Abstract:
A pressure relief system includes a closed space having fluid trapped therein, a chamber defined in the closed space, and a pressure responsive member for controlling fluid flow from the closed space to the chamber. Fluid flows from the closed space to the chamber when the pressure in the closed space exceeds a predetermined pressure.

Description:
This application claims the benefit of U.S. Provisional Application Ser. No. 60/101,206 filed on Sep. 21, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The invention relates generally to pressure relief systems and, more particularly, to a method and apparatus for relieving pressure in a pressurized space between two casings, or other members, in a well. 
     2. Background Art 
     Drilling of a well through subsurface formations typically involves progressively running casings into the well. Normally, the well is drilled to an initial depth and a conductor casing is run into the well and cemented to the well. A wellhead is typically mounted on the upper end of the conductor casing to provide means for suspending additional casings in the well. The rest of the well is drilled in sections with an intermediate casing run into the well after drilling of each section. The intermediate casings are concentrically arranged in the well with the innermost casing having the smallest diameter among all the casings and extending to a desired well depth, typically near a production zone. FIG. 1 shows a conductor casing  10  that is secured in a well  12  by a cement sheath  14 . Intermediate casings  16  and an innermost or production casing  18  are suspended in the well  12 . As shown, adjacent casings and the surrounding formation define annular spaces  20 . The annular spaces  20  are sealed at the top by the wellhead  22  and closed at the bottom by the formation. 
     The well is drilled by lowering an appropriately sized drill bit on the end of a drill string into the well and operating the drill bit to cut the formation. While operating the drill bit, drilling fluid is pumped through the drill string to move the earth cuttings away from the bottom of the well to the surface. The drilling fluid in the well also serves to control formation fluid influx into the well. The casings are run into the well with the drilling fluid in the well so that the drilling fluid is trapped in the annular spaces  20  between the casings. Typically, cement is pumped into the annular spaces  20  to displace the drilling fluid, secure the casings to the well, and prevent formation fluid influx into the annular spaces  20 . As can be appreciated, for casings extending several hundred feet into the well, substantial volumes of cement are required to fill the annular spaces. Thus, from an economic standpoint, it would be desirable to not displace the drilling fluid in the annular spaces with cement or to partially fill the annular spaces with cement, preferably the bottom portions of the annular spaces that are exposed to the surrounding formations. 
     The well is put to production after it is completed. Completion of the well may include suspending a liner  24  near the bottom end of the production casing  18 . The liner  24  includes perforations through which formation fluid may enter the liner  24  and flow into a production tubing  26 . A packer  28  isolates the section of the well to be produced by sealing an annular space between the production tubing  26  and the production casing  18 . During production, if drilling fluid is trapped in any one of the annular spaces  20 , the temperature of the drilling fluid trapped in the annular space rises to the temperature of the flowing formation fluids, resulting in expansion of the trapped drilling fluid. Because the annular space is closed, the pressure of the expanding drilling fluid also rises. When the pressure of the trapped drilling fluid exceeds the fracture pressure of the surrounding formation, the drilling fluid is forced into the formation adjacent the annular space and the pressure in the annular space stops rising. However, if the formation is plugged for some reason such that the fluid is unable to enter the formation, the fluid pressure in the annular space will continue to rise and may eventually cause the casings to burst or collapse. The formation may be plugged because it is cemented off. Even if the formation is not cemented off, the formation may still be plugged if the drilling fluid in the annular space is weighted with solids and the solids fall down and accumulate in the annular space as the temperature of the drilling fluid in the annular space rises. 
     It is undesirable to have the casings burst since this will lead to loss in control of the well. Thus, it has been the typical practice to completely fill the annular spaces with cement so that the pressure rise due to thermal expansion of trapped drilling fluid in the annular spaces is eliminated. However, for economical reasons, it is desirable to be able to produce the well with the annular spaces unfilled or partially filled with cement. One way of accomplishing this feat is to make the casings strong enough to withstand pressure increases that may occur due to thermal expansion of trapped drilling fluid in the annular spaces. This generally means heavier and more expensive casings, along with more expensive equipment for running the casings into the well, and may not result in cost savings over the typical practice of filling the annular spaces with cement. 
     Another method for preventing casings from bursting when the drilling fluid trapped in the annular spaces expands is to run a pressure relief valve between the casings. For example, a pressure relief valve may be run on the production casing such that fluid transfer from the annular spaces to the production casing occurs. The fluid in the production casing is maintained at a desired pressure and the pressure relief valve operates to equalize the pressure in the annular spaces with the pressure in the production casing. However, with the equalizing of pressure comes mixing of fluid in the annular spaces with the fluid in the production casing. This mixing of fluids, e.g., drilling fluid and completion fluid, may be undesirable. The drilling fluid in the annular spaces may contain solids which can accumulate in the production casing and settle on the packer  28 . Also, if the pressure relief valve seal fails, a leak path is created between the casings, creating a potential for uncontrolled fluid transfer between the casings. 
     SUMMARY OF THE INVENTION 
     In general, in one aspect, a pressure relief system comprises a closed space having fluid trapped therein and a chamber defined in the closed space. A pressure responsive member controls fluid flow from the closed space to the chamber. Fluid flows from the closed space to the chamber when the pressure in the closed space exceeds a first predetermined pressure. 
     In general, in another aspect, a method for relieving pressure in a closed space having fluid trapped therein comprises providing a chamber in the closed space for receiving fluid from the closed space and providing a pressure responsive member for controlling fluid communication between the closed space and the chamber. 
     Other advantages of the invention will become apparent from the following description and from the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of a cased well. 
     FIG. 2 is a partial cross-sectional view of a cased well with a pressure relief device mounted on a casing. 
     FIG. 3A is a cross-sectional view of the pressure relief device of FIG. 2 disposed between two casings. 
     FIG. 3B is a cross-sectional view of FIG. 3A along line A—A. 
     FIG. 4 shows a collapsible pressure relief device disposed between two casings. 
     FIG. 5 shows a pressure relief device with a plurality of cavities disposed between two casings. 
    
    
     DETAILED DESCRIPTION 
     Referring to the drawings wherein like characters are used for like parts throughout the several views, FIG. 2 depicts a well  100  extending from a surface  102  through a production zone  104 . A conductor casing  106  extends from the surface  102  into the well  100 . The conductor casing  106  is secured to the well  100  by a cement sheath  110 . A wellhead  108  is mounted on the conductor casing  106 . The wellhead  108  includes hangers for suspending additional casings in the well  100 . Intermediate casings  112  and a production casing  114  are hung off the wellhead  108  and suspended in the well  100 . A liner  116  disposed inside the well includes perforations which allow formation fluids from the production zone  104  to flow into the liner  116 . The formation fluid flowing into the liner  116  is directed into a production tubing  118  that is suspended in the production casing  114 . Packers  120  are positioned between the production casing  114  and the production tubing  118  and liner  116  to isolate the section of the well  100  which lies adjacent the production zone  104 . 
     The intermediate casings  112  and the production casing  114  are concentrically arranged in the well  100  such that annular spaces  122  and  124  are defined between adjacent casings. The bottom ends of the casings are secured to the well by cement. When the casings  112  and  114  are run into the well and set in place, drilling fluid fills and remains trapped in the annular spaces  122  and  124 . A pressure relief device  128  is disposed in the annular space  124 . The pressure relief device  128  includes a fluid dump chamber which receives excess fluid from the annular space  124  as the fluid trapped in the annular space  124  expands and pressure in the annular space rises above a predetermined level. The expected fluid volume increase in the annular space  124  due to thermal expansion is calculated by knowing the fluid volume in the annular space  124 , the temperature gradient, and the expected temperature increase due to formation fluid flow. The volume of the fluid dump chamber is designed to be larger than the expected volume increase due to thermal expansion. 
     Referring to FIGS. 3A and 3B, the pressure relief device  128  defines a chamber  126  in the annular space  124 . The pressure relief device  128  comprises a first end cap  130 , a second end cap  132 , and an annular housing  134  extending between the end caps  130  and  132 . The end caps  130  and  132  are mounted on a joint of the casing  114 . Casings are made of multiple joints that are linked together by casing couplings  136 . The casing  114 , the end caps  130  and  132 , and the annular housing  134  define a cavity or fluid dump chamber  138 . The fluid dump chamber  138  is arranged to receive fluid from the annular space  124  when the pressure of the fluid trapped in the annular space  124  reaches a predetermined pressure. Seal members  139  provide pressure seals between the end caps  130  and  132  and the casing  114  and between the end caps  130  and  132  and the annular housing  134 . Alternatively, the fluid dump chamber  138  can be made fluid-tight by welding the end caps  130  and  132  to the casing  114  and welding the annular housing  134  to the end caps, as shown at  141 . Although the pressure relief device  128  is shown as mounted on the casing  114 , it should be clear that the pressure relief device  128  may also be mounted on the casing  112 . 
     The end cap  130  includes a port  140  which allows fluid communication between the annular space  124  and the fluid dump chamber  138  when the pressure in the annular space  124  reaches a predetermined pressure. A pressure relief valve  142  is disposed in the port  140  to control fluid communication between the annular space  124  and the fluid dump chamber  138 . The pressure relief valve  142  may be selected to open when the pressure in the annular space reaches the predetermined pressure. This predetermined pressure may be selected as the design pressure of the casing  114  or  112  less a factor of safety. The end cap  130  may include multiple ports  140  and pressure relief valves  142  may be disposed in each port. The end cap  132  includes a port  144  which may also permit fluid communication between the annular space  124  and the fluid dump chamber  138  when the pressure in the annular space  124  reaches a predetermined pressure. A pressure vent device, e.g., rupture disc  146 , is disposed in the port  144 . The rupture disc  146  is arranged to burst to allow fluid in the annular space  124  to enter the fluid dump chamber  138  if the pressure in the annular space  124  reaches the disc burst pressure. Typically, the pressure in the annular space  124  will only reach the disc burst pressure if the pressure relief valve  142  fails. The end cap  132  may also have multiple flow ports similar to port  144  and pressure vent devices may be disposed in the flow ports. 
     In operation, when formation fluid starts to flow from the production zone  104  into the production casing  114 , the temperature of the drilling fluid trapped in the annular space  124  starts to increase to the temperature of the flowing formation fluid. As the temperature of the drilling fluid increases, the trapped drilling fluid starts to expand and the pressure in the annular space  124  increases. When the pressure in the annular space  124  reaches a predetermined value, the drilling fluid starts to flow into the fluid dump chamber  138  until the pressure in the annular space  124  drops below the predetermined value. The fluid trapped in the annular spaces  122 , shown in FIG. 2, also experience a similar pressure rise due to thermal expansion. Therefore, it should be clear that pressure relief devices, similar to the pressure relief device  128 , may be disposed in the annular spaces  122  to stop pressure rise due to thermal expansion of trapped fluid. The fluid dump chamber  138  thus provides a variable “available annulus volume” because the opening of the pressure relief valve  142  increases the volume in the annulus available for the annulus fluid. 
     The invention is not limited to the pressure relief device  128  having a fluid dump chamber  138  for receiving fluid from the annular space  124 . FIG. 4 shows an alternate pressure relief device  150  that collapses to define a fluid dump chamber. The pressure relief device  150  is a collapsible air bladder that is secured to the casing  114  by a strap  152 . Of course, other suitable means of securing the bladder to the casing  114  may be used. The bladder  150  is configured to collapse when the fluid trapped in the annular space  124  expands and the pressure in the annular space reaches a predetermined pressure. Like the pressure relief device  128  of FIG. 3A, the pressure relief device  150  also defines a chamber  126  in the annular space  124 . As the bladder  150  collapses, a fluid dump chamber is created within the chamber  126  to receive fluid from the annular space  124 . 
     Although the pressure relief device  150  is shown as an air bladder, it should be clear that other embodiments of a collapsing pressure relief device are possible. The collapsing pressure relief device is referred to generally herein as a variable volume body because it changes in shape to provide additional volume in the annulus. The changes in shape provide a change in the “available annulus volume” or volume of the annulus available for fluid within the annulus. As the fluid expands, the variable volume body contacts changing the available annulus volume. In an alternate embodiment, the pressure relief device may be a housing, e.g., cylinder, that is made of collapsible material, such as plastic foam. The cylinder may be secured to the casing  112  by a strap or any other suitable means, e.g., welding. The collapsible material is selected such that the cylinder collapses when the fluid trapped in the annular space expands and the pressure in the annular space  124  reaches a predetermined pressure. Like the air bladder, the collapsing cylinder will create a fluid dump chamber within the chamber  126  to receive excess fluid due to thermal expansion from the annular space  124 . The air bladder or collapsible cylinder should be designed to have a larger volume than the expected volume increase in fluid due to thermal expansion. 
     Referring to FIG. 5, another pressure relief device  158  is shown. The pressure relief device  158  defines a chamber  126  in the annular space  124 . The pressure relief device  158  includes a plurality of vessels  160  which are secured to the casing  114  by a strap  162 . Of course, other means of securing the vessels to the casing may also be used. The vessels  160  define fluid dump chambers which are linked together by a tubing  164 . One of the vessels has end caps with flow ports that permit communication between the annular space  124  and the fluid dump chambers defined within the vessels. As in the pressure relief device  128  shown in FIG. 3A, a pressure relief valve and a rupture disc are disposed in the flow ports  166  to control fluid flow from the annular space  124  to the fluid dump chambers. 
     The invention has many advantages. First by employing the pressure relief device in the annular space, the pressure of the drilling fluid in the annular space can be limited to a desired pressure. If this desired pressure is less than the casing design pressure, then the possibility of bursting the casing is eliminated. This makes it unnecessary to use heavy-weight casing. A light-weight casing will result in substantial cost savings in the casing program. Second, the pressure relief device is run into the annular space between two casings. Thus, a possible leak path between casings is not created. Third, the pressure relief device is easy to install and is run into the well on the casing. 
     While the invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous variations therefrom without departing from the spirit and scope of the invention. Any means of creating a fluid dump chamber within an annular space between two casings may be used with the invention. The fluid dump chamber will receive fluid from the annular space when the pressure in the annular space exceeds a predetermined pressure. In this way, the annular space can be maintained at a desired, safe pressure.