You are an expert at summarizing long articles. Proceed to summarize the following text:

You are an expert at summarizing long articles. Proceed to summarize the following text: 
BACKGROUND 
       [0001]    In the downhole drilling and completion industry, there is often need to contain fluid within a formation during various operations. Conventionally, a mechanical barrier is put in the system that can be closed to contain the formation fluid when necessary. One example of a system known in the art will use a valve in operable communication with an Electric Submersible Pump (ESP) so that if/when the ESP is pulled from the downhole environment, formation fluids will be contained by the valve. While such systems are successfully used and have been for decades, in an age of increasing oversight and fail safe/failure tolerant requirements, additional systems will be well received by the art. 
       SUMMARY 
       [0002]    Disclosed herein is a multi-barrier system including a first valve in fluid communication with a lower completion, and a second valve in fluid communication with the lower completion. The first valve and the second valve are positioned proximate an uphole extent of the lower completion, and a packer located proximate the first valve and the second valve is closable in response to retrieving an upper completion. 
         [0003]    Also disclosed herein is a method of redundantly closing a wellbore nonpermanently upon retrieval of an upper completion, including disengaging an upper completion from a lower completion, closing a first valve in response to the disengaging, closing a second valve in response to the disengaging, reengaging an upper completion with the lower completion, opening the first valve, and opening the second valve. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]    Referring now to the drawings wherein like elements are numbered alike in the several Figures: 
           [0005]      FIG. 1  is a schematic view of a stackable multi-barrier system; 
           [0006]      FIG. 2  is a schematic view of the system of  FIG. 1  in partial withdrawal from the borehole; 
           [0007]      FIG. 3  is a schematic view of a new stackable multi-barrier system engaged with the remains of the system illustrated in  FIG. 1 ; and 
           [0008]      FIG. 4  depicts a quarter cross sectional view of a portion of a hydraulically actuated valve employed in the stackable multi-barrier system of  FIGS. 1-3 . 
       
    
    
     DETAILED DESCRIPTION 
       [0009]    Referring to  FIG. 1 , a stackable multi-barrier system  10  is illustrated. Illustrated is a portion of a lower completion  12 , a packer  14  and a portion of an upper completion  16 . One of ordinary skill in the art will be familiar with the lower completion  12  and the packer  14  and the concept of an upper completion  16  in operable communication therewith. In the illustrated embodiment an electric submersible pump (ESP)  18  is included in the upper completion  16 , which is a device well known to the art. Between the illustrated ESP  18  and the lower completion  12  however, one of ordinary skill in the art will be surprised to see a number of mechanical barriers  20 ,  22  (sometimes referred to herein as “valves”) that is greater than one. As illustrated in the figures hereof there are two but nothing in this disclosure should be construed as limiting the number of mechanical barriers to two. Rather more could also be added, if desired. 
         [0010]    In one embodiment the more downhole valve  20  is a hydraulically actuated valve such as an ORBIT™ valve available commercially from Baker Hughes Incorporated, Houston Texas and the more uphole valve  22  is a mechanically actuated valve such as a HALO™ valve available from the same source. It will be appreciated that these particular valves are merely exemplary and may be substituted for by other valves without departing from the invention. 
         [0011]    Control lines  24  are provided to the valve  20  for hydraulic operation thereof. In the illustrated embodiment the lines also have a releasable control line device  28  in line therewith to allow for retrieval of the upper completion  16  apart from the lower completion  12 . Also included in this embodiment of the system  10  is a stroker  30  that may be a hydraulic stroker in some iterations. 
         [0012]    The components described function together to manage flow between the lower completion  12  and the upper completion  16 . This is accomplished in that the valve  20  is settable to an open or closed position (and may be variable in some iterations) based upon hydraulic fluid pressure in the control line  24 . The valve  22  is opened or closed based upon mechanical input generated by movement of the upper completion  16 , or in the case of the illustration in  FIG. 1 , based upon mechanical movement caused by the stroker  30  that is itself powered by hydraulic fluid pressure. Of course, the stroker  30  could be electrically driven or otherwise in other embodiments. In any condition, the valve  22  is configured to close upon withdrawal of the upper completion  16 . In normal production, both of the valves  20  and  22  will remain open unless there is a reason to close them. Such a reason occurs, for example, when it is required to retrieve the upper completion  16  for some reason. One such reason is to replace the ESP  18 . Regardless of the reason for closure, employment of the system  10  in a completion string provides more than one mechanical barrier  20 ,  22  at an uphole extent of the lower completion  12 . The barriers when closed prevent fluid flow after the upper completion is retrieved. 
         [0013]    Attention is directed to releasable control line devices  28  and  FIG. 2 . During a withdrawal of the upper completion  16 , the control lines  24  are subjected to a tensile load. The releasable control line devices will release at a threshold tensile load and seal the portion of the control lines  24  that will remain in the downhole environment as a part of the lower completion string  12 . The valve  20 , if not already closed, is configured to close in response to this release of the control lines  24 . This will complete the separation of the upper completion  16  from the lower completion  12  and allow retrieval of the upper completion  16  to the surface. With more than one mechanical barrier  20 ,  22  in place at the uphole extent of the lower completion  12 , there is improved confidence that fluids will not escape from the lower completion  12 . Important to note here briefly is that the system  10  also includes provision  44  for allowing the reopening of the valve  20  using tubing pressure after the upper completion  16  is reinstalled. This will be addressed further hereunder. 
         [0014]    In order to restore production, another system  110  is attached at a downhole end of upper completion  16  and run in the hole. This is illustrated in  FIG. 3 . The original system  10  has components such as packer  14 , valves  20  and  22  and control lines  24  are seen at the bottom of the drawing and a new system  110  stackable on the last is shown. The new system  110  includes a packer  114  valve  120 , valve  122 , lines  124 , stroker  13 , ESP  118  and releasable hydraulic line device  128 . In essence each of the components of system  10  is duplicated in system  110 . Moreover, it should be understood that the process of pulling out and stabbing in with new systems can go on ad infinitum (or at least until practicality dictates otherwise). 
         [0015]    Since the valves  20  and  22  will be in the closed position, having been intentionally closed upon preparing to retrieve the upper completion  16 , they will need to be opened upon installation of the new system  110 . This is accomplished by stabbing a mechanical shiftdown  142  into valve  22  and setting packer  114 . The mechanical shiftdown  142  mechanically shifts the valve  22  to the open position. It should be pointed out that, in this embodiment, the mechanical shiftdown  142  does not seal to the valve  22  and as such the inside of the upper completion  16  is in fluidic communication with annular space  146  defined between the packers  14  and  114 . Applying pressure to the tubing at this point will result in a pressure buildup that will act on the valve  20  through the string uphole thereof since all valves thereabove,  22 ,  120  and  122  are in the open position. Referring to  FIG. 4 , a view of valve  20  illustrates the provision  44  that includes a port  52  in operable communication with an optional shifter  50 . The shifter  50  is configured to open the port  52  in response to retrieval of the upper completion  16 . As illustrated the shifter  50  in this embodiment is a sleeve that is automatically actuated upon retrieval of the upper completion  16 . More specifically, when upper completion  16  begins to move uphole, the provision  44  is shifted to the open position. When the provision  44  is in the open position tubular fluid pressure is in communication with the port  52 . The port  52  includes an openable member  54  such as a burst disk or similar that when opened provides fluid access to an atmospheric chamber  56 . The member  54  opens upon increased tubing pressure and allows fluid to fill the atmospheric chamber  56 . Fluid in the atmospheric chamber causes one or more pistons  58  to urge the valve  20  to the open position. In one embodiment, ratcheting devices (not shown) may be provided in operable communication with the one or more pistons  58  to prevent the pistons from moving in a direction to allow the valve to close by serendipity at some later time. It may also be that the valve  20  itself is configured to be locked permanently open by other means if the atmospheric chamber floods. 
         [0016]    The foregoing apparatus and method for its use allows for the retrieval and replacement of an upper completion without the need for a wet connection. 
         [0017]    While one or more embodiments have been shown and described, modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustrations and not limitation.

Summary:
A multi-barrier system includes a first valve in fluid communication with a lower completion, and a second valve in fluid communication with the lower completion. The first valve and the second valve are positioned proximate an uphole extent of the lower completion, and a packer located proximate the first valve and the second valve is closable in response to retrieving an upper completion.