Abstract:
The frack plug has a sealing element that reforms when set to hold differential pressure. The element is granular with adhesive to hold the granular particles together but allow the shape to reform under setting force. The adhesive can be broken down with a chemical agent or in other ways to allow the seal to reform to the sealing position at the desired depth. As a result the structural components can disintegrate and the seal assembly can fragment into small pieces that can be circulated out of the well or allowed to drop to the hole bottom. The seal can have particles of controlled electrolytic materials (CEM), natural or synthetic sand, swelling or non-swelling rubber. The assembly can contain pellets that selectively release to initiate the breakdown of the structural components of the frack plug.

Description:
FIELD OF THE INVENTION 
       [0001]    The field of the invention is frack plugs that isolate treated zones so that additional zones can be perforated and fracked, and more particularly plugs that can be removed without drilling, or other intervention, so that the well can be rapidly put into production. 
       BACKGROUND OF THE INVENTION 
       [0002]    Fracking operations typically involve setting an isolation device and perforating and fracking above the set device. This process is repeated as the producing zone is perforated and fractured in a bottom up direction. At the conclusion of the perforating and fracturing of the producing zone the plugs need to be removed so that production can begin. Milling out what could be dozens of plugs can be very time consuming and thus expensive because production is delayed and the debris that is generated in the milling operation needs to be removed either with circulation or with capture devices to collect the debris. 
         [0003]    To aid the milling process the plugs can be made of non-metallic or composite materials. While this technique is workable, there was still a lot of time spent to mill out even the softer bridge plugs and remove that milling debris from the wellbore. 
         [0004]    In the past there have been plugs used that are milled out as described in U.S. Pat. No. 7,533,721. Some are forcibly broken to open a passage such as in U.S. Pat. No. 6,026,903. Other designs created a plug with material that responded to a magnetic field as the field was applied and removed when the field was removed. This design was described in U.S. Pat. No. 6,926,089 and 6,568,470. In a multi-lateral application a plug was dissolved from within the whipstock to reopen the main bore after the lateral was completed. This is described in U.S. Pat. No. 6,145,593. Barriers that assist in extending telescoping passages and then are removed for access to fracture the formation are described in U.S. Pat. No. 5,425,424. Longitudinally extending radially expanded packers to get them to release is shown in U.S. Pat. No. 7,661,470. 
         [0005]    In a variation of the above designs US Publication 2013/0000914 discusses a thin wall mandrel that is then expanded to enlarge the passage through the mandrel as a way of increasing production after sequential fracturing is over. While this design addressed the need for a larger bore diameter for subsequent production, the design still had issues with collapse resistance when the packer was set and the pressures used in fracturing were applied to the annular space causing an excessive compressive collapse force on the frack packer mandrel. 
         [0006]    More recently a design to temporarily support a shear component in a shear plane has been described by William Hered and Jason Barnard in an application called Reinforced Shear Components and Methods of Using Same. Here a disc was interposed in the shear plane and retained in position against a bias force. At a predetermined time the bias force was allowed to move the disc out of the shear plane so that the structure was weakened in the shear plane and the desired failure could occur in the shear plane to release two members to move relatively. 
         [0007]    Another design seeks to address the need for compressive strength against external pressures that would otherwise cause a collapse while at the same time addressing the later need for a larger flow diameter for subsequent production where the fracking was done and there no longer was a need to hold back against compressive collapse forces from outside the mandrel. This is accomplished without a need for expansion. A tubular insert is made of structural tubular materials preferable controlled electrolytic materials or CEM. Controlled electrolytic materials have been described in US Publication 2011/0136707 and related applications filed the same day. The related applications are incorporated by reference herein as though fully set forth. After the packer is set in tension and subjected to fracturing forces it no longer needs high collapse resistance and the CEM sleeve is removed to make a larger flow diameter for subsequent production. Other applications are envisioned where a tubular structure responds to differing pressure conditions at different times in a service life. For example in the fracking situation the anticipated tensile load for production is about 30,000 to 50,000 pounds force and for fracturing can be orders of magnitude higher. 
         [0008]    Various plug designs for subsequent removal by a variety of techniques are illustrated in the following U.S. Pat. Nos.: 5,607,017; 5,479,986; 7,093,664; 7,168,494; 7,353,879; 7,673,692; 7,958,940; 7,997,337; 8,151,895; 8,056,638; 8,235,102; 8,25,6521; 8,272,446; 8,291,969; 8,322,449; 8,327,926; 2012/0152524; 2012/0318513; 2013/0206425; 2013/02481945. 
         [0009]    Plug removal despite the use of composite components or components that dissolve can still lead to an incomplete removal of the plugs causing operational problems when going on production. Typically, plug design involving slips and a longitudinally compressed resilient seal such as rubber annular rings present such situations of incomplete removal. This is because the slips must withstand significant mechanical loads under the pressure differentials that are seen during the fracturing process. What is needed and provided by the present invention is a new design for the frack plugs where the structural body parts such as the mandrel can be made of readily dissolvable material such as CEM and the seal material is granular but with sufficient structural integrity for running in to the desired location and remaining structurally sound. However, when the desired location is reached the granular material is reconfigured, generally with axial compressive force to form a cohesive seal that can withstand the pressure differentials seen in the fracking process. The reconfiguration allows a reordering of the initial shape with sufficient residual binding for the granular material so that axial compression leaves much of the granular material cohesive to the point that on compression it stays together enough to be compressed into an impervious annular shape. The advantage lies in the speed of removal of such a plug without resorting to drilling. The body materials are dissolved with fluids introduced into the borehole. Exposure to wellbore conditions or materials brought into the borehole also weakens the binder for the granular material such that the undermining of the structural components coupled with the weakening of the binder and the granular nature of the material acting as the seal allows for a rapid degradation of the seal material into a loose granular pieces that can be readily circulated out of the borehole or alternatively allowed to drop to the borehole bottom or a further downhole location, depending on the configuration of the borehole. Those skilled in the art will better appreciate these and other aspects of the present invention from the detailed description and the associated drawing s while recognizing that the full scope of the invention can be obtained from the appended claims. 
       SUMMARY OF THE INVENTION 
       [0010]    The frack plug has a sealing element that reforms when set to hold differential pressure. The element is granular with adhesive to hold the granular particles together but allow the shape to reform under setting force. The adhesive can be broken down with a chemical agent or in other ways to allow the seal to reform to the sealing position at the desired depth. As a result the structural components can dissolve and the seal assembly can fragment into small pieces that can be circulated out of the well or allowed to drop to the hole bottom. The seal can have particles of controlled electrolytic materials (CEM), natural or synthetic sand, swelling or non-swelling rubber. The assembly can contain pellets that selectively release to initiate the breakdown of the structural components of the frack plug. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a section view of an embodiment of the present invention in half section and in the run in position; 
           [0012]      FIG. 2  is the view of  FIG. 1  in the set position; 
           [0013]      FIGS. 3   a - 3   c  are a perspective view of a wedge ( 3   a ) that is used to assemble an annular shape ( 3   b ) that is shown in a front view showing how the wedges are stacked ( 3   c ); 
           [0014]      FIG. 4  is the view of  FIG. 2  with a ball landed on a seat defined by a mandrel; 
           [0015]      FIG. 5  is an alternative embodiment in a run in position where relative movement crushes a cellular cement sealing element; 
           [0016]      FIG. 6  is a variation of  FIG. 5  where an inner layer of loose sand is located between a mandrel and the outer cellular cement exterior layer; 
           [0017]      FIG. 7  is an alternative embodiment shown in half section in the run in position; 
           [0018]      FIG. 8  is the view of  FIG. 7  in the set position. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0019]      FIG. 1  schematically illustrates a casing or tubular  1  that defines a borehole in which the packer mandrel  5  is supported on a setting tool that is in turn supported by a wireline that is not shown. The setting tool can be a model E-4 sold by Baker Hughes Incorporated. Part  3  is held to mandrel  5  with a shear ring  4 . The E-4 setting tool retains the mandrel  5  while moving the actuation sleeve  2  which results in axial movement of compression ring  7  to in turn compress the sealing element  8 . As a result of the setting as shown in  FIG. 2  part  3  is released from mandrel  5  as the shear ring is broken with piece  4   a  retained by part  3 . The actuation sleeve  2  remains with the setting tool and comes out to the surface. This leaves the ball seat AA exposed for later acceptance of a ball  9  as shown in  FIG. 4 . A lock ring  6  holds the  FIG. 2  set position of compression ring  7 . The lower end of the plug mandrel  5  has a tapered nose CC that acts as a guide for running in. A small gap BB can be used to avoid extrusion of the element  8  in the  FIG. 2  position or alternatively a flexible barrier schematically illustrated as  11  can be used. The barrier  11  can be retracted for running in and extended during the setting using a wedging action resulting from longitudinal compression of the element  8  to afford some protection to the barrier  11  when running in and to avoid slowing down the deployment speed of the plug P to the desired location. The sealing element  8  does not have to perfectly seal. It can be porous or impervious and needs to retain pressure differentials for a predetermined time which can be done with some leakage flow past the set element  8  or none. 
         [0020]      FIGS. 3   a - 3   c  show that the element can be formed from wedges  20  that can be stacked as shown in  FIGS. 3   b  and  3   c  to make an annular shape  22  that can be supported by the mandrel  5  and axially crushed with movement of ring  7  to assume the  FIG. 2  sealing position. The wedges  20  can be impermeable cellular concrete which is a combination of cementitious slurry with pre-formed foam meeting ASTM standard C869 and as is commercially available from the Geofill Cellular Concrete Company of Lincolnshire, Ill., USA. The crushing of the annular shape  22  reconfigures the annular shape to a shorter length with a diameter that grows to reach the casing or tubular  1 . The mandrel  5  is preferably made from CEM so that when it is time to remove the plug a dissolving fluid can be introduced into the borehole to structurally fail the mandrel  5  as it is dissolved or otherwise failed. With the structural support for the crushed annular shape removed it forms smaller pieces or simply loose granular shapes that are either removed from the location with circulation to the surface or allowed to move further downhole where they will either stay or be brought to the surface with subsequent production. An outer sheath  24  can be applied to the annular shape  22  so as to protect it during running in and to retain the crumbled element  8  until enough axial crushing force is applied to reshape the shape  22  to the set position in  FIG. 2 . The sheath  24  can be of a material that readily disappears with well fluid exposure or added materials or applied conditions in the well at a time before or during the setting action going from the  FIG. 1  to the  FIG. 2  position. The sheath  24  can optionally still be in the process of being removed as the element  8  is already set. 
         [0021]    Other configurations for the element  8  is an agglomerated material that can be principally sand but can also have gravel, tempered glass, proppant, clay, Teflon® or rubber or a combination of the foregoing where the granular material can be held together with an adhesive or with cement. The materials used for the element  8  are designed in part to enhance its grip in the set position. The surrounding sheath  24  can be knitted Kevlar® or nylon or a dissolvable material and can be wrapped about the exterior of the element  8  or all the way around all surfaces of the element  8  or some degree of coverage in between. Alternatively, the sheath  24  can be dissolved with well fluids, well temperatures or other intrinsic or applied well conditions to allow the element  8  to rapidly revert to loose granular form when the mandrel is undermined while at the same time providing protection during running in and cohesive structure to the element  8  as it is crushed and reformed for the sealing position in  FIG. 2 . Alternative ways to make the mandrel  5  fail is to use corrosion or melting from thermal exposure or a material that can dissolve such as CEM. 
         [0022]      FIGS. 5 and 6  show the use of relative movement to directly axially crush an element  30  with arrows  32  and  34  representing the relative axial movement induced by a wireline setting tool or the like. In  FIG. 5  the element  30  is preferably a cellular cement product alone and in  FIG. 6  a layer of loose sand  36  is contained by the outer layer  38  that is preferably cellular cement. 
         [0023]    On the other hand,  FIGS. 7 and 8  show the element  40  being pushed up a ramp  42  with opposed arrows  44  and  46  representing the opposed movement created by a setting tool which moves sleeve  48  to break shear pin  50 . Element  40  is preferably a sand with Teflon® mix which is held together with glue, epoxy or cement with the binder selected to break down after a predetermined time due to thermal exposure so as to make the element  40  easier to reform from the  FIG. 7  to the  FIG. 8  set position while still leaving enough cohesiveness to allow the element  40  to be run in without an optional sheath  52 . As before the sheath  52  can be over the exterior or can be all around the element  40  so that it contacts the ramp  42  which is preferably made of a dissolving material such as CEM. The sheath can start disappearing in temperatures as low as 200 degrees Fahrenheit. 
         [0024]    Another way to extend the sealing element  8  is to radially expand the mandrel  5  in the location adjacent the sealing element  8 . 
         [0025]    The binder for the sand that comprises the bulk of the described elements above can be a polyurethane that is impregnated into the sand. Some of the particles in the mixture can store a material that is released on crushing of the element so as to act on the binder and break it down to facilitate ultimate removal of the plug as the mandrel dissolves and the element reverts to loose granular material for relocation in the well or removal to the surface with circulation. The crushing during set can release chemicals or start a reaction that breaks the binder down and allows the mixture to return to a state of mostly granular sand for ultimate plug removal. The crushing of the element can also release an acid that starts to work on the mandrel that is preferably CEM so that by the time the fracking is done there is less time needed to ultimately fail the mandrel and make it dissolve for displacement in the well or removal of any remnants to the surface. The ends of the mandrel can have interlocking components so that they do not relatively rotate in the event they need to be milled out for any reason. Another way to hold the granular material together is in a porous or impervious enclosure such as a mesh, a flexible film, a foam barrier that can optionally also be combined with binder for the granular material that is preferably sand. The enclosure or cover can be degraded as can the granular binder using the same or different agents that are either introduced in the borehole, already present in the borehole or stored in the granular material for a release on setting or before or after the set position for the sealing element is obtained. Some sharp and hard particles can also be used for the multiple purposes of enhancing grip in the set position as the granular material is dewatered from being compacted with a potential added benefit of starting to undermine the covering physically as the set position is obtained. Such particles can be rubber, CEM chips, swelling rubber or deformable synthetic sand. The cover can then be more fully removed with other means such as thermal exposure, chemical exposure or simply mechanical damage from compaction of the element that such a cover surrounds. 
         [0026]    The above description is illustrative of the preferred embodiment and many modifications may be made by those skilled in the art without departing from the invention whose scope is to be determined from the literal and equivalent scope of the claims below: