Patent Publication Number: US-6213204-B1

Title: High load, thin slip system

Description:
FIELD OF THE INVENTION 
     The field of this invention relates to slip systems for downhole packers, particularly those that require a high load from uphole or downhole directions. 
     BACKGROUND OF THE INVENTION 
     Slip systems are typically used to anchor packers to the casing. A typical slip system comprises a cone, slips and a body. The cone is typically a cylindrical component which has a shallow angle cut on the outside diameter of one end. The slips are segments cut from a cylinder and have the same angle as the cone on the inside diameter, as well as sharp teeth on the outside face. The cone and slips slide over the body, which is also cylindrical. When the packer is set, the cone pushes against the slips through the shallow angle, causing them to move radially until the sharp teeth contact the casing. Load applied to the packer is transmitted to the cone, which causes the slips to bite deeper into the casing to prevent the packer from moving. Therefore, in most slip systems, a radial load is applied to the cone when the packer is loaded due to the angles cut on the cone and slips. If the load applied to the packer is great enough, the cone will collapse until the inside diameter of the cone contacts the outside diameter of the body. At times, the applied load can cause the body to collapse. The limitation of the amount of load a packer can hold is often determined by when the cone collapses onto the body, causing it to collapse. Thinner slip systems, because of their reduced cross-section, are less resistant to collapse from the applied radial load and hold less force than thicker systems. However, thick slip systems have a disadvantage of requiring additional space, which decreases the available bore size in the packer for a given casing size. 
     Another design which has been used in the past on packers is illustrated in FIGS. 1-3, as well as in U.S. Pat. No. 4,711,326. FIG. 1 is a perspective of a slip without the wickers, illustrating opposed beveled surfaces  10  and  12 . Each of those surfaces has an elongated tab  14  and  16 , respectively. Referring to FIGS. 2 and 3, the elongated tabs  14  and  16  ride in grooves  18  and  20 . Grooves  18  and  20  are wider than the width of the tabs  14  and  16  to allow easy movement for guiding the slip  22  along the cone  24 . As seen in FIG. 3, the cone  24  has opposed surfaces  26  and  28  which are disposed to engage the beveled surfaces  10  and  12  on slip  22  shown in FIG.  1 . Thus, the extension of the tabs  14  and  16  into grooves  18  and  20  serves to guide the slip  22  with respect to cone  24 , while at the same time the engagement of the beveled surfaces  10  and  12  on slip  22  to surfaces  26  and  28  of cone  24  acts to transfer the radial load from the casing through the slip  22  into the cone  24 . Because of the beveled cut on surfaces  10  and  12 , a near-circumferential component of the radial force applied to the slips  22  is communicated into the cone  24 . This design has been used traditionally to hold forces from only one direction and in permanent installations. The present invention is more suitable for retrievable packers and systems which need to hold forces from both directions (bidirectional). The present invention retrieves because there is only one angle between the slip and cone instead of the combined angles in the prior art shown in FIGS. 1-3. This combined angle causes a wedging effect between the slips and cone which increases the retrieval force. Tests have shown that in some cases, the retrieval force is so high that the tails  15  are pulled off the ends of the slips due to a tensile failure at narrow region  17  (see FIG.  1 ). When this happens, the slips cannot be retrieved. 
     In the preferred embodiment, the present invention uses bidirectional slips which have a ramp angle on each end. The prior art slips of FIGS. 1-3 only have a ramp angle on one end. The prior art system of FIGS. 1-3 is not readily convertible to a bidirectional design, and even if it could be, it would still be very costly, highly complex, and not as reliable as the present invention. 
     These and other advantages of the present invention will be more readily understood by those skilled in the art from a review of the preferred embodiment described below. 
     SUMMARY OF THE INVENTION 
     A high-load slip system allows better transmission of loads from the slips to the body. The cone comprises longitudinal slots and the body comprises tabs which are disposed in those slots. The load is transferred from the slips to the cone and into the tabs which reside in the slots. The arrangement can be configured to share the load between the tabs extending from the body and the actual body itself after a small amount of collapse on the cone, leaving the body to support the cone, both through the tabs and on the outside diameter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a slip, without wickers, of the prior art, showing opposed beveled surfaces. 
     FIG. 2 is the cone to be used with the slips shown in FIG. 1 in the prior art, illustrating the matching surfaces to the beveled surfaces of the slip. 
     FIG. 3 is an end view of the cone in FIG. 2, again showing the disposition of opposed surfaces which accept the slip of FIG.  1 . 
     FIGS. 4 a-b  are a sectional view of the present invention shown in the run-in position. 
     FIGS. 4 c-d  are the sectional view of the present invention shown in the set position. 
     FIG. 5 is a section view of one of the cones shown in FIGS. 4 a-b , taken along lines  5 — 5  of FIG.  6 . 
     FIG. 6 is an end view of the cone in FIG.  5 . 
     FIG. 7 is a section view of a portion of the body of the downhole tool shown in FIGS. 4 a-b  and taken along lines  7 — 7  of FIG.  8 . 
     FIG. 8 is an end view of FIG.  7 . 
     FIG. 9 is a sectional view showing the tab extending into the slot of the cone. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 4, the slip system for any given downhole tool, such as a packer or bridge plug, is illustrated. In the run-in position shown in FIG. 4, the body  30  supports a bidirectional slip  32 , which is disposed between an upper cone  34  and a lower cone  36 . Wickers  33  and  35  are opposite to each other to secure the packer against loads from opposed directions. FIGS. 5 and 6 illustrate the cones in more detail. Each cone is cylindrically shaped with a tapered surface  38 . The discussion of FIGS. 5 and 6 will focus on lower cone  36  and the lower end of slip  32 , although it is equally applicable to the upper cone  34  and the upper end of slip  32 . The slip  32  has a tapered surface  40  matching the angle of tapered surface  38  on cone  36 . The cone  36  has a series of elongated slots  42  which extend from end  44  where the tapered surface  38  begins. Referring to FIG. 6, the orientation of slots  42  can readily be seen. Referring to FIGS. 7 and 8, it can be seen that the body  30  has a series of tabs  46 , each one being disposed in slot  42  of the cone  36 . Referring to FIG. 4, a slip cage  48  helps to retain the slips  32  and pull the cones from under the slips  32  for release. At its lower extremity  50 , the slip cage  48  extends into grooves  52  of cone  36  (see FIG.  6 ). 
     The essential components of the thin slip system for high loads now having been described, its operation can be explained in greater detail. Setting the slips  32  involves relative movement with the result that cones  34  and  36  are brought closer together. Referring to FIGS. 5-8, as the slips are wedged against the tubular or casing  54 , a radial load is transmitted through the slips  32  into the tapered surfaces  38  of each of the cones  34  and  36 . In view of the fact that the cones, such as  36 , have the elongated slots  42  with tabs  46  from body  30  extending therein, the radial load from the slips is transmitted through the cones, such as  36 , and into circumferential loads on the tabs  46  extending from body  30 . The load on the cone  36  from the slips  32  is illustrated by arrow  56  as acting on tapered surfaces  38 . That force is in turn translated into opposed circumferential loads as indicated by arrows  58  (see FIGS.  6  and  8 ). Depending on the design parameters for the cone  36 , varying amounts of movement of the segments of cone  36  between slots  42  can occur as a result of loading from the slips  32 . The design of the cone  36  can be such that all of the applied load from the slips  32  can be transferred into the tabs  46  on body  30 . The parameters which will dictate whether the load is taken entirely by tabs  46  or shared between tabs  46  and the remainder of the body  30  include the relationship of the width of slots  42  to tabs  46 , as well as the thickness of the cone  36 . The cone  36  can be designed to flex or somewhat buckle between slots  42  to come into a load-bearing relationship with the body  30  between the tabs  46 . In the preferred embodiment, the radial loading from the slips  30  pushes the broad fingers defined between slots  42  sufficiently inwardly to make edge contact with the tabs  46  such that further loading radially from the slips goes directly to the tabs  46  on body  30 . 
     Those skilled in the art will appreciate that relatively thin slips can be used compared to those illustrated in the prior art, such as FIGS. 1-3. The cone configuration, such as for cone  36 , permits the high loading with a thin slip by virtue of the use of the narrow slots  42 . The cone  36  has greater structural rigidity for a given thickness than the designs for the cone shown in FIGS. 2 and 3. Because of the use of longitudinal slots  42 , coupled with tabs  46 , release of the slips from the casing  54  is also facilitated. The slips  32  do not tend to get stuck to the cone  36 . The design illustrated for the cone in FIGS. 5 and 6 also separates the regions of loading from the slips at tapered surfaces  38  from the transfer of load to the body  30  via tabs  46  which extend into the narrow slots  42 . There is, thus, less of a tendency to stick or jam the slips in the cone, as in the prior art FIGS. 1-3, where guidance of the slip and transfer of load from the slip to the cone occurred in close proximity. The capability of handling a high load comes from the ability to transfer load through the cone  36  into the tabs  46  appended to the body  30 , as opposed to the design of FIGS. 1-3 where the slip loading was transferred entirely into the cone, where loading on the body in the design of FIGS. 1-3 only occurred upon complete collapse of the cone onto the body. In view of the configuration of the cone in FIGS. 2 and 3 to accommodate the slips shown in FIG. 1, limited loading was possible on the cone  24  before it would be collapsed. 
     As shown in FIG. 4, the slip system can employ a unitary slip with two cones, making the entire assembly shorter than the design shown in FIGS. 1-3, which required two distinct slips oriented in opposite directions with a slip ring in between to engage the T-shaped ends of the opposing slips. The designs depicted in FIGS. 4-8 are considerably cheaper to manufacture and provide a greater assurance of release, making the system of the present invention ideal for retrievable packers and bridge plugs requiring high differential loads. 
     The foregoing disclosure and description of the invention are illustrative and explanatory thereof, and various changes in the size, shape and materials, as well as in the details of the illustrated construction, may be made without departing from the spirit of the invention.