An annulus collar around a well production tube is cast in cement by a procedure that axially delineates the collar between two expandable well packers in the production tube string. Between the packers are a pair of cementing valves. An ingress valve is most proximate to the lower packer whereas an egress valve is most proximate to the upper packer. Additionally, the egress valve is modified to enclose the egress valve with a screen having mesh or slot openings that correspond with a screen plugging material that is mixed with the cement.

RELATED APPLICATION 
     This application is related to a U.S. provisional application titled “Positive Indication System for Well Annulus Cement Displacement” filed on Apr. 24, 2001, Ser. No. 60/286,100, and from which priority is claimed for the present application. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the tools and methods for earth boring and deep well completion. In particular, the invention relates to tools, materials and operational methods for placing an annulus of cement around a pipe or tube along a defined length of well bore. 
     2. Description of the Related Art 
     A well annulus is that generally annular space within a wellbore that may be between the raw borehole wall and the outside of a casing pipe suspended within the borehole. The term may also be applied to the annular space between the raw borehole wall and the outside surface of a fluid production tube. The well annulus may also be that annular space between the casing inside surface and the outer surfaces of a pipe or tube that is suspended within the casing. 
     Packers are well completion tools that are used to segregate axially adjacent sections of the well annulus to prevent the transfer of fluids, liquid or gas, from flowing along the length of an annulus from one section to another or migrating from one earth strata to another. More generally, the packer is a structural barrier across an annulus section that usually extends along a short length of the annulus. 
     Characteristically, inflatable packers comprise an elastomer or rubber sleeve element around the outer perimeter of a tubular mandrel. Opposite ends of the elastomer sleeve are secured to the mandrel. The tubular mandrel wall provides structural strength to physically link elements of a tubular work string above and below the packer. Additionally, the open bore along the mandrel center provides working fluid (hydraulic oil, etc.) flow continuity from surface located pumps to other tools below the packer. 
     The opposing ends of a packer sleeve may be overlaid by collar elements. One or both collars may include valve devices to admit pressurized fluid from the mandrel flow bore into the interface between the elastomer sleeve and the outer surface elements of the mandrel. Sufficient pressure within the interface expands the elastomer radially from the mandrel surface out to a tight, pressure seal against the internal walls of the annulus to prevent fluid flow in either direction along the annulus past the packer. 
     A wellbore zone to be produced through the flow bore of a production tube or casing liner is often isolated by an annular collar that is cast in cement around the production tube or casing liner. The cement collar is also cast in intimate contact with the surrounding borehole wall or inside surface of the casing bore. This collar seals the wellbore annulus around the casing liner and also secures the casing liner within the wellbore. 
     A prior art procedure for placement of the uncured collar cement within the well annulus includes placement of form packers in the well annulus above and below the collar segment. For downhole placement, the packers are tool segments of the well casing liner that are secured within the casing liner pipe string at positions of axial separation corresponding to the desired length of the cement collar. Between the packers, the casing liner (or production tube) may also include a pair of selectively opened and closed cement valve elements for providing respective cement flow paths between the flow bore of the casing liner and the surrounding annulus. By means of a cementing tool, a cement flow path between one of the cement valves and the tubular flow bore of the cement tool is isolated. Cement is pumped from the surface, along the cementing tool flow bore, through transverse flow ports in the cement tool, and into the annulus around the casing liner. The other cement valve in the casing liner string receives the material in the collar annulus that is displaced by the uncured cement. This displaced material is received into an inner annulus between the cementing tool and the interior of the casing liner. 
     A raw borehole profile often is irregular. Although the exact dimension of the outside casing liner dimensions are known, the unknown volume within the borehole prevents a precise determination of the annulus volume between the collar packers. Consequently, a considerable excess of cement is pumped into the collar annulus simply to assure that the collar annulus is filled. Any excess cement flows through the second cement valve into the inner annulus between the casing liner interior and the cementing tool exterior. Removal of the cementing tool swabs the casing liner bore of the excess cement. 
     A major difficulty of the foregoing prior art process is the unknown. Notwithstanding delivery of volumetrically excessive cement, there is no certainty that the collar annulus is completely filled. It is therefor, an objective of the present invention to provide equipment and procedures to positively conclude a volumetric filling of a collar annulus. 
     SUMMARY OF THE INVENTION 
     This and other objects of the invention as will become apparent from the following detailed description are obtained by a procedure that includes a shrouding screen over the cement return (ingress) valve. The cement egress valve is positioned along the casing liner or production string, as the case may be, between the pair of collar delineating packers but closely proximate of one. The screen shrouded return valve is also positioned between the packers but closely proximate of the other packer. 
     In cooperation with a liner casing or production tube having a shrouding screen over the cement ingress valve, the cement injected into the collar annulus is blended with a particulate or compatible thixotropic material that is matched to the mesh or slot opening of the shrouding screen. 
     Fluids within the collar annulus that are volumetrically displaced by a pressure driven influx of cement have a traditional drain route through the cement ingress valve and covering screen. However, when the particulate blended cement reaches the screen element over the cement ingress valve, the particulates will not pass through the screen openings. In due time, most of the screen mesh or slot opening will be bridged over by the cement borne particulates. A well working crew at the surface will recognize the condition by an increase in the cement pump discharge pressure as a consequence. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The advantages and further aspects of the invention will be readily appreciated by those of ordinary skill in the art as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference characters designate like or similar elements throughout the several figures of the drawing and wherein: 
     FIG. 1 is a partial section view of a well casing liner suspended within an uncased wellbore. 
     FIG. 2 is a line schematic of the invention in operation. 
     FIG. 3 is a partial section view of a well casing liner suspended within a cement collar. 
     FIG. 4 is a partial section view of a single acting, egress cementing valve. 
     FIG. 5 is a detailed enlargement of the egress cementing valve illustrated by FIG.  4 . 
     FIG. 6 is a partial section view of the double-acting ingress cementing valve. 
     FIG. 7 is a partial section view of the cementing and shifting tool. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A representative application of the invention is illustrated by FIG. 1 to include an open bore hole  10  having a casing liner  12  suspended therein. The casing liner may be a continuous pipe string that is supported at or near the surface, or, alternatively, may be concentrically sleeved within a larger diameter casing and suspended from an intermediate depth. An internal flow bore  13  of the casing liner is accessible at the surface as a conduit for well working fluids or as a mechanical guide channel for other tools and instruments suspended from the surface into and along the casing liner flow bore. Other applications of the invention may include, for example, a production tube within a cased and perforated bore hole. 
     The lower end of the casing liner may include an upper packer  14  and a lower packer  16 . Although fluid inflatable packers are preferred, it should be understood that the term “packer” is merely a convenience reference to any form of selectively engaged annulus barrier that obstructs the continuity of the annulus  18 . The packers  14  and  16  are separated by a distance D corresponding to the desired length of an annulus production collar  20  and linked by a casing liner subsection  22 . The packers  14  and  16  are located, for example, along the length of the borehole  10  in relation to a particular well fluid production zone. 
     Within the casing liner subsection  22 , and preferably adjacent to the lowermost packer  16 , is an egress cementing valve  24  for channeling a discharge flow of uncured, fluidized cement from a cementing tool into the collar annulus  20 . The material described herein as “cement” may also be or include other phase changing materials such as epoxies, polyesters, etc. An ingress cementing valve  26  for the return of fluid and other matter displaced by the cement occupation of the collar  20  annulus volume is preferably provided in the subsection  22  adjacent to the uppermost packer  14 . 
     Although the preferred sequence and order of the cementing valves is to locate the egress valve  24  in the proximity of the lower packer  16  and to locate the ingress valve  26  in the proximity of the uppermost packer  14 , those skilled in the art will understand and appreciate the fact that the sequence and order may be reversed. 
     With respect to FIGS. 4 and 5, the egress cementing valve  24  comprises a tubular housing  30  subtended at opposite ends by threaded connecting subs  32  and  34 . Near the upper connecting sub  32 , the housing  30  is perforated by one or more orifices  35 . The orifices are initially sealed by respective rupture discs  36 . Internally of the housing  30 , a closing sleeve  38  is provided with a close sliding fit against the inside wall surface of the tubular housing  30 . The closing sleeve has a limited freedom of axial translation in opposite directions along the housing for opening and closing the orifice  35  to fluid flow after the rupture discs  36  are discharged and the orifice  35  opened. A circumferential rib  40  flanked by glide ramps  42  around the inside circumference of the closing sleeve provides an operational connection to a shifting tool  106  that will be described subsequently. 
     Integral with and positioned between the closing sleeve  38  and the guide sleeve  46  are a plurality of axially extended, resilient collet reeds  44 . The outside perimeter of the collet reeds carries a latching shoulder  45 . 
     A locking piston  47  displaced by internal bore pressure is secured against axial translation by a calibrated shear pin  48 . A displacement space  49  is provided to receive the piston  47 . A radially biased piston skirt  50  closes against the end surface  52  of the guide sleeve  46 . However, the locking piston  47  will not secure the closed position of the closing sleeve  38  over the orifice  35  until the locking piston is translated into the displacement space  49 . Such translation is selectively actuated by sufficient fluid pressure within the internal flow bore  13  bearing on the end of the locking piston to shear the pin  48 . The actuation pressure is normally imposed by surface pumps not illustrated. The outer perimeter of the guide sleeve  46  carries a latching shoulder  54  that cooperates with the end of the biased skirt  50  to prevent reopening of the orifices  35  once the closing sleeve  38  has been translated to the closed position and the locking sleeve  47  has been translated into the displacement space  49 . 
     The ingress cementing valve  26  is described by reference to FIG. 6 which illustrates an upper connecting sub  62  and a lower connecting sub  64 . In threaded assembly between the two connecting subs is a tubular housing  60 . The housing  60  is perforated by orifices  66 . For downhole run-in, the orifices are closed by pressure rupture discs  67 . Internally, the housing  60  confines a closing sleeve  68 . The sleeve  68  is assembled to the internal bore of the housing  60  with a close sliding fit that overlies the orifices  66 . Collet reeds  70  carry a detent ridge  72 . The collet reeds resiliently bias the ridge into a circumferential detent channel  74  to releasably restrain the collet and closing sleeve at the open orifice position illustrated. The internal bore of the closing sleeve may include a circumferential tool rib  76  flanked by guide ramps  78 . The outer perimeter of the closing sleeve includes a radially expansible lock ring  80 . 
     Between the ingress valve upper sub  62  and the housing  60  is a lock piston  82  that is axially secured by a calibrated shear pin  83 . Predetermined fluid pressure within the flow bore  13  applied to the inside cross-section of the bore shears the lock pins  83 . Upon failure of the lock pins  83 , the lock piston  82  shifts into the displacement space  84  and removes the piston skirt  86  from the housing counterbore shoulder  88 . When the counterbore shoulder  88  is exposed and the closing sleeve  68  is shifted to the orifice  66  closure position, the lock ring  80  expands into the channel between the counterbore shoulder  88  and the end of the lock piston skirt  86 . This meshing of the lock ring  80  against the counterbore shoulder  88  secures the sleeve  68  from subsequent opening. 
     Secured around the external perimeter of the housing  60  is a calibrated screen  90 . The term screen is used herein to include all forms of sized flow paths which, for examples, may include meshed wire, parallel slots and drilled or punched orifices. Orifice or mesh opening dimensions or gage is highly dependent upon the material to be used with the collar forming cement. If the material blended with the cement is particulate, the orifices are sized to barely but confidently retain the particulate in a bridged position across the mesh or slot opening. An objective is to close the cement ingress path through the orifices  66  when the collar annulus is packed with cement. As a consequence of the operative cooperation between the screen mesh size and the cement blended particulate size, the collar annulus  20  must be filled with cement before all openings in the screen  90  are closed. 
     A specific example of the foregoing might include a 12 ga. meshed or slotted screen around the ingress orifices  66  to receive a collar annulus cement blended with resieved 20/40 U.S. Mesh Gravel. Appropriate particulates may include sand or ground glass. However, non-particulate cement additives may also be used to exploit flow properties such jelling or congealing under dynamic conditions. 
     With respect to FIG. 7, the cementing tool  100  comprises a threaded assembly of three sectors including upper sealing elements  102  and lower sealing elements  104 . Between the sealing elements is a shifting tool  106 . The sealing elements may be substantially passive swab seals. The shifting tool  106  comprises a plurality of cylindrically distributed collet reeds  108  having symmetric ramp faces  110  flanking a tool ridge engagement slot  112 . 
     The reed base sleeve  114  is secured to an upper collar  116  having a concentrically sliding fit about an outer mandrel  118 . A lower collar  120  is threadably assembled with the outer mandrel but loosely overlies free tips  122  of the collet reeds  108 . An annular, spring compliance space  124  spans beneath the collet reeds. 
     The outer mandrel  118  is a static, threaded assembly of tube between an upper collar  126  and a lower collar  128 . The upper collar  126  assembles with the terminal end of a cement delivery conduit not illustrated. The cement delivery conduit extends to the wellbore surface and is connected at the surface to a pumped delivery system. 
     Between the upper and lower collars  126  and  128  is a cooperative box joint  130  and pin joint  132 . The box joint is penetrated by an inner cement discharge orifice  134 . An inner mandrel  136  extends from the upper collar  126  to the lower collar  128 . An inner cement discharge orifice  138  aligns with the outer discharge orifice  134 . Below the inner discharge orifice  138  is a bore plug seat  140  adapted to receive a surface launched bore sealing element  142  such as a ball, rod or dart. 
     The invention method sequence is most conveniently understood from the schematic of FIG. 2 which illustrates a raw borehole wall  10  having a collar annulus  20  between a casing liner  12  and the borehole wall  10 . The collar annulus extends along the borehole length between the upper packer  14  and the lower packer  16 . Between the packers  14  and  16  is the egress cementing valve  24  and the ingress cementing valve  26 . The flow orifice  66  of the ingress valve  26  is shielded by a calibrated mesh screen  90 . 
     The cementing tool  100  is suspended within the internal bore of the casing liner  12  thereby providing an internal annulus  13 . This internal annulus  13  is internal of the collar annulus  20 . The cementing tool is positioned along the borehole length relative to the egress valve  35 . The sealing elements  102  and  104  are located on opposite sides of the egress valve  35  and expanded to isolate the inner annulus section  92 . This isolated inner annulus  92  provides a channel for the cement flow down the cementing tool flow bore from the orifices  138  to the orifices  35  of the egress valve  24 . The annulus  92  between the cementing tool  100  and the casing liner  12  is isolated between the sealing elements  102  and  104 . Consequently, the forced flow of cement is routed further through the egress valve  35  into the collar annulus  20 . 
     When the tool  100  is positioned as required and the inner annulus sealing elements  102  and  104  are expanded, the dart  142  is deposited in the tool flow bore to seal the tube bore at the seat  140 . Pump pressure within the flow bore may thereafter be increased to open the rapture disc in the egress valve  35 . 
     The ingress valve rupture disc  67  may also be opened at this time and the collar annulus  20  proceed to receive cement. 
     As the collar annulus fills with cement from the egress valve  35 , downhole formation fluids, drilling fluids and other debris is forced from the collar annulus  20  through the screen  90  and into the ingress orifice  66  until the cement reaches the screen  90 . Fluids and other materials passing through the ingress orifice  66  are channeled uphole along the annulus  13  between the cementing tool  100  and the casing liner  12 . As the aggregate laden cement attempts to penetrate the screen  90 , the particulates correspondingly plug the protective mesh thereby effectively closing the ingress valve  26 . The fact that the screen  90  enclosing the ingress valve  26  has plugged is objectively reported at the well surface by the discharge pressure in the cement displacement pump. The pump discharge pressure against the fluid column bearing on the cement abruptly rises. That fluid column is carried in the tubing bore of cementing tool  100 . 
     With the cement collar  20  in place, the orifice  35  of egress valve  24  is closed by a translated shift of the sleeve  38 . The cementing tool sealing elements  102  and  104  are retracted and the shifting tool  106  is manipulated to engage the shifting tool engagement slot  112  with the sleeve  38  rib  40 . When engaged, the sleeve  38  is shifted to underlie the orifice  35  and thereby isolate it from the interior bore. 
     When the sleeve  38  shifts, the radially inward spring bias of the locking piston  47  skirt  50  contracts the locking piston radially to present an abuttment obstacle to the sleeve  38  latching shoulder  54  thereby caging the sleeve at the orifice closed position. 
     If desired, the orifice  55  may be reopened once by the shifting tool  106 . Again the tool slots  112  engage the ribs  40  of the ingress valve sleeve  38 . Force is applied on the tool  100  to shear the retaining pin  48  and displace the locking piston into the space  49 . 
     After the ingress orifice  38  is closed, the shifting tool  106  is manipulated to engage the ingress valve  26  sleeve ridge  76 . The closing sleeve  68  is shifted to underlie and close the orifice  66 . The closing sleeve  68  is held at the open position by the collet reed detent ridge  72  resting in the housing detent channel  74 . When shifting force is applied to the sleeve  68 , the detent ridge  72  resiliently yields from the channel  74 , but expands to abut the housing shoulder  75 . 
     Shifting of the sleeve  68  to the orifice closure position also places the sleeve lock ring  80  contiguously within the piston skirt  86  of the lock piston  82 . Opening and closing of the egress orifice  66  by reverse shifting of the sleeve  68  is optional until the lock piston  82  is shifted by fluid pressure within the internal flow bore  13 . Sufficient flow bore pressure on the interior end of the lock piston  82  shears the retaining pin  84  to allow translation of the lock piston into the displacement space  84 . Such translation extracts the piston skirt from around the resiliently biased lock ring  80  which consequently expands into the circumferential channel evacuated by the piston skirt  86 . 
     Although the invention has been described in terms of specified embodiments which are set forth in detail, it should be understood that this is by illustration only and that the invention is not necessarily limited thereto. Alternative embodiments and operating techniques will become apparent to those of ordinary skill in the art in view of the present disclosure. Accordingly, modifications of the invention are contemplated which may be made without departing from the spirit of the claimed invention.