A system allows for sequential treatment of sections of a zone. Access to each portion can be with a sliding sleeve that has a specific internal profile. Pump down plugs can be used that have a specific profile that will make a plug latch to a specific sleeve. Pressure on the plug when latched allows a sequential opening of sleeves while zones already affected that are below are isolated. The pump down plugs have a passage that is initially obstructed by a material that eventually disappears under anticipated well conditions. As a result, when all portions of a zone are handled a flow path is reestablished through the various latched plugs. The plugs can also be blown clear of a sliding sleeve after operating it and can feature a key that subsequently prevents rotation of the plug on its axis in the event is later needs milling out.

PRIORITY CLAIM 
   This application is a divisional application claiming priority from U.S. patent application Ser. No. 11/388,847, filed on Mar. 24, 2006 now U.S. Pat No. 7,325,617. 


   FIELD OF THE INVENTION 
   The field of the invention is completion techniques and more particularly those involving sequential procedures in a zone which need periodic obstruction of the flow bore to conduct the operation and need the flow bore cleared thereafter for production. More specifically the invention focuses on plugs that temporarily block a flow path and then at least in part disappear to allow flow to resume. 
   BACKGROUND OF THE INVENTION 
   Some completion methods require sequential isolation of adjacent zones in an interval to perform treatments such as fracing. Typically the zones are isolated with packers and in between them there are sliding sleeves that can be selectively opened to provide access. Typically, this assembly is run in to position, and then a ball or plug is pumped down to the bottom which closes off the flow path through the bottom end of the liner. Pressure is applied and the packers are set, creating multiple isolated zones. The tubular string is pressurized and the lowermost sliding sleeve is opened. After the lowermost zone is treated a ball is dropped on a lowermost seat to close off the zone just treated and the pressure is built up on this first dropped ball to open the next sliding sleeve up. After that treatment an even bigger ball lands on an even bigger seat to close off the second zone just treated. The process is repeated until all zones are treated using a progression of bigger and bigger seats as the treatment moves toward the surface. At the end, the balls on all the seats are either floated to the surface when the flow commences from the treated formation or the assembly of all the seats and the balls that are respectively on them are milled out so as not to impede subsequent production from the treated zone. This technique is shown in U.S. Pat. No. 6,907,936. The problem with it is that different sized seats are required at specific locations to make the isolation system work and in the end there are some rather small passages through the smallest of the seats even if the balls are floated out that then requires a discrete step of milling out the seat and ball near all but one sliding sleeve. 
   Techniques have been developed to temporarily block wellbores using dissolving or other wise disappearing plugs. Such devices are illustrated in U.S. Pat. Nos. 6,220,350; 6,712,153 and 6,896,063. Some packers are built to be disposable involving the use of degradable polymers as illustrated in US Publication No. 2005/0205264; 2005/0205265 and 2005/0205266. Some assemblies involve landing collars that can be changed from a go to a no go orientation with a shifting tool that also doubles as a tool to operate sliding sleeves. This is illustrated in US Publication No. 2004/0238173. Yet other designs that create selective access into a formation by using perforating charges that blow out plugs in casing or pressure actuated pistons with internal rupture discs are illustrated in U.S. Pat. Nos. 5,660,232 and 5,425,424. U.S. Pat. No. 6,769,491 illustrate a typical anchor assembly for a downhole tool. 
   The present invention seeks to streamline certain downhole operations by matching profiles on plugs to those on sliding sleeves or nipple profiles. This allows a specific plug to be located at a certain location and bypass other potential landing locations. The flow path can be identical in size for the duration of different portions can be addressed in a particular sequence. Apart from that, the plugs, after having served their purpose, reopen the flow path for further operations. These and other benefits of the present invention will be more readily understood by those skilled in the art from a review of the description of the preferred embodiment that appears below, as well as the drawings and the claims, which define the full scope of the invention. 
   SUMMARY OF THE INVENTION 
   A system allows for sequential treatment of sections of a zone. Access to each portion can be with a sliding sleeve that has a specific internal profile. Pump down plugs can be used that have a specific profile that will make a plug latch to a specific sleeve. Pressure on the plug when latched allows a sequential opening of sleeves while zones already affected that are below are isolated. The pump down plugs have a passage that is initially obstructed by a material that eventually disappears under anticipated well conditions. As a result, when all portions of a zone are handled a flow path is reestablished through the various latched plugs. The plugs can also be blown clear of a sliding sleeve after operating it and can feature a key that subsequently prevents rotation of the plug on its axis in the event it later needs milling out. 


   
     DETAILED DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a section view of a pump down plug before it is pumped downhole; 
       FIG. 2  is the plug of  FIG. 1  with the passage through the plug open after the nose plug has disappeared; 
       FIG. 3  is a section view of a typical sliding sleeve in the closed position; 
       FIG. 4  is a section view of the pump down plug landed on the sliding sleeve; 
       FIG. 5  is the view of  FIG. 4  with pressure applied and the sleeve shifted to an open position; 
       FIG. 6  is a section view of an alternative embodiment showing the sliding sleeve closed and the profile to receive the pump down plug; 
       FIG. 7  is the view of  FIG. 6  with the pump down plug landed creating a piston around the sliding sleeve; 
       FIG. 8  is the view of  FIG. 7  with pressure applied that results in shifting the sliding sleeve; 
       FIG. 9  is a section of a pump down plug showing the disappearing portion in the nose; 
       FIG. 10  is a closer view of  FIG. 9  showing how the disappearing portion is attached to the pump down plug; 
       FIG. 11  is a section of an alternative design of the disappearing component; 
       FIGS. 12   a - c  are a section view of an alternative pump down plug design showing the plug landed in the sliding sleeve; 
       FIGS. 13   a - c  are the view of  FIGS. 12   a - c  with the sliding sleeve shifted; 
       FIGS. 14   a - c  are the view of  FIGS. 13   a - c  with the plug released from the sliding sleeve and captured on a landing collar; 
       FIG. 15  is a part section perspective view showing the sliding sleeve and a groove that holds the pump down plug against turning if the plug is milled out; 
       FIG. 16  is the pump down plug in perspective showing the lug that resists turning if the plug is milled out. 
   


   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  shows a typical pump down plug  10  that has wiper seals  12  and  14  to make contact with the surrounding tubular so that it can be pumped down. Although cup seals are shown, other types and quantities of seals can be used. The plug  10  has a tubular body  16  with a through passage  18 . Near end  20  is a fishing neck  22  to be used if the plug  10  is to be fished out for any reason. A series of longitudinal grooves  22  define flexible collet fingers  24  that are attached at opposed ends to body  16 . Cantilevered fingers can be alternatively used or any other structure that can maintain a cylindrical shape with sufficient strength and still allow flexing. The flexing feature allows the protrusions  26  and  28  to move radially as the plug  10  is pumped downhole. While the preferred plug  10  has seals  12  and  14  the invention envisions a plug  10  that simply is dropped making the use of seals  12  and  14  optional. Looking at  FIG. 3 , there is a sliding sleeve  30  that has depressions  32  and  34  that are designed to match the shape of protrusions  26  and  28  on the plug  10 . As the plug  10  approaches the sliding sleeve  30  the fingers  24  flex to let the protrusions  26  and  28  jump up on the sleeve  30  and then spring out into depressions  32  and  34  as radial surface  36  on projection  28  registers with radial surface  38  on depression  32 . 
   Those skilled in the art will appreciate that while 2 protrusions  26  and  28  are shown on the plug  10  to match similarly shaped depressions on the sliding sleeve  30  there are many different ways to execute the inventive concept. The concept is to create a unique match between a given plug  10  and a given downhole location which happens to be a sliding sleeve such as  30 . For example, when treating a long zone there will be a plurality of sliding sleeves such as  30  that have packers such as  40  and  42  to isolate a surrounding annulus (not shown). The idea is to progressively isolate parts of a zone working uphole so that the next sliding sleeve between a pair of packers can be opened for treating the formation between those two packers while the portions below already treated are isolated. 
   To better understand how this happens reference is again made to  FIG. 1  where the passage  18  is shown to be blocked by what will generically be referred to as a disappearing material  44 . In this application, the phrase disappearing material is intended to encompass a wide variety of materials used alone or in combination that can retain structural integrity during the pump down procedure but over time when subjected to well conditions whether existing or artificially created will lose that integrity and no longer block the passage  18 , as shown in  FIG. 2 . Threads  46  are visible in  FIG. 2  after the disappearing material  44  has gone away. They are used to initially retain the material  44  in position as shown in  FIG. 1 . The preferred material  44  is a biopolymer that responds to well temperature. Generally when a plug is pumped down from the surface, the fluids used and the flow keeps the material  44  in a plug  10  strong enough to withstand that applied pumping pressures. After a particular portion of a zone is treated through an open sleeve such as  30 , another plug lands in the next sleeve. That cuts off all the lower plugs from flow and allows them to come to equilibrium with well temperatures. Over time the material  44  in the lower plugs disappears opening a path  18  through the lower plugs as plugs land above them in another sliding sleeve. 
     FIGS. 4 and 5  show how a plug  10  with projections  26  and  28  registered with depressions  34  and  32  respectively can be used to shift sleeve  30  from the closed position with ports  48  closed in  FIG. 4  and where they are open in  FIG. 5 . By design, the material  44  continues to block passage  18  with ports  48  open so that a frac job for example can be accomplished through ports  48  with a zone isolated between two external packers  40  and  42 . 
   One aspect of the invention is that a given plug has a profile on the fingers  24  that registers with a specific sliding sleeve profile in the embodiment of  FIGS. 1-5 . The concept is related to a key in a lock cylinder. Combinations of protrusions and depressions can be used with either one being on the plug or the sleeve and the mating profile on the other member. The registration can be determined by having a protrusion and mating depression have similar longitudinal lengths to make them register. There can be more than one pair of protrusions and matching depressions and their spacing from each other can be unique to a given sliding sleeve and a plug that will match. 
   If fracing is to be done for example, using sliding sleeves A, B and C where A is furthest from the surface, the procedure would be to run the assembly into position and set packers between A,B and C and another above C. All sleeves would be run in closed. To frac the zone adjacent sliding sleeve A the string is simply pressurized to open sleeve A to treat the furthest zone from the surface. Sleeve A can be a pressure to open design. When that zone is done a plug is pumped down into sleeve B and that effectively isolates the zone just treated through sliding sleeve A. This plug has a pattern on its fingers to register only with sleeve B. Pressure is built up again and sleeve B opens and treatment of the zone through open sleeve B takes place. When that treatment is done, another plug specially configured to register only with sleeve C is pumped down. Pressure is again built up and the zone is treated through open sliding sleeve C. While that is going on the plug in sleeve B is isolated by virtue of the plug above it and it starts to warm to well temperature and the material  44  in that plug disappears. When pumping is stopped against the plug in sliding sleeve C, it too warms up and the material  44  in it disappears. What are then left are the open passages in the two plugs  18  with all sleeves open and the need to go in and drill out is not there. The treated formation can simply be produced. Should it be desired, the plugs could be fished out using necks  20 . 
   While a procedure with 3 sleeves A, B and C has been described those skilled in the art will understand any number of sleeves that have external isolation devices can be used. The only difference among the sleeves is the profile on them is unique to each and the plugs pumped down have matching profiles to properly land in the sleeves in the desired sequence. In the preferred bottom up sequence each successive plug isolates an already treated zone while the material  44  in that now isolated plug just disappears. What's left is a fully treated interval and a fully open passage to the entire treated interval with no need to drill or mill ball seats as in the past. In the preferred embodiment the sleeves that span the zone can all have similar internal diameters and the unique patterns that register between a plug and a sleeve will ensure that similarly dimensioned plugs wind up at the right sleeve. After it is all done each plug now with its material  44  disappeared presents a consistent flow path  18  to the entire treated interval. 
   In an optional variation, instead of using the material  44  an easily milled disc can be provided. While this way will require subsequent intervention after all the plugs are in place, the milling should go quickly if only the discs themselves are milled out and not the plugs that retain them. Thereafter, with the passage in each plug open, production can flow through them all. Any remnants from milling can be brought to the surface with this production. 
   While the embodiment in  FIGS. 1-5  registered with a given sleeve, the embodiment in  FIGS. 6-8  registers with grooves  50  and  52  in the housing  54 . The sliding sleeve  56  initially covers ports  58  as seals  60  and  62  straddle the ports  58 . Projection  68  initially registers with depression  64  to hold the sleeve  56  in the  FIG. 6  closed position. Eventually when lower end  70  of sleeve  56  hits shoulder  72 , the projection  68  will register with depression  66  as shown in  FIG. 8 .  FIG. 7  shows a plug  74  that has projections  76  and  78  to match depressions  50  and  52  fully registered. Since material  80  is intact and closes passage  82 , and seal  84  contacts sleeve  56  any applied pressure on plug  74  now moves sleeve  56  because sleeve  56  is now turned into a piston. The final position of sleeve  56  is shown in  FIG. 8  with ports  58  open. 
   In this embodiment a given plug has a unique profile or pattern than is matched in the housing adjacent to a sleeve as opposed to literally on the sleeve in the case of  FIGS. 1-5  to be sure a plug lands adjacent a desired sleeve to turn it into a piston so that pressure above it can force it to shift to open the associated ports. Again the plug uses a disappearing material  80  that goes away after it is isolated by another plug latched above it. As in the case of the procedure described above for  FIGS. 1-5  the  FIGS. 6-8  procedure is similar with the main difference being that in  FIGS. 1-5  the plug literally moves the sleeve and in  FIGS. 6-8  the latched plug allows pressure to force the sleeve open in a piston effect. In other respects the procedure is similar. 
     FIGS. 9 and 10  illustrate an embodiment for the disappearing material plug  44  or  80  illustrated in use in  FIGS. 1-8 . Since the material needs some structural strength to withstand differential pressure during pumping procedures like a frac job, the design features alternating layers of a biopolymer  86  alternating with water soluble metal discs  88 . In the assembly, the discs  88  are all internal. The biopolymer  86  has a relatively slow dissolving rate coupled with poor creep resistance. The discs  88  are fast dissolving but add strength and creep resistance. A retaining sleeve  90  engages thread  92  on housing  94  to compress the assembly within passage  96  for run in. Longitudinal compression creates a better peripheral seal in housing  94 . 
     FIG. 11  represents another construction for such a plug as an alternative to the one illustrated in  FIGS. 9 and 10 . Here the end components  98  and  100  are preferably a biopolymer with a relatively slow dissolving rate and poor creep resistance. Sandwiched in between is a granular substance such as, for example, sand, frac proppant or glass micro spheres  102 . When a directional load is placed on either end component  98  or  100  the applied stress is transferred to the layer  102  and due to shifting of the granular material the load is shifted outward against ring  104  that is secured to the housing  106  at thread  108  before it can migrate to the opposite end component. This helps to retain the sealing integrity of the assembly. As before in  FIGS. 9 and 10 , the ring  104  is used to initially longitudinally squeeze the assembly for better sealing. After exposure to well temperatures for a long enough period, the end components dissolve and production can be used to deliver the granular substance to the surface. 
   While two specific embodiments have been described as a unique way to block a passage in a plug that disappears, those skilled in the art will appreciate that independent of the specific execution of the disappearing member the invention encompasses the use of other assemblies that disappear by a variety of mechanisms apart from dissolving when used in the contexts that here described in the application and covered in the claims. 
   Referring now to  FIG. 16  another optional feature of a plug  110  is illustrated. Here there is a leading section  112  that has one or more projections  114  that are designed to enter a matching depression  116  seen in section in  FIG. 15 . Although not shown, those skilled in the art will appreciate that alignment ramps to interact between a plug  110  and the surrounding housing  118  to get the projection  114  to properly align with a depression  116  can be used. However, since the projection is on a flexible finger  120  and the purpose of the registration of parts is to prevent rotation if the plug is to be milled out for any reason, alignment device will not be necessary because some rotation induced from milling will result in registration of  114  with  116  as long as they are supported at the same elevation from the registration of projections  122  and  124  above. 
     FIGS. 12-14  show the plug illustrated in  FIG. 16  (where the disappearing material is not shown in passage  126 ) used to shift a sleeve and then get off the sleeve and latch to a body just below the sleeve. In  FIG. 12   b  projection  128  is just below the bottom of sleeve  130  while projection  132  has engaged a radial surface  134  on the sleeve  130 .  FIG. 12   c  shows the offset at this time between the torque resisting projection  114 ′ and the receiving recess  116 ′. In  FIG. 12  the sleeve  130  has not been shifted. Moving on to  FIG. 13   b  the sleeve  130  is now shifted to travel stop  136  with plug  138  still engaged at radial surface  134  of sleeve  130 . In  FIG. 14   b  the fully shifted sleeve  130  is no longer engaged by the pumped plug  138 . Instead, projections  128  and  132  are now registered with recesses  140  and  142  while torque resisting projection  114 ′ is registered with recess  116 ′. Those skilled in the art will realize that the torque resistance feature is optional and that it can be used regardless of whether the pumped plug  138  remains connected to the sleeve  130  after shifting it or, as shown in  FIGS. 12-14  leaves the sleeve  130  to register with housing  144 . 
   It is worthy of mention again that all types of ways to obtain a unique registering location between a given plug and a given sleeve or a given downhole location are part of the invention. While projections and depressions have been used as an example with either member capable of having one or the other, other combinations that result in registrations of selected pump down plugs at different locations are within the scope of the invention. The sleeves or landing locations can be all the same diameter but what makes them unique is the ability to register with a specific plug that has a profile that registers with it. 
   Yet another aspect of the present invention is to use progressively larger seats as described in U.S. Pat. No. 6,907,936 except to make the obstructing members of a disappearing material so that when all zones are treated, all the seats are reopened. While this embodiment has the disadvantage that without milling there are well obstructions that vary in size, it does retain an advantage over the method in the aforementioned patent in that production can begin without milling out balls on seats. 
   In another technique, a plurality of nipple profiles that are unique can be placed in a casing string. A pump down plug that supports a perforating gun can be delivered to register with a particular nipple profile whereupon registering at the proper location pressure above the now supported plug can fire the gun. In that manner an interval can be perforated in a specific order and intervals already perforated can be isolated as other portions of the interval are perforated. 
   In another embodiment the sliding sleeves that have explosive charges to open access to the formation as described in U.S. Pat. No. 5,660,232 can be selectively operated with the pump down plugs described above that register with a discrete sleeve to open access to the formation in a desired order. The technique can also be grafted to the sliding sleeves used in combination with telescoping pistons as described in U.S. Pat. No. 5,425,424 to selectively shift them in a desired order using the techniques described above. 
   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.