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This application is a continuation of application Ser. No. 10/004,956, filed Dec. 5, 2001, now U.S. Pat. No. 6,722,440, which claims the benefit of U.S. Provisional Application Ser. No. 60/251,293, filed Dec. 5, 2000. U.S. Pat. No. 6,722,440 is also a continuation-in-part of U.S. application Ser. No. 09/378,384, filed on Aug. 20, 1999, now U.S. Pat. No. 6,347,949, which claims the benefit of U.S. Provisional Application Ser. No. 60/097,449, filed on Aug. 21, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to the field of well completion assemblies for use in a wellbore. More particularly, the invention provides a method and apparatus for completing and producing from multiple mineral production zones, independently or in any combination. 
     The need to drain multiple-zone reservoirs with marginal economics using a single well bore has driven new downhole tool technology. While many reservoirs have excellent production potential, they cannot support the economic burden of an expensive deepwater infrastructure. Operators needed to drill, complete and tieback subsea completions to central production facilities and remotely monitor, produce and manage the drainage of multiple horizons. This requires rig mobilization (with its associated costs running into millions of dollars) to shut off or prepare to produce additional zones from the central production facility. 
     Another problem with existing technology is its inability to complete two or more zones in a single well while addressing fluid loss control to the upper zone when running the well completion hardware. In the past, expensive and often undependable chemical fluid loss pills were spotted to control fluid losses into the reservoir after perforating and/or sand control treatments. A concern with this method when completing upper zones is the inability to effectively remove these pills, negatively affecting the formation and production potential and reducing production efficiency. Still another problem is economically completing and producing from different production zones at different stages in a process, and in differing combinations. The existing technology dictates an inflexible order of process steps for completion and production. 
     Prior systems required the use of a service string, wire line, coil tubing, or other implement to control the configuration of isolation valves. Utilization of such systems involves positioning of tools down-hole. Certain disadvantages have been identified with the systems of the prior art. For example, prior conventional isolation systems have had to be installed after the gravel pack, thus requiring greater time and extra trips to install the isolation assemblies. Also, prior systems have involved the use of fluid loss control pills after gravel pack installation, and have required the use of through-tubing perforation or mechanical opening of a wire-line sliding sleeve to access alternate or primary producing zones. In addition, the installation of prior systems within the wellbore require more time consuming methods with less flexibility and reliability than a system which is installed at the surface. Each trip into the wellbore adds additional expense to the well owner and increases the possibility that tools may become lost in the wellbore requiring still further operations for their retrieval. 
     While pressure actuated valves have been used in certain situations, disadvantages have been identified with such devices. For example, prior pressure actuated valves had only a closed position and an open position. Thus, systems could not reliably use more than one such valve, since the pressure differential utilized to shift the first valve from the closed position to the open would be lost once the first valve was opened. Therefore, there could be no assurance all valves in a system would open. 
     There has therefore remained a need for an isolation system for well control purposes and for wellbore fluid loss control, which combines simplicity, reliability, safety and economy, while also affording flexibility in use. 
     SUMMARY OF THE INVENTION 
     The present invention provides a system which allows an operator to, perforate, complete, and produce multiple production zones from a single well in a variety of ways allowing flexibility in the order of operation. An isolation system of the present invention does not require tools to shift the valve and allows the use of multiple pressure actuated valves in a production assembly. 
     According to one aspect of the invention, after a zone is completed, total mechanical fluid loss is maintained and the pressure-actuated circulating (PAC) and/or pressure-actuated device (PAD) valves are opened with pressure from the surface when ready for production. This eliminates the need to rely on damaging and sometimes non-reliable fluid loss pills being spotted in order to control fluid loss after the frac or gravel pack on an upper zone (during the extended time process of installing completion production hardware). 
     According to another aspect of the present invention, the economical and reliable exploitation of deepwater production horizons that were previously not feasible are within operational limits of a system of the invention. 
     A further aspect of the invention provides an isolation sleeve assembly which may be installed inside a production screen and thereafter controlled by generating a pressure differential between the valve interior and exterior. 
     According to a still another aspect of the invention, there is provided a string for completing a well, the string comprising: a base pipe comprising a hole; at least one packer in mechanical communication with the base pipe; at least one screen in mechanical communication with the base pipe, wherein the at least one screen is proximate the hole in the base pipe; an isolation pipe concentric within the base pipe and proximate to the hole in the base pipe, wherein an annulus is defined between the base pipe and the isolation pipe, and an annulus-to-annulus valve in mechanical communication with the base pipe and the isolation pipe. 
     Another aspect of the invention provides a system for completing a well, the system comprising: a first string comprising: a first base pipe comprising a hole, at least one first packer in mechanical communication with the first base pipe, at least one first screen in mechanical communication with the first base pipe, wherein the at least one first screen is proximate the hole in the first base pipe, a first isolation pipe concentric within the first base pipe and proximate to the hole in the first base pipe, wherein a first annulus is defined between the first base pipe and the first isolation pipe, and a first annulus-to-annulus valve in mechanical communication with the first base pipe and the first isolation pipe; and a second string which is stingable into the first string, the second string comprising: a second base pipe comprising a hole, at least one second screen in mechanical communication with the second base pipe, wherein the at least one second screen is proximate the hole in the second base pipe, a second isolation pipe concentric within the second base pipe and proximate to the hole in the second base pipe, wherein a second annulus is defined between the second base pipe and the second isolation pipe, and a second annulus-to-annulus valve in mechanical communication with the second base pipe and the second isolation pipe. 
     According to an aspect of the invention, there is provided a system for completing a well, the system comprising: a first string comprising: a first base pipe comprising a hole, at least one first packer in mechanical communication with the first base pipe, at least one first screen in mechanical communication with the first base pipe, wherein the at least one first screen is proximate the hole in the first base pipe, a first isolation pipe concentric within the first base pipe and proximate to the hole in the first base pipe, wherein a first annulus is defined between the first base pipe and the first isolation pipe, and a first annulus-to-annulus valve in mechanical communication with the first base pipe and the first isolation pipe; and a second string which is stingable into the first string, the second string comprising: a second base pipe comprising a hole, at least one second screen in mechanical communication with the second base pipe, wherein the at least one second screen is proximate the hole in the second base pipe, a second isolation pipe concentric within the second base pipe and proximate to the hole in the second base pipe, wherein a second annulus is defined between the second base pipe and the second isolation pipe, and a second annulus-to-annulus valve in mechanical communication with the second base pipe and the second isolation pipe; and a third string which is stingable into the second string, the third string comprising: a third base pipe comprising a hole, at least one third screen in mechanical communication with the third base pipe, wherein the at least one third screen is proximate the hole in the third base pipe, a third isolation pipe concentric within the third base pipe and proximate to the hole in the third base pipe, wherein a third annulus is defined between the third base pipe and the third isolation pipe, and a third annulus-to-annulus valve in mechanical communication with the third base pipe and the third isolation pipe. 
     According to a further aspect of the invention, there is provided a method for completing multiple zones, the method comprising: setting a first string in a well proximate a first production zone, wherein the first string comprises: a first base pipe comprising a hole, at least one first packer in mechanical communication with the first base pipe, at least one first screen in mechanical communication with the first base pipe, wherein the at least one first screen is proximate the hole in the first base pipe, a first isolation pipe concentric within the first base pipe and proximate to the hole in the first base pipe, wherein a first annulus is defined between the first base pipe and the first isolation pipe, and a first annulus-to-annulus valve in mechanical communication with the first base pipe and the first isolation pipe: performing at least one completion operation through the first string, isolating the first production zone with the first string; and producing fluids from the first production zone. 
     According to a further aspect of the invention, there is provided a method for completing multiple zones, the method comprising: setting a first string in a well proximate a first production zone, wherein the first string comprises: a first base pipe comprising a hole, at least one first packer in mechanical communication with the first base pipe, at least one first screen in mechanical communication with the first base pipe, wherein the at least one first screen is proximate the hole in the first base pipe, a first isolation pipe concentric within the first base pipe and proximate to the hole in the first base pipe, wherein a first annulus is defined between the first base pipe and the first isolation pipe, and a first annulus-to-annulus valve in mechanical communication with the first base pipe and the first isolation pipe; performing at least one completion operation through the first string; isolating the first production zone with the first string; and producing fluids from the first production zone; stinging a second string into the first string and setting the second string proximate a second production zone, wherein the second string comprises: a second base pipe comprising a hole, at least one second screen in mechanical communication with the second base pipe, wherein the at least one second screen is proximate the hole in the second base pipe, a second isolation pipe concentric within the second base pipe and proximate to the hole in the second base pipe, wherein a second annulus is defined between the second base pipe and the second isolation pipe, and a second annulus-to-annulus valve in mechanical communication with the second base pipe and the second isolation pipe; performing at least one completion operation through the second string; and producing fluids from the second production zone through the second string. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention is better understood by reading the following description of non-limitative embodiments with reference to the attached drawings wherein like parts in each of the several figures are identified by the same reference characters, and which are briefly described as follows. 
         FIGS. 1A through 1I  illustrate a cross-sectional, side view of first and second isolation strings. 
         FIGS. 2A through 2L  illustrate a cross-sectional, side view of first, second and third isolation strings, wherein the first and second strings co-mingle production fluids. 
         FIGS. 3A through 3K  illustrate a cross-sectional, side view of first, second and third isolation strings, wherein the second and third strings co-mingle production fluids. 
         FIGS. 4A through 4N  illustrate a cross-sectional, side view of first, second, third and fourth isolation strings, wherein the first and second strings co-mingle production fluids and the third and fourth strings co-mingle production fluids. 
         FIGS. 5A through 5B  are a cross-sectional side view of a pressure actuated device (PAD) valve shown in an open configuration. 
         FIGS. 6A through 6E  are a cross-sectional side view of the PAD valve of  FIG. 5A through 5F , shown in a closed configuration so as to restrict flow through the annulus. 
         FIGS. 7A through 7D  are a side, partial cross-sectional, diagrammatic view of a pressure actuated circulating (PAC) valve assembly in a locked-closed configuration. It will be understood that the cross-sectional view of the other half of the production tubing assembly is a mirror image taken along the longitudinal axis. 
         FIGS. 8A through 8D  illustrate the isolation system of  FIG. 7  in an unlocked-closed configuration. 
         FIGS. 9A through 9D  illustrate the isolation system of  FIG. 8  in an open configuration. 
         FIG. 10  is a cross-sectional, diagrammatic view taken along line A—A of  FIG. 9C  showing the full assembly. 
         FIGS. 11A through 11D  illustrate a cross-sectional side view of a first isolation spring. 
         FIGS. 12A through 12I  illustrate a cross-sectional side view of a second isolation string stung into the first isolation string shown in FIG.  11 . 
         FIGS. 13A through 13L  illustrate a cross-sectional side view of a third isolation string stung into the second isolation string shown in  FIG. 12 , wherein the first isolation string is also shown. 
         FIGS. 14A through 14L  illustrate a cross-sectional side view of the first, second and third isolation strings shown in  FIGS. 11 through 13 , wherein a production string is stung into the third isolation string. 
       It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, as the invention may admit to other equally effective embodiments. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiment illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated device, and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur to one skilled in the art to which the invention relates. 
     Referring to  FIGS. 1A through 1I , there is shown a system for production over two separate zones. A first isolation string  11  is placed adjacent the first production zone  1 . A second isolation string  22  extends across the second production zone  2 . The first isolation string  11  enables gravel pack, fracture and isolation procedures to be performed on the first production zone  1  before the second isolation string  22  is placed in the well. After the first production zone  1  is isolated, the second isolation string  22  is stung into the first isolation string  11 . Without running any tools on wire line or coil tubing to manipulate any of the valves, the second isolation string  22  enables gravel pack, fracture and isolation of the second production zone  2 . The first and second isolation strings  11  and  22  operate together to allow simultaneous production of zones  1  and  2  without co-mingling the production fluids. The first production zone  1  produces fluid through the interior of the production pipe or tubing  5  while the second production zone  2  produces fluid through the annulus between the production tubing  5  and the well casing (not shown). 
     The first isolation string  11  comprises a production screen  15  which is concentric about a base pipe  16 . At the lower end of the base pipe  16  there is a lower packer  10  for engaging the first isolation string  11  in the well casing (not shown). Within the base pipe  16 , there is a isolation or wash pipe  17  which has an isolation valve  18  therein. A pressure-actuated device (PAD) valve  12  is attached to the tops of both the base pipe  16  and the isolation pipe  17 . The PAD valve  12  allows fluid communication through the annuluses above and below the PAD valve. A pressure-actuated circulating (PAC) valve  13  is connected to the top of the PAD valve  12 . The PAC valve allows fluid communication between the annulus and the center of the string. Further, an upper packer  19  is attached to the exterior of the PAD valve  12  through a further section of base pipe  16 . This section of base pipe  16  has a cross-over valve  21  which is used to communicate fluid between the inside and outside of the base pipe  16  during completion operations. 
     Once the first isolation string  11  is set in the well casing (not shown) by engaging the upper and lower packers  19  and  10 , fracture and gravel pack operations are conducted or may be conducted on the first production zone. To perform a gravel pack operation, a production tube (not shown) is stung into the top of a sub  14  attached to the top of the PAC valve  13 . Upon completion of the gravel pack operation, the isolation valve  18  and the PAD valve  12  are closed to isolate the first production zone  1 . The tubing is then withdrawn from the sub  14 . The second isolation string  22  is then stung into the first isolation string  11 . The second isolation string comprises a isolation pipe  27  which stings all the way into the sub  14  of the first isolation string  11 . The second isolation string  22  also comprises a base pipe  26  which stings into the upper packer  19  of the first isolation string  11 . The second isolation string  22  also comprises a production screen  25  which is concentric about the base pipe  26 . A PAD valve  23  is connected to the tops of the base pipe  26  and isolation pipe  27 . The isolation pipe  27  also comprises isolation valve  28 . Attached to the top of the PAD valve  23  is a sub  30  and an upper packer  29  which is connected through a section of pipe. Production tubing  5  is shown stung into the sub  30 . The section of base pipe  26  between the packer  29  and the PAD valve  23  also comprises a cross-over valve  31 . 
     Since the second isolation string  22  stings into the upper packer  19  of the first isolation string  11 , it has no need for a lower packer. Further, since the first isolation string  11  has been gravel packed and isolated, the second production zone  2  may be fractured and gravel packed independent of the first production zone  1 . As soon as the completion procedures are terminated, the isolation valves  28  and the PAD valve  23  are closed to isolate the second production zone  2 . 
     The production tubing  5  is then stung into the sub  30  for production from either or both of zones  1  or  2 . For example, production from zone  1  may be accomplished simply by opening isolation valve  18  and allowing production fluid from zone  1  to flow through the center of the system up through the inside of production tubing  5 . Alternatively, production from only zone  2  may be accomplished by opening isolation valve  28  to similarly allow production fluids from zone  2  to flow up through the inside of production tubing  5 . 
     Non-commingled simultaneous production is accomplished by closing isolation valve  18  and opening PAD valve  12  and PAC valve  13  to allow zone  1  production fluids to flow to the inside of the system and up through the center of production tubing  5 . At the same time, PAD valve  23  may be opened to allow production fluids from zone  2  to flow through the annulus between production tubing  5  and the casing. 
     The first isolation string  11  comprises a PAD valve  12  and a PAC valve  13 . The second isolation string  22  comprises a PAD valve  23  but does not comprise a PAC valve. PAD valves enable fluid production through the annulus formed on the outside of a production tube. PAC valves enable fluid production through the interior of a production tube. These valves are discussed in greater detail below. 
     Referring to  FIGS. 2A through 2L , an isolation system is shown comprising three separate isolation strings. In this embodiment of the invention, the first production string  11  comprises a lower packer  10  and a base pipe  16  which is connected to the lower packer  10 . A production screen  15  is concentric about the base pipe  16 . A isolation pipe  17  extends through the interior of the base pipe and has an isolation valve  18  thereon. The PAD valve  12  of the first isolation string is attached to the tops of the base pipe  16  and isolation pipe  17 . In this embodiment of the invention, a sub  14  is attached to the top of the PAD valve  12 . The first isolation string  11  also comprises an upper packer  19  which is connected to the top of the PAD valve  12  through a length of base pipe  16 . The length of base pipe  16  has therein a cross-over valve  21 . 
     The second isolation string  22  is stung into the first isolation string  11  and comprises a base pipe  26  with a production screen  25  therearound. Within the base pipe  26 , there is a isolation pipe  27  which is stung into the sub  14  of the first isolation string  11 . The isolation pipe  27  comprises isolation valve  28 . Further, the base pipe  26  is stung into the packer  19  of the first isolation string  11 . The second isolation string  22  comprises a PAD valve  23  which is attached to the tops of the base pipe  26  and isolation pipe  27 . A PAC valve  24  is attached to the top of the PAD valve  23 . Further, a sub  30  is attached to the top of the PAC valve  24 . An upper packer  29  is attached to the top of the PAD valve  23  through a section of base pipe  26  which further comprises a cross-over valve  31 . 
     The third isolation string  32  is stung into the top of the second isolation string  22 . The third isolation string  32  comprises a base pipe  36  with a production screen  35  thereon. Within the base pipe  36 , there is a isolation pipe  37  which has an isolation valve  38  therein. Attached to the tops of the base pipe  36  and isolation pipe  37 , there is a PAD valve  33 . A sub  40  is attached to the top of the PAD valve on the interior, and a packer  39  is attached to the exterior of the PAD valve  33  through a section of base pipe  36 . A production tubing  5  is stung into the sub  40 . 
     The first isolation string  11  comprises a PAD valve  12  but does not comprise a PAC valve. The second isolation string  22  comprises both a PAD valve  23  and a PAC valve  24 . The third isolation string  32  only comprises a PAD valve  33  but does not comprise a PAC valve. This production system enables sequential grave pack, fracture and isolation of zones  1 ,  2  and  3 . Also, this system enables fluid from production zones  1  and  2  to be co-mingled and produced through the interior of the production tubing, while the fluid from the third production zone is produced through the annulus around the exterior of the production tube. 
     The co-mingling of fluids produced by the first and second production zones is effected as follows: PAD valves  12  and  23  are opened to cause the first and second production zone fluids to flow through the productions screens  15  and  25  and into the annulus between the base pipes  16  and  26  and the isolation pipes  17  and  27 . This co-mingled fluid flows up through the opened PAD valves  12  and  23  to the bottom of the PAC valve  24  is also opened to allow this co-mingled fluid of the first and second production zones  1  and  2  to flow from the annulus into the center of the base pipes  16  and  26  and the sub  30 . All fluid produced by the first and second production zones through the annulus is forced into the production tube  5  interior through the open PAC valve  24 . 
     Production from the third production zone  3  is effected by opening PAD valve  33 . This allows production fluids to flow up through the annulus between the base pipe  36  and the isolation pipe  37 , up through the PAD valve  33  and into the annulus between the production tube  5  and the well casing (not shown). 
     Referring to  FIGS. 3A through 3K , a system is shown wherein a first isolation string  11  comprises a PAD valve  12  and a PAC valve  13 . This first isolation string  11  is similar to that previously described with reference to FIG.  1 . The second isolation string  22  comprises only a PAD valve  23  and is similar to the second isolation string described with reference to FIG.  1 . The third isolation string  32  comprises only a PAD valve  33  but no PAC valve and is also similar to the second isolation string described with reference to FIG.  1 . This configuration enables production from zone  1  to pass through the PAC valve into the interior of the annulus of the production tubing. The fluids from production zones two and three co-mingle and are produced through the annulus about the exterior of the production tube. 
     The co-mingling of fluids produced by the second and third production zones is effected as follows: Opening PAD valves  23  and  33  creates an unimpeded section of the annulus. Fluids produced through PAD valves  23  and  33  are co-mingled in the annulus. 
     Referring to  FIGS. 4A through 4N , a system is shown comprising four isolation strings. The first isolation string  11  comprises a PAD valve  12  but no PAC valve. The second isolation string  22  comprises a PAD valve  23  and a PAC valve  24 . The third isolation string  32  comprises a PAD valve  33  but does not comprise a PAC valve. Similarly the fourth isolation string  42  comprises a PAD valve  43  but does not comprise a PAC valve. In this particular configuration, production fluids from zones one and two are co-mingled for production through the PAC valve into the interior of the production tube  5 . The fluids from production zones three and four are co-mingled for production through the annulus formed on the outside of the production tube  5 . 
     In this embodiment, the first isolation string  11  is similar to the first isolation string shown in FIG.  2 . The second isolation string  22  is also similar to the second isolation string shown in FIG.  2 . The third isolation string is also similar to the third isolation string shown in FIG.  2 . However, rather than having a production tubing  5  stung into the top of the third isolation string, the embodiment shown in  FIG. 4 , comprises a fourth isolation string  42 . The fourth isolation string comprises a base pipe  46  with a production screen  45  therearound. On the inside of the base pipe  46 , there is a isolation pipe  47  which has an isolation valve  48 . Attached to the tops of the base pipe  46  and the isolation pipe  47 , there is a PAD valve  43 . To the interior of the top of the PAD valve  43 , there is attached a sub  50 . To the exterior of the PAD valve  43 , there is attached through a section of base pipe  46 , an upper packer  49 , wherein the section of base pipe  46  comprises a cross-over valve  51 . A production tubing  5  is stung into the sub  50 . 
     Referring to  FIGS. 5A through 5E  and  6 A through  6 E, detailed drawings of a PAD valve are shown. In  FIG. 5 , the valve is shown in an open position and in  FIG. 6 , the valve is shown in a closed position. In the open position, the valve enables fluid communication through the annulus between the interior and exterior tube of the isolation string. Essentially, these interior and exterior tubes are sections of the base pipe  16  and the isolation pipe  17 . The PAD valve comprises a shoulder  52  that juts into the annulus between two sealing lands  58 . The shoulder  52  is separated from each of the sealing lands  58  by relatively larger diameter troughs  60 . The internal diameters of the shoulder  52  and the sealing lands  58  are about the same. A moveable joint  54  is internally concentric to the shoulder  52  and the sealing land  58 . The moveable joint  54  has a spanning section  62  and a closure section  64 , wherein the outside diameter of the spanning section  62  is less than the outside diameter of the closure section  64 . 
     The valve is in a closed position, when the valve is inserted in the well. The PAD valve is held in the closed position by a shear pin  55 . A certain change in fluid pressure in the annulus will cause the moveable joint  54  to shift, opening the PAD valve by losing the contact between the joint  54  and the shoulder  52 . Since the relative diameters of the spanning section  62  and closure section  64  are different, the annulus pressure acts on the moveable joint  54  to slide the moveable joint  54  to a position where the spanning section  62  is immediately adjacent the shoulder  52 . Since the outside diameter of the spanning section  62  is less than the inside diameter of the shoulder  52 , fluid flows freely around the shoulder  52  and through the PAD valve. 
     As shown in  FIG. 6 , in the closed position, the PAD valve restricts flow through the annulus. Here, the PAD valve has contact between the shoulder  52  and the moveable joint  54 , forming a seal to block fluid flow through the annulus at the PAD valve. 
     Referring to  FIGS. 7A through 7D , there is shown a production tubing assembly  110  according to the present invention. The production tubing assembly  110  is mated in a conventional manner and will only be briefly described herein. Assembly  110  includes production pipe  140  that extends to the surface and a production screen assembly  112  with PAC valve assembly  108  controlling fluid flow through the screen assembly. In a preferred embodiment production screen assembly  112  is mounted on the exterior of PAC valve assembly  108 . PAC valve assembly  108  is interconnected with production tubing  140  at the uphole end by threaded connection  138  and seal  136 . Similarly on the downhole end  169 . PAC valve assembly  108  is interconnected with production tubing extension  113  by threaded connection  122  and seal  124 . In the views shown, the production tubing assembly  110  is disposed in well casing  111  and has inner tubing  114 , with an internal bore  115 , extending through the inner bore  146  of the assembly. 
     The production tubing assembly  110  illustrates a single preferred embodiment of the invention. However, it is contemplated that the PAC valve assembly according to the present invention may have uses other than at a production zone and may be mated in combination with a wide variety of elements as understood by a person skilled in the art. Further, while only a single isolation valve assembly is shown, it is contemplated that a plurality of such valves may be placed within the production screen depending on the length of the producing formation and the amount of redundancy desired. Moreover, although an isolation screen is disclosed in the preferred embodiment, it is contemplated that the screen may include any of a variety of external or internal filtering mechanisms including but not limited to screens, sintered filters, and slotted liners. Alternatively, the isolation valve assembly may be placed without any filtering mechanisms. 
     Referring now more particularly to PAC valve assembly  108 , there is shown outer sleeve upper portion  118  joined with an outer sleeve lower portion  116  by threaded connection  128 . For the purpose of clarity in the drawings, these openings have been shown at a 45° inclination. Outer sleeve upper portion  118  includes two relatively large production openings  160  and  162  for the flow of fluid from the formation when the valve is in an open configuration. Outer sleeve upper portion  118  also includes through bores  148  and  150 . Disposed within bore  150  is shear pin  151 , described further below. The outer sleeve assembly has an outer surface and an internal surface. On the internal surface, the outer sleeve upper portion  118  defines a shoulder  188  ( FIG. 7C ) and an area of reduced wall thickness extending to threaded connection  128  resulting in an increased internal diameter between shoulder  188  and connection  128 . Outer sleeve lower portion  116  further defines internal shoulder  189  and an area of reduced internal wall thickness extending between shoulder  189  and threaded connection  122 . Adjacent threaded connection  138 , outer sleeve portion  118  defines an annular groove  176  adapted to receive a locking ring  168 . 
     Disposed within the outer sleeves is inner sleeve  120 . Inner sleeve  120  includes production openings  156  and  158  which are sized and spaced to correspond to production openings  160  and  162 , respectively, in the outer sleeve when the valve is in an open configuration. Inner sleeve  120  further includes relief bores  154  and  142 . On the outer surface of inner sleeve there is defined a projection defining shoulder  186  and a further projection  152 . Further inner sleeve  120  includes a portion  121  having a reduced external wall thickness. Portion  121  extends down hole and slidably engages production pipe extension  113 . Adjacent uphole end  167 , inner sleeve  120  includes an area of reduced external diameter  174  defining a shoulder  172 . 
     In the assembled condition shown in  FIGS. 7A through 7D , inner sleeve  120  is disposed within outer sleeves  116  and  118 , and sealed thereto at various locations. Specifically, on either side of production openings  160  and  162 , scals  132  and  134  seal the inner and outer sleeves. Similarly, on either side of shear pin  151 , seals  126  and  130  seal the inner sleeve and outer sleeve. The outer sleeves and inner sleeve combine to form a first chamber  155  defined by shoulder  188  of outer sleeve  118  and by shoulder  186  of the inner sleeve. A second chamber  143  is defined by outer sleeve  116  and inner sleeve  120 . A spring member  180  is disposed within second chamber  143  and engages production tubing  113  at end  182  and inner sleeve  120  at end  184 . A lock ring  168  is disposed within recess  176  in outer sleeve  118  and retained in the recess by engagement with the exterior of inner sleeve  120 . Lock ring  168  includes a shoulder  170  that extends into the interior of the assembly and engages a corresponding external shoulder  172  on inner sleeve  120  to prevent inner sleeve  120  from being advanced in the direction of arrow  164  beyond lock ring  168  while it is retained in groove  176 . 
     The PAC valve assembly of the present invention has three configurations as shown in  FIGS. 7 through 9 . In a first configuration shown in  FIG. 7 , the production openings  156  and  158  in inner sleeve  120  are axially spaced from production openings  160  and  162  along longitudinal axis  190 . Thus, PAC valve assembly  108  is closed and restricts flow through screen  112  into the interior of the production tubing. The inner sleeve is locked in the closed configuration by a combination of lock ring  168  which prevents movement of inner sleeve  120  up hole in the direction of arrow  164  to the open configuration. Movement down hole is prevented by shear pin  151  extending through bore  150  in the outer sleeve and engaging an annular recess in the inner sleeve. Therefore, in this position the inner sleeve is in a locked closed configuration. 
     In a second configuration shown in  FIGS. 8A through 8D , shear pin  151  has been severed and inner sleeve  120  has been axially displaced down hole in relation to the outer sleeve in the direction of arrow  166  until external shoulder  152  on the inner sleeve engages end  153  of outer sleeve  116 . The production openings of the inner and outer sleeves continue to be axial displaced to prevent fluid flow there through. With the inner sleeve axial displaced down hole, lock ring  168  is disposed adjacent reduced outer diameter portion  174  of inner sleeve  120  such that the lock ring may contract to a reduced diameter configuration. In the reduced diameter configuration shown in  FIG. 8 , lock ring  168  may pass over recess  176  in the outer sleeve without engagement therewith. Therefore, in this configuration, inner sleeve is in an unlocked position. 
     In a third configuration shown in  FIGS. 9A through 9D , inner sleeve  120  is axially displaced along longitudinal axis  190  in the direction of arrow  164  until production openings  156  and  158  of the inner sleeve are in substantial alignment with production openings  160  and  162 , respectively, of the outer sleeve. Axial displacement is stopped by the engagement of external shoulder  186  with internal shoulder  188 . In this configuration, PAC valve assembly  108  is in an open position. 
     In the operation of a preferred embodiment, at least one PAC valve according to the present invention is mated with production screen  112  and, production tubing  113  and  140 , to form production assembly  110 . The production assembly according to  FIG. 7  with the PAC valve in the locked-closed configuration, is then inserted into casing  111  until it is positioned adjacent a production zone (not shown). When access to the production zone is desired, a predetermined pressure differential between the casing annulus  144  and internal annulus  146  is established to shift inner sleeve  120  to the unlocked-closed configuration shown in FIG.  8 . It will be understood that the amount of pressure differential required to shift inner sleeve  120  is a function of the force of spring  180 , the resistance to movement between the inner and outer sleeves, and the shear point of shear pin  151 . Thus, once the spring force and resistance to movement have been overcome, the shear pin determines when the valve will shift. Therefore, the shifting pressure of the valve may be set at the surface by inserting shear pins having different strengths. 
     A pressure differential between the inside and outside of the valve results in a greater amount of pressure being applied on external shoulder  186  of the inner sleeve than is applied on projection  152  by the pressure on the outside of the valve. Thus, the internal pressure acts against shoulder  186  of to urge inner sleeve  120  in the direction of arrow  166  to sever shear pin  151  and move projection  152  into contact with end  153  of outer sleeve  116 . It will be understood that relief bore  148  allows fluid to escape the chamber formed between projection  152  and end  153  as it contracts. In a similar fashion, relief bore  142  allows fluid to escape chamber  143  as it contracts during the shifting operation. After inner sleeve  120  has been shifted downhole, lock ring  168  may contract into the reduced external diameter of inner sleeve positioned adjacent the lock ring. Often, the pressure differential will be maintained for a short period of time at a pressure greater than that expected to cause the down hole shift to ensure that the shift has occurred. This is particularly important where more than one valve according to the present invention is used since once one valve has shifted to an open configuration in a subsequent step, a substantial pressure differential is difficult to establish. 
     The pressure differential is removed, thereby decreasing the force acting on shoulder  186  tending to move inner sleeve  120  down hole. Once this force is reduced or eliminated, spring  180  urges inner sleeve  120  into the open configuration shown in FIG.  9 . Lock ring  168  is in a contracted state and no longer engages recess  176  such the ring now slides along the inner surface of the outer sleeve. In a preferred embodiment spring  180  has approximately 300 pounds of force in the compressed state in FIG.  8 . However, varying amounts of force may be required for different valve configurations. Moreover, alternative sources other than a spring may be used to supply the force for opening. As inner sleeve  120  moves to the open configuration, relief bore  154  allows fluid to escape chamber  155  as it is contracted, while relief bores  148  and  142  allow fluid to enter the connected chambers as they expand. 
     Shown in  FIG. 10  is a cross-sectional, diagrammatic view taken along line A—A of  FIG. 9C  showing the full assembly. 
     Although only a single preferred PAC valve embodiment of the invention has been shown and described in the foregoing description, numerous variations and uses of a PAC valve according to the present invention are contemplated. As examples of such modification, but without limitation, the valve connections to the production tubing may be reversed such that the inner sleeve moves down hole to the open configuration. In this configuration, use of a spring  180  may not be required as the weight of the inner sleeve may be sufficient to move the valve to the open configuration. Further, the inner sleeve may be connected to the production tubing and the outer sleeve may be slidable disposed about the inner sleeve. A further contemplated modification is the use of an internal mechanism to engage a shifting tool to allow tools to manipulate the valve if necessary. In such a configuration, locking ring  168  may be replaced by a moveable lock that could again lock the valve in the closed configuration. Alternatively, spring  180  may be disengageable to prevent automatic reopening of the valve. 
     Further, use of a PAC valve according to the present invention is contemplated in many systems. One such system is the ISO system offered by BJ Services Company U.S.A. (successor to OSCA, Inc.) and described in U.S. Pat. No. 5,609,204; the disclosure therein is hereby incorporated by reference. A tool shiftable valve disclosed in the above patent is a type of isolation valve and may be utilized within the production screens to accomplish the gravel packing operation. Such a valve could be closed as the crossover tool string is removed to isolate the formation. The remaining production valves adjacent the production screen may be pressure actuated valves according to the present invention such that inserting a tool string to open the valves is unnecessary. 
       FIGS. 11 through 14  illustrate several steps in the construction of an isolation and production system according to an embodiment of the present invention. 
       FIGS. 11A through 11D  show a first isolation string  211 . The isolation string comprises a PAD valve  212 . At the lower end of the isolation string  211 , there is a lower packer  210  and at the upper end of the isolation string  211  there is an upper packer  219 . A base pipe  216  is connected to the lower packer  210  and has a production screen  215  therearound. The isolation string  211  further comprises an isolation valve  218  on a isolation pipe  217 . The PAD valve  212  enables fluid communication through the annulus between the isolation pipe  217  and the isolation string  211 . The first isolation string  211  also comprises a sub  214  attached to the top of the PAD valve  212 . Further, in the base pipe section between the PAD valve  212  and the upper packer  219 , there is a cross-over valve  221 . This configuration of the first isolation string  211  enables the first production zone  1  to be fractured, gravel packed, and isolated through the first isolation string  211 . Upon completion of these procedures, the isolation valve  218  and PAD valve  212  are closed to isolate the production zone  1 . 
       FIGS. 12A through 12I  show cross-sectional, side views of two isolation strings. In particular, a second isolation string  222  is stung inside an isolation string  211 . Isolation string  222  comprises a PAD valve  223  and a PAC valve  224 . The isolation string  211 , shown in this figure, is the same as the isolation string shown in FIG.  11 . After the gravel/pack and isolation function are performed on the first zone with the isolation string  211 , the isolation string  222  is stung into the isolation string  211 . The second isolation string  222  comprises a base pipe  226  having a production screen  225  therearound. The base pipe  226  is stung into the packer  219  of the first isolation string  211 . The second isolation string  222  also comprises a isolation pipe  227  which is stung into the sub  214  of the first isolation string  211 . The isolation pipe  227  also comprises an isolation valve  228 . At the tops of the base pipe  226  and isolation pipe  227 , there is connected a PAD valve  223 . A PAC valve  224  is connected to the top of the PAD valve  223 . Also, a sub  230  is attached to the top of the PAC valve  224 . An upper packer  229  is also connected to the exterior portion of the PAD valve  223  through a section of base pipe  226  which also comprises a cross-over valve  231 . 
     Referring to  FIGS. 13A through 13L , the isolation strings  211  and  222  of  FIG. 12  are shown. However, in this figure, a third isolation string  232  is stung into the top of isolation string  222 . In this particular configuration, isolation strings  211  and  222  produce fluid from respective zones  1  and  2  up through the annulus between the isolation strings and the isolation sleeves until the fluid reaches the PAC valve  224 . The co-mingled production fluid from production zones  1  and  2  pass through the PAC valve  224  into the interior of the production string. The production fluids from zone  3  is produced through the isolation string  232  up through the annulus between the isolation string  232  and the isolation pipe  237 . In the embodiment shown in  FIG. 13 , the PAD valves  212 ,  223  and  233  are shown in the closed position so that all three of the production zones are isolated. Further, the PAC valve  224  in isolation string  222  is shown in a closed position. 
     The third isolation string  232  comprises a base pipe  236  which is stung into the packer  229  of the second isolation string. The base pipe  236  also comprises a production screen  235 . Inside the base pipe  236 , there is a isolation pipe  237  which is stung into the sub  230  of the second isolation string  222 . The isolation pipe  237  comprises isolation valve  238 . A PAD valve  233  is connected to the tops of the base pipe  236  and isolation pipe  237 . A sub  234  is connected to the top of the PAD valve  233 . An upper packer  239  is also connected through a section of base pipe  236  to the PAD valve  233 . This section of base pipe also comprises a cross-over valve  241 . 
     Referring to  FIGS. 14A through 14L , the isolation strings  211 ,  222  and  232  of  FIG. 13  are shown. In addition to these isolation strings, a production tube  240  is stung into the top of isolation string  232 . With the production tube  240  stung into the system, pressure differential is used to open PAD valves  212 ,  223 , and  233 . In addition, the pressure differential is used to set PAC valve  224  to an open position. The opening of these valves enables co-mingled production from zones  1  and  2  through the interior of the production tube while production from zone  3  is through the annulus on the outside of the production tube  240 . 
     The packers, productions screens, isolations valves, base pipes, isolations pipes, subs, cross-over valves, and seals may be off-the-shelf components as are well known by persons of skill in the art. 
     While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only the preferred embodiment has been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected.

Summary:
An isolation system for producing oil and gas from one or more formation zones and methods of use are provided comprising one or more pressure activated valve and one or more tool shiftable valve. The tool shiftable valve may be actuated before or after actuation of the pressure activated valve.