Abstract:
A single bidirectional slip for a downhole tool reduces the volume of metal and allows an increased drill up speed. A dual sealing element system, with one sealing element above and one below the bi-directional slip, provides boost forces going through the sealing elements to the slip. The wickers of the slip can be separated by a substantially flat circumference section for placing a band around the slip to improve fracturing uniformity. The wickers can have any of a variety of configurations, including orientations axially away from the central portion of the slip and orientations axially toward the central portion of the slip.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to the field of downhole tools, and in particular to downhole tools such as bridge plugs, frac-plugs, and packers. 
       BACKGROUND ART 
       [0002]    An oil or gas well includes a wellbore extending into a well to some depth below the surface. Typically, the wellbore is lined with tubulars or casing to strengthen the walls of the borehole. To strengthen the walls of the borehole further, the annular area formed between the casing and the borehole is typically filled with cement to set the casing permanently in the wellbore. Perforating the casing allows production fluid to enter the wellbore and flow to the surface of the well. 
         [0003]    Downhole tools with sealing elements are placed within the wellbore to isolate the production fluid or to manage production fluid flow through the well. For example, a bridge plug or frac-plug placed within the wellbore can isolate upper and lower sections of production zones. Bridge plugs and frac-plugs create a pressure seal in the wellbore to allow pressurized fluids or solids to treat an isolated formation. 
         [0004]    Packers are typically used to seal an annular area formed between two co-axially disposed tubulars within a wellbore. For example, packers may seal an annulus formed between production tubing disposed within wellbore casing. Alternatively, packers may seal an annulus between the outside of a tubular and an unlined borehole. Routine uses of packers include the protection of casing from pressure, both well and stimulation pressures, as well as the protection of the wellbore casing from corrosive fluids. Other common uses include the isolation of formations or leaks within a wellbore casing or multiple producing zones, thereby preventing the migration of fluid between zones. Packers may also be used to hold kill fluids or treating fluids within the casing annulus. 
         [0005]    The downhole tools are usually constructed of cast iron, aluminum, or other alloyed metals, but can be made of non-metallic materials, such as composite materials. A sealing member is typically made of a composite or synthetic rubber malleable material that seals off an annulus within the wellbore to prevent the passage of fluids. The sealing member is compressed or swells, thereby expanding radially outward from the tool to engage and seal with a surrounding tubular. Conventional bridge plug, frac plugs, and packers typically comprise a synthetic sealing member located between upper and lower metallic retaining rings, commonly known as slips, that prevent the downhole tool from moving up or down in the wellbore. 
         [0006]    One problem associated with conventional element systems of downhole tools arises when the tool is no longer needed to seal an annulus and must be removed from the wellbore. For example, plugs and packers are sometimes intended to be temporary and must be removed to access the wellbore. Rather than de-actuate the tool and bring it to the surface of the well, the tool is typically destroyed with a rotating milling or drilling device. As the mill contacts the tool, the tool is “drilled up” or reduced to small pieces that are either washed out of the wellbore or simply left at the bottom of the wellbore. The more metal parts making up the tool, the longer the milling operation takes. Metallic components also typically require numerous trips in and out of the wellbore to replace worn out mills or drill bits. 
         [0007]    Slips have been designed to reduce the amount of metal to reduce drill up time. Although some have attempted to create composite material slips, often pressure holding at temperature has been sacrificed to gain up drill up speed. 
       SUMMARY OF INVENTION 
       [0008]    The conventional two slips of a downhole tool are combined into a single bi-directional slip, thereby reducing the volume of metal and allowing an increased drill up speed. A dual sealing element system, with one sealing element above and one below the bidirectional slip, provides boost forces going through the sealing elements to the slip. In some embodiments, teeth sections of the slip are separated by a substantially flat circumference section for placing a band around the slip to improve fracturing uniformity. The teeth sections can have any of a variety of configurations, including orientations axially away from the central portion of the slip and orientations axially toward the central portion of the slip. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0009]    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, 
           [0010]      FIG. 1  is a cutaway view of a bridge plug according to the prior art; 
           [0011]      FIG. 2  is a cutaway view of a bridge plug according to one embodiment; 
           [0012]      FIG. 3  is a cutaway view of a bi-directional slip according to one embodiment; 
           [0013]      FIG. 4  is a cross-sectional view of a bi-directional slip according to another embodiment; and 
           [0014]      FIG. 5  is a cross-sectional view of a bi-directional slip according to yet another embodiment. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0015]      FIG. 1  is a cutaway view of a conventional bridge plug  100  according to the prior art. The bridge plug  100  comprises a mandrel  110 , about which are disposed various elements, which are typically formed of metal, but can be made of a composite material, such as is described in U.S. Pat. No. 7,124,831, which is incorporated herein by reference in its entirety for all purposes. Even when most of the bridge plug  100  is made of composite materials, the two slips  130  are typically made of metal, such as a ductile cast iron. 
         [0016]    As shown in  FIG. 1 , a sealing member  120  and other related elements  125  are disposed about the mandrel  110 . Axial force through the slips  130  and the other elements  125  compress the sealing member  120 , causing it to expand and to seal with the surrounding tubular (not shown). The two slips  130 , oriented opposite to each other, expand to engage with the surrounding tubular and help retain the downhole tool in place in the wellbore. Boost forces from the sealing member  120  on the slips  130  increase their holding ability. As described above, the two slips  130  are made of metal, typically a ductile cast iron, and increase the mill up time of the downhole tool  100 . 
         [0017]    Turning to  FIG. 2 , a cutaway view illustrates an improved downhole tool  200  that uses a single bidirectional slip with reduced metal volume, allowing faster mill up time, but providing sufficient retaining ability against uphole and downhole forces when engaged with the surrounding tubular. Instead of confining a single sealing member  120  with two slips  130 , in this embodiment, a single slip  210  is confined by two sealing systems  220 , as described in detail below. The downhole tool  200  can be configured as a bridge plug, a frac plug, a packer, or any other desired downhole tool. 
         [0018]    As with the conventional downhole tool  100 , the downhole tool  200  uses a mandrel  110 , disposing the remainder of the downhole tool  200  about the mandrel  110 . Although shown in  FIG. 2  as a hollow core mandrel  110 , the mandrel  110  can be a solid core mandrel or can be a hollow core mandrel that is plugged with a core plug (not shown in  FIG. 2 ) as desired. Most of the downhole tool  200 , including the mandrel  110  and the sealing systems  220 , is typically non-metallic, and the non-metallic elements are usually manufactured from one or more composite materials. 
         [0019]    Instead of two unidirectional slips  130 , each of which resists movement in a single axial direction and tends to disengage from the surrounding tubular in the opposite axial direction, the embodiment illustrated in  FIG. 2  uses a single bi-directional slip  210  that resists movement in either axial direction once activated by the sealing systems  220  to engage with the surrounding tubular. Although made of metal, such as a ductile cast iron, the single slip  210  contains less metal than the conventional pair of slips  130 . 
         [0020]    To provide sealing with the tubular, the downhole tool  200  uses two sealing systems  220 , one axially on either end of the slip  210 . Each sealing system  220 , in addition to sealing the downhole tool  200  with the surrounding tubular, also provides boost forces to the slip  210 , increasing its ability to engage with the tubular and hold the downhole tool  200  in place under high pressure. 
         [0021]    Each sealing system  220  includes a sealing member  225 , which is a malleable, synthetic element. The sealing member  225  can have any desired configuration to seal an annulus within the wellbore. For example, the sealing member  225  can include grooves, ridges, indentations, or protrusions designed to allow the sealing member  225  to conform to variations in the shape of the interior of the surrounding tubular. The sealing member is capable in one embodiment of withstanding temperatures of 232° C. (450° F.) and pressure differentials of up to 103,000 kPa (15,000 psi). Other temperature and pressure configurations can be used, as well. 
         [0022]    As illustrated in  FIG. 2 , according to one embodiment each sealing system  220  comprises, in addition to the sealing member  225 , a cone  221  and two each of cones  224 , expansion rings  223 , and support rings  222 . In other embodiments, a second cone  221  is also included. In the embodiment illustrated in  FIG. 2 , axial force is applied to the sealing system distal to the slip  210  by either a mule shoe  250  or a setting ring  240  and a setting tool (not shown), as described below. In the embodiment with a second cone  221 , the additional cone  221  is interposed between the setting ring  240  and support ring  230 . Such an embodiment would allow manufacture of a sealing system  220  that could be used unchanged both in downhole tools such as the downhole tool of  FIG. 2  and also in conventional downhole tools such as illustrated in  FIG. 1 . 
         [0023]    The sealing system  220 , as illustrated in  FIG. 2 , transfers axial force onto the sealing member  225 , compressing the sealing member  225  and thereby sealing with the surrounding tubular. Each cone  224  transfers axial force from the rest of the sealing system  220  onto the sealing member  225 , compressing the sealing member  225 , and causing the sealing member  225  to expand radially toward the inner surface of the surrounding tubular, sealing with the surrounding tubular. 
         [0024]    The expansion ring  223  flows and expands in one embodiment across a tapered surface of the cone  224 , applying a collapse load through the cone  224  on the mandrel  110 , which helps prevent slippage of the system  220  one activated. The collapse load also prevents the cone  224  and sealing member  225  from rotating when milling up the downhole tool  200 , reducing the mill up time. The cone  224  thus transfers axial force from the expansion ring  223  to the sealing member  225  to cause radial expansion of the sealing member  225 . 
         [0025]    The support ring  222  transfers axial force to the expansion ring  223 . In one embodiment, the support ring  222  comprises sections that are designed to hinge radially outwardly, toward the surrounding tubular as sections are forced across a tapered section of the expansion ring  223 . At full deployment, the sections expand outwardly sufficient to engage the surrounding tubular. 
         [0026]    The cone  221  transfers axial force to the support ring  222 , forcing it across the expansion ring  223  and causing the support ring  222  to expand as described above. The cone  221  proximal to the slip  210  in turn receives axial force in the opposite direction, and transfers boost force to the end of the slip  210 . 
         [0027]    The mule shoe  250  is positioned on the downhole end of the downhole tool  200 , and is typically threadedly attached to the mandrel  110 . The mule shoe  250  is typically pinned in position on the mandrel  110 . 
         [0028]    The setting ring  240  is an annular member that provides a substantially flat surface for use with a setting tool (not shown), and transfers axial force from the setting tool to the sealing system  220  through the other elements of the sealing system  220 . In one embodiment, the support ring  230  has an outer diameter less than the outer diameter of the setting ring  240 , providing a shoulder for engagement with the setting tool, which thus slips over the support ring  240  to engage with the shoulder of the setting ring  240  to transfer force from the setting tool to the sealing system  220 . 
         [0029]    The downhole tool  200  can be installed in a wellbore with any desired non-rigid system, such as electric wireline or coiled tubing. A setting tool, such as a Baker E- 4  Wireline Setting Assembly commercially available from Baker Hughes, Inc., connects to an upper portion of the mandrel  110 . Specifically, an outer movable portion of the setting tool is disposed about the outer diameter of the support ring  230 , abutting the first end of the setting ring  240 . An inner portion of the setting tool is fastened about the outer diameter of the support ring  230 . The setting tool and downhole tool  200  are then run into the well casing to the desired depth where the downhole tool  200  is to be installed. 
         [0030]    To set or activate the downhole tool  200 , the mandrel  110  is held by the wireline, through the inner portion of the setting tool, as an axial force is applied through the outer movable portion of the setting tool to the setting ring  240 . The axial forces cause the outer portions of the downhole tool  200  to move axially relative to the mandrel  110 . 
         [0031]    The force asserted against the setting ring  240  is transmitted by the setting ring  240 . An equal and opposite force is asserted by the stationary mule shoe  250  on the other end of the downhole tool  200 . The force from both ends is transmitted to the sealing systems  220 , which causes the sealing members  225  to expand and to seal with the surrounding tubular. The force is further transmitted to the slip  210 , activating each end of the slip  210  by causing each end of the slip  210  to expand and to engage with the surrounding tubular, setting the slip  210  and thus the downhole tool as a whole. 
         [0032]    The slip  210  fractures under radial stress. The slip  240  in one embodiment, illustrated in  FIG. 2 , has a pineapple configuration that includes at least one and typically a plurality of recessed grooves  212 , milled or otherwise formed therein as fracture zones, allowing the slip  210  to fracture along the grooves  212  to engage the teeth of the slip  210  with the inner surface of the surrounding tubular. 
         [0033]    The slip  210  is disposed between the sealing systems  220  about the mandrel  110 . An outer surface of the slip can include two or more outwardly, extending wickers, comprising serrations or edge teeth, with at least one of the wickers oriented toward the setting ring  240 , to resist uphole axial movement, and at least one of the wickers oriented toward the mule shoe  250 , to resist downhole axial movement. As each end of the slip  210  is driven across the cones  221 , the cone  221  drives that end of the slip radially outward from the mandrel  110 , in addition to providing axial force through the cones  221  to the support rings  222 , and thus to the sealing member  225 . 
         [0034]    In another embodiment, the sealing system  220  illustrated proximal to the mule shoe  250  in  FIG. 2  can be replaced with a bias piston. Because such a biasing piston is more complicated than the sealing system  220 , the use of a second sealing system  220  instead of a biasing piston is preferred. 
         [0035]    The slip  210  of  FIG. 2  is illustrated with circumferential rows of wickers  211  oriented to resist axially downhole movement adjacent to rows of teeth  213  oriented to resist axially uphole movement. The tooth configuration of the slip  210  in  FIG. 2  is illustrative and by way of example only. In particular, the number, shape, and configuration of teeth are illustrative and by way of example only, and other numbers, shapes, and configurations of teeth can be used as desired. For example, instead of biasing the teeth  211  and  213  towards either end of the downhole tool  200 , the teeth  211  and  213  can extend radially perpendicular from a central axis of the slip  210  and resist movement in either axial direction. 
         [0036]      FIG. 3  is a cutaway view that illustrates another embodiment of a slip  300 . In this embodiment, teeth sections  310  and  330  are separated by a central portion with a substantially flat circumferential configuration around which a band  340  is disposed. The band or strap  340  helps ensure that the slip  300 , when axial force is exerted on it from both directions, fractures more evenly, providing substantially equal expansion and engagement of the teeth section  310  and  330 . Thus, the slip  300  can provide substantially equal resistance to axial movement in either direction. 
         [0037]      FIGS. 4 and 5  are cross-sectional views along the central axis A-A of two other embodiments of a slip  400  and  500 . In  FIG. 4 , the teeth  410  and  430  are oriented toward each end of the slip  400 . As in the slip  300  of  FIG. 3 , a central section can be surrounded with a strap  340 , not shown in  FIG. 4 , for improved evenness of fracture and engagement with the surrounding tubular, although embodiments without the central section can be used. Unlike the teeth of the embodiments of  FIGS. 2 and 3 , which have a complex shape, the teeth  410  and  430  of slip  400  are simple triangular teeth, forming a right triangle perpendicular to the axis A-A with the right angle oriented toward the end of the slip  400 . 
         [0038]    An alternate embodiment is shown in  FIG. 5 . As in the embodiment of  FIG. 4 , the teeth sections  510  and  530  comprise simple triangular teeth separated by a central section  520  for the attachment of the band  340 . The orientation of the teeth  510  and  530  are reversed from the orientation of the teeth  410  and  430 , so that the teeth are oriented toward the central portion  520 . 
         [0039]    The number, shape, and configuration of teeth illustrated in  FIGS. 2-5  are illustrative and by way of example only, and any number, shape, and configuration of teeth can be used as desired, including orientation axially either toward the ends of the slip or toward the center. The use of wickers or teeth is illustrative and by way of example only. In some embodiments, other surface treatments other than wickers or teeth can be used to provide gripping ability for the slip. 
         [0040]    The bi-directional slip and dual sealing systems described herein may be used in conjunction with any downhole tool used for sealing an annulus within a wellbore, such as frac-plugs, bridge plugs, or packers, for example. 
         [0041]    In conclusion, by using a single bidirectional slip surrounded by two sealing systems, various embodiments provide a downhole tool with reduced metal content, allowing faster mill up and less metal waste to fall downhole, but without sacrificing the ability to hold the downhole tool in place under high temperature and pressure conditions. The two sealing systems, in addition to doubly sealing with the surrounding tubular, provide boost force on the single slip in both directions, thus increasing the holding power of the single slip. 
         [0042]    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.