outer annulus. A tubular is between the inner and outer annuli that has a relief valve set in a sidewall. When closed, the relief valve forms a pressure seal between the inner annulus and outer annulus. The relief valve can selectively opened to allow venting from the higher pressure of the inner annulus and outer annulus. After the inner and outer annuli are substantially pressure equalized, the relief valve then closes. A designated pressure differential between the inner annulus and outer annulus can cause the relief valve to open. In an example embodiment, the relief valve includes a valve seat having a surface in pressure communication with one of the inner annulus or the outer annulus and that is biased to a closed position by a spring. The wellhead assembly may also include a passage leading through the wellhead from one of the annuli. Optionally, a pressure sensor can be set in one of the inner annulus or outer annulus. In an alternative embodiment, the inner annulus can be a tubing annulus and the outer annulus can be a casing annul us and the pressure relief valve allows flow from the casing annulus to the tubing annulus when in the open position. In an alternate example, the wellhead assembly includes a blocking sleeve selectively mounted within one of the annuli and into sealing contact with a vent side of the relief valve to block flow through the relief valve. 
     Also disclosed herein is a method of managing wellbore annulus pressure, in an example embodiment the method involves suspending a tubular in the wellbore that creates an inner annulus in the tubular and an outer annulus around the tubular. In the example method the tubular has a vent valve set in its sidewall, the vent valve opens in response to a pressure difference across the sidewall of the tubular. The pressure difference can be created when one of the inner annulus or outer annulus experiences an increase in pressure. The vent valve opens when the pressure difference is above a designated pressure differential. When open, pressure vents across the tubular to equalize the pressure in the inner and outer annuli. Thus when the pressure difference between the annuli falls below a set value, the vent valve closes. This example can also include monitoring pressure in the inner or outer annulus via non-intrusive means. The inner annulus can be a tubing annulus and the outer annulus can be a casing annulus. In an example embodiment, the annulus having a higher pressure is the outer annulus. In an alternative step, a bridging sleeve may be set in the tubular adjacent the vent valve to override the vent valve function. The wellhead assembly can include a vent passage for venting flow from the inner or outer annulus having the higher pressure through a wellhead and out of the wellbore. 
     An alternative embodiment of a wellhead assembly is described herein that is set over a well. Tubing is suspended in the well and circumscribed by a string of inner casing, that is surrounded by a string of outer casing. The tubing and inner and outer casings define an inner annulus between the tubing and inner casing and an outer annulus between the inner and outer strings casing. Also included is a pressure relief valve set in a passage in a side wall of the inner casing that blocks flow through the passage when a pressure difference between the inner annul us and outer annulus is less than a designated pressure differential and is selectively moveable out of the passage when a pressure difference between the inner annulus and outer annulus is greater than a designated pressure differential so that flow communicates through the passage from the outer annulus to the inner annulus. Optionally included with the wellhead assembly is a tubing annuls passage leads from the inner annuls and to an exterior of the wellhead assembly. Yet further optionally, a pressure sensor can be included in one of the inner annulus or outer annulus. Communication between the outer annulus and the exterior of the wellhead assembly may be limited to a flow path through the pressure relief valve. A blocking sleeve can be included that is selectively installable within the tubing annulus and into sealing contact with a side of the passage (during for instance a planned well workover). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic partial cross sectional view of an embodiment of a wellhead assembly having an automated vent system. 
         FIG. 2  is a schematic side sectional view of a vent valve in, a closed position. 
         FIG. 3  illustrates the vent valve of  FIG. 2  in a open position. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  provides a side partial cross-sectional view of an embodiment of a wellhead assembly  10  in accordance with the present disclosure. The wellhead assembly  10  can be used with a subsea well for controlling production fluid from within a hydrocarbon producing wellbore  11 . An outer wellhead housing  12  is provided having an annular outer conductor pipe  14  extending from its bottom end into formation  15  intersected by the wellbore. Coaxially disposed within the outer wellhead housing  12  is a high pressure/inner wellhead housing  16 . A string of surface casing  18  depends downward from the inner wellhead housing  16  and coaxial within the outer conductor pipe  14 . An outer annulus  19  is formed between the outer conductor pipe  14  and surface casing  18 . 
     The wellhead housing  16  coaxially circumscribes a tubing hanger  20  and production tubing  22  supported by the tubing hanger  20 . A casing hanger  24  is also coaxially landed on a shoulder  26  within the wellhead housing  16 . The shoulder  26  is formed on the inner radius of the wellhead housing  16  and projects inward towards the wellhead assembly axis A X . Casing  28 , which is supported from the bottom end of the casing hanger  24 , depends downward circumscribing the production tubing  22 . The casing  28  defines a casing annulus  30  between it and the wellhead housing  16  and surface casing  18 . A tubing annulus  32  is defined between the casing  28  and tubing  22 . A seal  34  is shown disposed, in the space between the casing hanger  24  and high pressure housing  16 , thereby isolating the casing annulus  30  from the tubing annulus  32 . 
     A typical production tree  36  is shown mounted on the upper end of the high pressure housing  16 ; although this may take many alternative forms and is not intrinsic to the disclosure. The production tree  36  includes a main bore  38  that is axially formed through the production tree  36  and in fluid communication with the production tubing  22 . A sealingly engaged sleeve  39  projects between the upper end of the tubing hanger  20  and the main bore  38 . The main bore  38  is selectively opened or closed with a swab valve  40  shown disposed at its upper end. A production port  42  projects laterally from the main bore  38  through the outer circumference of the production tree  36 . Flow through the production port  42  is regulated with an inline wing valve  44 . 
     The pressure rating of the outer conductor pipe  14  and outer wellhead housing  12  is less than the surface casing  18  and high pressure wellhead housing  16 . Pressure rating of the intermediate casing  28  is compatible with the pressure rating of the surface casing  18  and often higher. However, a leak may occur in the intermediate casing  28  or associated seals (typified by  34 ) and/or (most probably) thermal transients can cause undue pressure to become present in the annulus  30 . Under some conditions, this can cause collapse of the casing  28  (i.e. if caused by thermal transient conditions) or rupture of surface casing  18  releasing wellbore fluids directly to the adjacent environment in the latter case 
     An optional pressure sensor  50  is shown mounted on the outer conductor pipe  14 . The pressure sensor  50  would typically be a non-intrusive device, capable of monitoring pressure level in the annulus  30  without being in direct communication with the annulus  30 . An example of a sensor  50  is depicted in U.S. Pat. No. 5,492,017 assigned to the assignee of the present application. Measurements made by the pressure sensor  50  can be conveyed to the controller  48  via a communication link  51  connected between the sensor  50  and controller  48   
     A vent valve  52  is illustrated that selectively allows communication through the intermediate casing  28  between the outer annulus  30  and inner annulus  32 . In this embodiment, the vent valve  52  operates as a pressure relief valve and opens at a specific set pressure to allow communication between the casing annulus  30  and tubing annulus  32 . An embodiment of the vent valve  52  is shown in a side sectional view in  FIG. 2 , wherein the valve  52  includes a cylindrical body  70  set in a port  71  formed through the casing  28 . The valve  52  may also be mounted in a special casing sub or coupling (not shown). In the embodiment of  FIG. 2 , the body  70  has an inner end substantially flush, with the internal surface of the casing  28  facing the tubing annulus  32 . An outer end of the body  70  projects into the casing annulus  30 . 
     Still referring to  FIG. 2 , a valve seat  72  is shown coaxially provided in the body  70  set in a profiled channel on the side of the body  70  in the casing annulus  30 . The valve seat  72  mid section is cylindrical having an open end facing the casing  28 . The valve seat  72  includes an “L” shaped flange that projects radially outward from the open end of the mid section and then extends axially away from the mid section and towards the casing  28 . A ring shaped metal seal  74  is set in the body  70  in a groove  75  shown circumscribing the mid section of the valve seat  72  to form a sealing surface between the valve seat  72  and body  70 . An annular cavity  76  is shown in the body  70  oriented transverse to the casing  28 ; a spring  77  is disposed in the cavity  76 . The spring  77  extends between the end of cavity  76  proximate the casing  28  and to the portion of the valve seat  72  projecting radially outward from the opening at the mid-section. Thus when compressed, the spring  77  pushes the valve seat  72  away from the casing  28 . 
     A channel  78  is formed in the side of the seal  74  opposite the casing annulus  30  thereby defining a space  79  between the seal  74  and bottom of the groove  75 . Flow passages  80  are shown in the body  70  that provide communication between the space  79  and the tubing annulus  32 . The sealing interface between the seal  74  and valve seat  72  and body  70  as shown in  FIG. 2  blocks pressure communication between the space  79  and the casing annulus  30 . The passages  80  in the body  70  puts the side of the valve seat  72  facing the casing  28  in pressure communication with the tubing annulus  32 . The valve seat  72  is therefore exposed to any pressure differentials that may occur between the casing annulus  30  and tubing annulus  32 . Thus if the pressure in the casing annulus  30  sufficiently exceeds the pressure in the tubing annulus  32 , so that a resultant force is applied to the valve seat  72  that overcomes the force in the spring  77 . As depicted in the schematic of  FIG. 3 , the pressure differential will push the valve seat  72  inward and compress the spring  77 A. Continued movement of the valve seat  72  eventually moves the mid-section of the valve seat  72  past the seal  74  thereby removing the sealing interface between the valve seat  72  and seal  74 . As such, the casing annulus  30  is in pressure communication with the tubing annulus  32  via a path that that travels through the space  79  and passage  80 . The path allows the higher pressurize fluid in the casing annulus  30  to flow through the valve  52 A to the tubing annulus  32 . 
     Fluid flow during venting from the casing annulus  30  to the tubing annulus  32  reduces the pressure in the casing annulus  30 ; and also reduces the pressure differential between the easing annulus  30  and the tubing annulus  32 . Removing the pressure different allows the spring  77  to reseat the valve seat  72  and reinstate the sealing interface as illustrated in  FIG. 2 . This would be typified by a nominal relief setting of 500 psi on the valve, the actual value being predetermined by operator preference. 
     In one example of use, when pressure in the casing annulus  30  approaches a designated pressure that may potentially damage wellbore assembly  10  hardware, the vent valve  52 , automatically reverts to the open position of  FIG. 3  (casing annulus  30  vented into tubing annulus  32 ) until pressure in the casing annulus  30  is below a potentially damaging pressure. The casing annulus  30  is vented until the pressure therein is no greater than 500 pounds per square inch (or some other value of the pressure setting of the valve  52 ) less than the minimum differential rating of the wellhead assembly  10  and surface casing  18  when considered together. Optionally, the pressure could be reduced yet further (for instance down to ambient pressure) in an attempt to compensate for a slow leak downhole past for instance a production packer (not shown) or tubing joint. 
     As a contingency, later in field life if desired, during for instance recompletion, the vent valve  52  can be overridden by installation of a contingency “patch” or sleeve  64  ( FIG. 1 ) inside the intermediate casing  18 , bridging the vent assembly. The blocking sleeve  64  is shown coaxially within the casing  28  and illustrated at an axial location adjacent the vent valve  52 . This sleeve  64  may be set in a number of ways that are typified by casing patch technology, more recent versions of this being as typified by expandable tubular systems, wherein metal casing is plastically deformed to expand out radially into contact with the casing inner diameter. 
     In an alternative embodiment, the production tree  36  includes an annulus line  82  that extends from the tubing annulus  32 , through the tubing hanger  20 , and to the annular space  84  between the tubing hanger  20  and the production tree  36 . The annulus line  82  has a valve that can be opened to bleed off pressure it receives from the pressurized (or leaking) casing annulus  30  in an example of use, the valve  52  allows flow only from the casing annulus  30  to the tubing annulus  32 , and not vice-versa. As indicated above, the casing annulus  30  is closed and sealed at its supper end by the seal  34 , also referred to as a casing hanger packoff. Optionally, the production tree  36  could be in a horizontal configuration, in which case the tubing annulus line  82  would bypass the tubing hanger  20 . 
     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. For example, the vent valve  52  can be of the form found in Fenton et al. U.S. Pat. No. 6,840,323, which is assigned to the assignee of the present application and incorporated by reference herein. Optionally, the vent valve  52  can be made of a valve member urged closed by a resilient member, such as a spring, that compresses in response to a designated pressure differential.

Technology Classification (CPC): 4