Patent Publication Number: US-2011048740-A1

Title: Securing a composite bridge plug

Description:
TECHNICAL FIELD 
     The present invention relates to the field of downhole sealing systems, and in particular to downhole tools such as bridge plugs, frac-plugs, and packers having a non-metallic mandrels. 
     BACKGROUND ART 
     An oil or gas well includes a borehole extending into a well to some depth below the surface, typically lined with tubulars or casing to strengthen the walls of the borehole. Drillers typically fill the annular area formed between the casing and the borehole with cement, to strengthen the borehole walls further and to set the casing permanently in the wellbore. They then perforate the casing to allow production fluid to enter the borehole for retrieval at the surface of the well. 
     Downhole tools with sealing elements are placed within the wellbore to isolate the production fluid or to manage production fluid flow through the well. The tools, such as plugs or packers for example, can be constructed of cast iron, aluminum, other alloyed metals, or from composite materials, and have a malleable, synthetic element system. An element system typically includes a sealing element of a composite or synthetic rubber material that seals off an annulus within the wellbore to prevent the passage of fluids. The element system is compressed or swells, thereby expanding radially outward from the tool to seal with a surrounding tubular. For example, a bridge plug or frac-plug is placed within the wellbore to isolate upper and lower sections of production zones. By creating a pressure seal in the wellbore, bridge plugs and frac-plugs allow pressurized fluids or solids to treat an isolated formation. 
     Where downhole tools use mandrels made of a filament wound composite material, the mandrel has an inner diameter bore that must be sealed then the bridge plug or frac-plug is used. Typically, a solid core is placed into the mandrel, using O-rings to seal with the inner diameter of the mandrel. The core is held in place with pins that fix it axially to the mandrel. These pins create a tensile weak point in the core, which limits the pressure and temperature to which the composite plug can be rated. If the pins fail, the core falls downhole, and the bridge plug or frac-plug must be replaced. 
     Metallic pins and cores have been used to attempt to eliminate this failure mode. But metal pins and cores can cause problems when milling out the installed plug, because the metallic components are not as easily milled as their composite counterparts. In addition, milling is least effective at the center of the plug where the core is. 
     SUMMARY OF INVENTION 
     An element is disposed below a core in a mandrel, providing support for the core against top annulus pressure. In one embodiment, a filament wound composite tube is inserted into the mandrel below the core to support the core when holding pressure in the top annulus. The tube is secured in place with pins through a mule shoe disposed with the mandrel, such that the inner diameter of the tube is not impeded. In some embodiments, the tube has a close-fit tolerance to the existing inner diameter of the mandrel. 
     In another embodiment, a retainer with wickers is disposed below the core to support the core when holding pressure in the top annulus. The wickers are biased to engage with the inner diameter of the mandrel, holding the insert in place under pressure. Axial movement of the core downward furthers engage the wickers with the mandrel, helping to support the core. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate an implementation of apparatus and methods consistent with the present invention and, together with the detailed description, serve to explain advantages and principles consistent with the invention. In the drawings, 
         FIG. 1A  is a cutaway view of a conventional composite bridge plug illustrating a sealing the bore of a mandrel; 
         FIG. 1B  is a cross-sectional view of the bridge plug of  FIG. 1A  at line C-C; 
         FIG. 2A  is a cutaway view of the composite bridge plug of  FIG. 1 , with a tube providing support to the core according to one embodiment; 
         FIG. 2B  is a cross-sectional view of the bridge plug of  FIG. 2A  at line D-D; 
         FIG. 3  is a cutaway view of the composite mandrel of the bridge plug of  FIG. 1 , illustrating installation of a retainer providing support to the core according to a second embodiment, 
         FIG. 4  is a cutaway view of the composite mandrel of  FIG. 3 , illustrating a core supported by the retainer of  FIG. 3 ; 
         FIG. 5A  is a perspective view of the retainer of  FIGS. 3-4  according to one embodiment; 
         FIG. 5B  is a cutaway view of the retainer of  FIG. 5A ; 
         FIG. 5C  is an end view of the retainer of  FIG. 5A ; 
         FIG. 6A  is a perspective view of the retainer of  FIGS. 3-4  according to another embodiment; 
         FIG. 6B  is a cutaway view of the retainer of  FIG. 5A ; 
         FIG. 6C  is an end view of the retainer of  FIG. 5A ; and 
         FIG. 7  is a cross-sectional view of the composite mandrel of  FIG. 3  along line B-B. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Although illustrated in the following with a bridge plug, the present invention is not so limited and embodiments can be used with other types of downhole tools as desired. 
       FIG. 1  is a cutaway view illustrating one embodiment of a downhole tool  100  according to the prior art, with a non-metallic mandrel  110  and an element system  105  disposed about the mandrel  110  configured to seal the downhole tool  100  to a surrounding tubular (not shown). In addition, the downhole tool comprises cones  145 , slips  140 , a sleeve  120 , and a mule shoe  170 . However, the components of the downhole tool  110  are illustrative and by way of example only, and other element systems or components of the downhole tool  100  can be used as desired for a particular application. 
     A bore  180  of the mandrel  110  is sealed with a core  130 , a solid, typically non-metallic member. As illustrated in  FIG. 1A , one or more sealing elements  135 , such as O-rings, seal the core  130  to the inner diameter of the mandrel  110 , preventing fluid flow through the bore  180  of the mandrel  110 . Other techniques for sealing the core  130  to the inner diameter of the mandrel  170  can be used as desired. In one embodiment, the downhole end of the core  130  is chamfered to aid in insertion of the core  130  into the mandrel  110 . 
     The core  130  is typically pinned in place with pins  127  through the sleeve  120  and mandrel  110  and through the core  130 . As illustrated in  FIG. 1B , a cross-sectional view along line D-D, multiple pins  125  are typically spaced apart around the circumference of the sleeve  120  to hold the sleeve  120  in place, and a plurality of spaced-apart pins  127  extend through the core  130 , only one of which is illustrated in  FIG. 1B . The pins  125  and  127  are typically metallic, such as an aluminum-bronze alloy, to provide sufficient tensile strength to hold the core in place under pressure. In some embodiments, the pins  125  and the pins  127  are the same pins. 
     Top annulus pressure (or bottom annulus pressure) can cause deformation of the core  130  and the sealing elements  135 , resulting in a loss of seal, thus leaking fluids around the core  130 . In more extreme cases, excess top annulus pressure breaks the pins  127  or the core  130 , causing the core  130  to be pushed out of the mandrel  110  and lost down hole. 
     Turning now to  FIG. 2A , a downhole tool  200  according to one embodiment comprises all of the components of  FIG. 1A , but additionally provides support for the core  130  by an additional retainer member disposed in the bore of the mandrel  110 . The additional support allows increasing the pressure rating of the downhole tool  200  over that of the downhole tool  100 , and reduces the likelihood of loss of the core  130  downhole. 
     In one embodiment, a non-metallic tube  210  is composed of a filament wound polymeric composite material identical or similar to the polymeric composite material used to construct the element system  105  members, the mule shoe  170 , and the mandrel  110 . In other embodiments, the tube  210  can be formed from other high-temperature and pressure resistant non-metallic materials, such as a polyetheretherketone (PEEK) thermoplastic material. The tube  210  is inserted below the core  130 , abutting a lower surface  205  of the core  130 , as illustrated in  FIG. 2A . The tube  210  extends along the mandrel  110  and is affixed to the mandrel  110  to support the tube  210  and thus the core  130 . The tube  210  can be of any length, and affixed to the mandrel in any location or manner desired. In one embodiment, illustrated in  FIG. 2A , the tube  210  extends along the bore  180  of the mandrel  110  to the mule shoe  170 , where the tube  210  is pinned with pins  175  to affix the tube  210  to the mandrel  110 . 
     The mule shoe  170  of  FIGS. 1A and 2A  is typically threadedly connected to the mandrel  110 , and affixed in place by pins  175 , usually made of an aluminum-bronze alloy, through holes formed through the mule shoe  170  and mandrel  110 , preventing rotational and axial movement of the mule shoe  170  relative to the mandrel  110 . In one embodiment, at least one of the pins  175  also extend through at least a portion of the tube  210 , as illustrated in the cross-sectional view along line C-C of  FIG. 2B , preventing axial and rotational movement of the tube  210  relative to the mandrel  110 . In one embodiment, all of the pins  175  extend into the tube  210 , allowing all of the pins  175  to be made the same length. 
     The mule shoe  170  end of the tube  210  in one embodiment is straight cut for convenience, but can have any other desired configuration. In one embodiment, the pins  175  can be non-metallic pins, such as a polymeric composite, or elastomeric, such as TEFLON® pins. In other embodiments, metallic pins may be used. 
     Because drilling the pins  175  all of the way through the tube  210  would impede fluid flow through the smaller inner diameter of the tube  210 , holes for the pins  175  are preferably drilled nearly all of the way through the wall of the tube  210 , but not all the way through to break the surface of the inner diameter of the tube  210 . In other embodiments, typically with large diameter mandrels, holes for the pins  175  can be drilled all the way through the tube  210 &#39;s wall. 
     The tube  210  is open at the mule shoe  170  end, allowing fluid movement and pressure through the inner diameter of the tube  210  to the surface  205  of the core  130 . To allow easier insertion of the core  130  into the mandrel  110 , edges of the surface  205  can be chamfered if desired. 
     Because the tube  210  is non-metallic, it can be milled out easily. With the added support of the tube  210 , in one embodiment, the pins  127  holding the core  130  are non-metallic, allowing easier milling, while still providing a core  130  that can hold against high top annulus pressures. 
     In one embodiment, the tube  210  is configured with a close tolerance to the inner diameter  180  of the mandrel  110 , providing frictional support to the tube  210 , and reducing annual pressure between the tube  210  and the mandrel  110 . Close fitting of the tube  210  to the mandrel  110  also provides support for the mandrel  110 . 
     In another embodiment, a retainer member  320  provides support for the core  130 , engaging the inner diameter of the mandrel  110 , preventing axial movement of the inner retainer member  320 , and therefore the core  130 . 
     As illustrated in  FIG. 3 , in this embodiment, an installation tool  310  is used to set the retainer  320  in the mandrel  110  at a desired location. A frusto-conical section  315  of the installation tool  310  allows insertion of the retainer  320  onto a reduced diameter section  317  of the installation tool  310 . The installation tool  310  then positions the retainer  320  at the desired location and sets the retainer  320  to engage the mandrel  110 . In one embodiment, the retainer  320  may be compressed prior to insertion into the mandrel  110 , and held in the compressed position by a positioning sleeve  330  shown in dotted lines in  FIG. 3 , to indicate its removal after positioning. In a further embodiment, assembly tooling may be used to expand the retainer  320  after insertion. 
     The installation tool  310  is then withdrawn and the core  130  is inserted into the mandrel  110  to abut the retainer  320  as illustrated in  FIG. 4 . The retainer  320  thus supports the core  130 . Instead of abutting a flat lower surface  205  of the core  130 , as described above, the core  130  has a frusto-conical section  410 , similar to the conical section  315  described above for the installation tool  310 , allowing positioning the retainer  320  on a reduced diameter section  420  of the core  130 . The section  3  is sealed to the inner diameter  180  of the mandrel  110  with O-rings  135 . Alternate techniques for sealing the core  130  to the mandrel  110  can be used as desired. 
     The retainer  320  is typically made of an aluminum-bronze alloy, but can be made of a soft cast iron or other materials as desired, for ease of milling. 
       FIG. 5A  is a perspective view illustrating one embodiment of the inner retainer  320 . As described above, hardened wicker sections  510  are spaced around the circumference of the inner retainer  320 . The inner retainer  320  is roughly C-shaped in cross-section, best illustrated in  FIG. 5C , allowing radial expansion of the inner retainer  320  by widening the gap  530  between two wicker sections  510  under pressure from the installation tool  310 &#39;s frusto-conical section  315  and the frusto-conical section  410  of the core  130 . As best illustrated in  FIG. 5C , grooves  620  are manufactured longitudinally in the inner retainer  510 , providing break points that allow the wicker sections  510  to separate from each other at least in part as they move radially outwardly. The wickers  510 , best illustrated in  FIG. 5B , are biased against downhole movement. The specific configuration of the wickers  510  is illustrative and by way of example only, and any desired wicker configuration can be used. As illustrated best in  FIG. 5B , the inner diameter of the retainer  320  is constant across the length of the retainer  320 , so that the frusto-conical portion  315  of the installation tool  310  and the similar section  420  of core  130  can cause radial expansion of the retainer  320 . 
     As illustrated in  FIGS. 5A and 5B , in one embodiment a portion  540  of the retainer  320  has a smooth outer diameter, except for the longitudinal grooves  520  for ease of handling. In other embodiments, the portion  540  may be omitted. 
       FIGS. 6A-C  illustrate a second embodiment of the retainer  320  that eliminates the smooth section  540 , providing wickers  510  along the entire axial length of the retainer  320 . As illustrated in  FIG. 6B , the inner diameter  610  of this embodiment decreases along the axis of the retainer  320  in a downhole direction. The installation tool  310  and the core  320  can thus be formed without the frusto-conical sections  315  and  410 , respectively, and use the reduced diameter sections  317  and  420  to engage with and force the wicker section  510  radially outward to engage with the mandrel  110 . 
       FIG. 7  a cross-sectional view along line B-B of the system of  FIG. 3 . As the frusto-conical section  315  (or, in the embodiment of  FIG. 6A-C , the reduced diameter section  317 ) engages the retainer  320 , the installation tool  310  urges hardened wicker sections  510  radially outward to engage the mandrel  110 . Once engaged, the wicker sections  510  prevent further axial movement of the inner retainer  320  and the downhole tool  200 , and the core  130  can be disposed with and supported by the retainer  320 . 
     Although described above with a separate installation tool  310 , in some embodiments, the core  130  is used as an installation tool to position and set the retainer  320  in the bore  180  of the mandrel  110 . 
     While certain exemplary embodiments have been described in details and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative of and not devised without departing from the basic scope thereof, which is determined by the claims that follow.