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This application claims the benefit of an earlier filing date from U.S. Ser. No. 60/106,794, filed Nov. 3, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to the oil field industry. More particularly, the invention relates to hydrocarbon production systems in highly deviated (&gt;55° deviation) wellbores. 
     2. Prior Art 
     Highly deviated or horizontally disposed wellbores have been employed in growing numbers in recent years to access oil reservoirs not previously realistically productible. In an open hole completion however, and especially where there is water closely below the oil layer or gas closely above, highly deviated or horizontal wells are much more difficult to produce. 
     Pressure drop produced at the surface to extract oil from the formation is as its highest at the heel of the highly deviated or horizontal well. In an open hole well, this causes water or gas coning and early breakthrough at the heel of (or any part of) the highly deviated or horizontal well. Such a breakthrough is a serious impediment to hydrocarbon recovery because once water has broken through, all production from the highly deviated or horizontal is contaminated in prior art systems. Contaminated oil is either forsaken or separated at the surface. Although separation methods and apparatuses have become very effective they still add expense to the production operation. Contamination always was and still remains undesirable. 
     Another inherent drawback to open hole highly deviated or horizontal wells is that if there is no mechanism to filter the sand or formation solids prior to being swept up the production tubing, a large amount of solids is conveyed through the production equipment effectively sand blasting and damaging the same. A consequent problem is that the borehole will continue to become larger as sand is pumped out. Cave-ins are common and over time the sand immediately surrounding the production tubing will plug off and necessitate some kind of remediation. This generally occurs before the well has been significantly depleted. 
     To overcome this latter problem the art has known to gravel (gravel being used according to the vernacular; gravel, sand, and similar particulate matter) pack the highly deviated or horizontal open hole wells to filter out the sand and support the bore hole. As will be recognized by one of skill in the art, a gravel packing operation generally comprises running a screen in the hole and then pumping gravel therearound in known ways. While the gravel (such as gravel, ceramic beads, sand etc.) effectively alleviates the latter identified drawbacks, water or gas coning and breakthrough are not alleviated and the highly deviated or horizontal well may still be effectively occluded by a water breakthrough. 
     To achieve zonal isolation, the art has known to gravel pack multiple stages between pre-activated isolation devices (such as external casing packers (ECP) etc.). This operation is known to be complex, time consuming and at high risk. 
     Since prior attempts at enhancing productivity in highly deviated or horizontal wellbores have not been entirely successful, the art is still in need of a system capable of reliably and substantially controlling, monitoring and enhancing production from open hole highly deviated or horizontal wellbores. 
     SUMMARY OF THE INVENTION 
     The invention teaches a system that effectively creates a gravel pack on both sides of a non-activated annular seal (NAAS), allowing the seal to be activated to set against a casing or open hole. More specifically, the gravel when placed by the system of the invention, skips over the NAAS and leaves virtually no gravel around the NAAS when the annular velocity is above critical settling velocity. The beneficial effects of the invention are obtained by causing the gravel to stall in an area upstream of the NAAS by preventing leak-off downstream of the NAAS. When sufficient pressure builds in the gravel carrier fluid, due to flow restriction caused by the tightly packed gravel upstream of the NAAS, a valve opens upstream of the NAAS and gravel begins to pack the downstream section. 
     This invention allows the gravel placement in continuous pumping operation, prior to activation of the AS devices. 
     An additional benefit of the valve structure of the invention is that prior art limits on the length of a gravel pack are avoided. More specifically, because of the valves of the invention pump pressures do not continue to climb as they do in the prior art. Thus with the invention pressures do not reach the fracturing pressures, the avoidance of which limited prior art pack lengths. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic cross section view of an open hole zonal isolation and control system of the invention; 
     FIG. 2 is a schematic cross section view of a gravel packing zonal isolation embodiment of the invention where a secondary valve is closed; 
     FIG. 3 is the embodiment of FIG. 2 where the secondary valve is open; 
     FIG. 4 is one embodiment of the valve for use in the embodiment of FIGS. 2 and 3; 
     FIG. 5 prior art pressure—time plot; 
     FIG. 6 is the new invention pressure—time plot; 
     FIGS. 7-14 is another valve embodiment of the invention in a closed position; 
     FIGS. 15-22 is another valve embodiment of the invention in an unlocked position; and 
     FIGS. 23-30 is another valve embodiment of the invention in an open position. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, in order to most effectively produce from a hydrocarbon reservoir where a highly deviated or horizontal wellbore in an open hole formation is indicated, a gravel pack is ideally constructed. Moreover the gravel packed area is most desirably zonally isolatable. Such zonal isolation is, pursuant to the invention, by way of annular seal (AS) (i.e hydraulic packer, ECP or mechanical packer) at selected intervals or hydraulically isolated with composite material or cement (curable materials). To complete the system, a production string including flow control devices may be run into the hole, each zone being isolated by a locator and a seal. This production string may be omitted, allowing for subsequent internal zonal isolation in the life of the well. The various components of the system are illustrated in FIG. 1 wherein those of skill in the art will recognize a liner hanger or sand control packer  10  near heel  12  of highly deviated or horizontal wellbore  14 . From liner hanger  10  hangs a production string that may include flow control device  16  which may be hydraulic, mechanical, electrical, electromechanical, electromagnetic, etc. operated devices such as sliding sleeves and seal assembly  18 . Seal assembly  18  operates to create selectively controllable zones within highly deviated or horizontal wellbore  14 . Seal assemblies  18  (in most cases there will be more than one though only one is depicted in FIG. 1) preferably seal against a polished bore in the original gravel packing basepipe  22  which remains in the hole from the previous gravel packing operation. Also visible are ports  24  in basepipe  22  with screen  26  thereover. Roller  30  is illustrated in the net position evidencing substantially no gravel between its outer perimeter and the borehole well  31 . 
     Referring to FIGS. 2-4, an annular seal (AS) is employed to create the zonal isolation. Traditionally, AS&#39;s are expanded (set) against the gravel pack because gravel will have settled thereover in the packing operation. The gravel between the open hole or casing and the AS is a leak path and is undesirable. To render the AS more effective, the present inventors have developed a system which effectively packs both uphole and downhole of an AS and deposits virtually no gravel over the AS. 
     Referring to FIG. 2, basic components will first be identified for frame of reference. Washpipe  80  is located inside base pipe  82  which is screened  84 ,  86  in a generally conventional manner. AS  88  is located centrally. In a preferred arrangement a blank section  90  is located immediately downhole of AS  88  to collect overflow gravel from the uphole edge of the downhole screen. Without the blank section, the overflow would spill out over the AS and reduce the effectiveness of the invention. Washpipe  80  preferably includes a valve  92  with a seal  94  just downhole of the valve  92 , the seal spanning the annulus defined by the OD of washpipe  80  and the ID base pipe  82 . It should be understood that only a section of the portion of the well being gravel packed is illustrated and that the gravel packing activities of pumping a loose slurry of gravel downhole through a crossover, through a screen and back uphole through the end of the washpipe should still be considered the operation undertaken relative to the invention. The difference being shown in the figures and disclosed hereunder. 
     Again referring to FIG. 5, the normal gravel packing action starts with the α wave and leak-off fluid being drawn through screen  86  and to the end of washpipe  80  (end not shown). As is known the α wave will continue to the bottom of washpipe  80  and then begin a β wave back uphole. The β wave propagates gravel deposition back up and over the top of the annulus around screen  86 . As the β wave nears the AS however, movement uphole thereof stops because there is no leak-off (necessary for deposition) above AS  88 . The result is that the gravel pack  96  below AS  88  is very tight and the pressure of the gravel carrier fluid increases on the area uphole of AS  88 . Since there is no leak-off uphole of AS  88  no more gravel is deposited. One should understand that there is no leak-off under screen  84  because of seal  94 . Without seal  94 , leak-off would occur from under screen  84  and simply flow to the end of washpipe  80 . Seal  94  prevents such flow and creates the above described condition. 
     As pressure increases in the annulus  100  to a preselected differential over the pressure in annulus  102 , the valve  92  opens which in effect moves the end of the washpipe  80  to uphole of seal  94 . Immediately upon opening of the valve  92  there is a leak-off path (see flow lines  108  in FIG. 3) from under screen  84  to washpipe  80  and the β wave progresses thereto. Since the annular area  104  between AS  88  and the open hole  106  is relatively narrow, the velocity of fluid traveling therethrough is high which prevents the deposition of gravel. Thus gravel is not deposited until it reaches screen  84  where leak-off is present and the velocity of the fluid slows. Thus, the β wave skips over the AS  88  and resumes over screen  84 . Such skipping will occur in any location where the construction is as stated regardless of the number of AS&#39;s used. Because of the valve structures used, the pressure across the valve actuator will always be balanced until the downhole section is packed up and pressure thereabove increases. This allows multiple units to be run simultaneously. This will be more clear from the following discussion of the valve embodiments. 
     The ASs can then be inflated conventionally with assurance that the OD thereof will be in contact with the formation at open hole boundary  106  and not a segment of packed gravel. Hereby a reliable isolation between zones is established. 
     Referring to FIG. 4, one embodiment of the valve for the zonal isolation system of FIGS. 2 and 3 is illustrated. For clarity, only the valve structure itself and seal  94  are illustrated. It should be understood that the intended environment for the valve is as shown in FIGS. 2 and 3. 
     Valve  92  includes flow port  110  which connects the interior of washpipe  80  to the annulus  100  allowing fluid from annulus  100  to go to the washpipe  80 . The valve will be initially closed by sleeve  112  having seals  114 . Such position (closed) is preferably ensured by a shear out member  116  such as a bolt. The sleeve  112  is connected to and operable in response to a piston  118  which rides in a bore  120  that is bifurcated into chamber  120   a  and  120   b  by the piston  118 . Provision is made to allow chamber  120   a  to “see” annulus  100  pressure while chamber  120   b  “sees” annulus  102  pressure. When annulus  100  pressure exceeds annulus pressure by a preselected amount of about 20 to about 500 psi, the bolt  116  shears and the sleeve  112  shifts to open port  110 . In the drawing, chamber  120   a  is provided with the pressure information through channel  122  and chamber  120   b  is provided with the pressure information through channel  124 . These are but examples of channels that can be employed and it is important to note only that the channels or other “pressure sensors” (computer sensors being an alternative where the sleeve is opened electrically or mechanically other than simply hydraulically) should be exposed to pressure on opposite sides of the seal  94 . 
     An additional benefit of the invention is that long runs of gravel material can be installed without gravel fluid carrier pressure increase because of the valves employed in the invention. The pump pressure difference for the beta wave is illustrated in FIGS. 5 and 6 where the invention (FIG. 6) shows a saw tooth pressure pattern which keeps pressure low. 
     In another embodiment of the valve component of the invention, reference is made to FIGS. 7-30, which are broken up to FIGS. 7-14;  15 - 22 ; and  23 - 30  to illustrate three distinct conditions of the same valve. For frame of reference, seal  94  in this embodiment of the valve of the invention can be found in FIGS. 12,  20  and  28  and preferably is a bonded seal stack. A bonded seal stack is a phrase known to the art and requires no specific discussion. Such a seal arrangement is commercially available from a wide variety of sources. 
     Referring now to FIGS. 7-14, the valve portion of the invention is illustrated in a closed position. This is the position for run in of the washpipe and it is the position in which the valve will remain until the gravel packing operation causes pressure to rise in the area uphole of seal  94  as hereinbefore described. 
     The valve is locked closed by lock piston  150  which prevents lock ring  152  from disengaging with groove  154  on washpipe  156 . The lock piston is also biased in the locked position by spring  158  which is what preselects the pressure differential required to unlock the tool. Spring  158  is bounded by nut  159  which is threadedly attached to sleeve  160 . One will note that annulus  161  (FIG. 11) has been left open for receipt of the sleeve  160  and its actuation assemblies when opened. More specifically, pressure in the area uphole of the seal  94  is “seen” by the uphole end of lock piston  150 ; pressure downhole of seal  94  is “seen” by the downhole side of piston  150 . Thus, the pressure downhole in addition to the spring  158  bias must be overcome for uphole pressure to unlock the tool. The pressure path for the uphole pressure is along the OD of the closing sleeve  160 . Downhole pressure is accessed downhole of seal  94  at port  162  (FIG.  13 ). 
     Referring to FIGS. 15-22, once the pressure uphole of seal  94  reaches the preselected differential to that downhole thereof, the tool will be in the condition set forth in FIGS. 15-22, i.e, the lock piston  150  will move downhole off of lock ring  152  which then disengages from groove  154 . There is no longer anything holding the closing sleeve  160  closed and the same pressure that opened lock piston  150  will, in conjunction with spring  168  which bears against spring stop  169 , urge the closing sleeve  160  into the open position by shifting the sleeve downhole of the ports  164 . The open condition is illustrated in FIGS. 23-30 where the sleeve has moved completely off ports  164  and has come to rest on land  170  with shoulder  172  of sleeve  160  bearing thereagainst. Suitable seals  174  have been placed throughout the tool to contain pressure where desired. 
     The operable components noted are contained between a sleeve cover  180  and the washpipe  156 . Cover  180  is threadedly attached to seal sub  182  which then is attached via a acme thread to lower sub  184 . One of skill in the art should note the lack of a seal  174  at the uphole junction of cover  180  and upper sub  188 . This is part of the pressure path to the uphole area discussed above. 
     Since the provision of different zones and flow control devices in the invention allow the metering of the pressure drop in the individual zones, the operator can control the zones to both uniformly distribute the pressure drop available to avoid premature breakthrough while producing at a high rate. Moreover, the operator can shut down particular zones where there is a breakthrough while preserving the other zones&#39; production. 
     After construction of one of the assemblies above described, and the washpipe has been removed, a production string is installed having preferably a plurality of the seal assemblies with at least one tool stop mechanism to locate the seal assemblies at points where the basepipe is smooth and the inner diameter is not reduced. Location may also be assured based upon the liner hanger  10 . The seal assemblies allow different zones to be created and maintained so that selective conditions may be generated in discrete zones. 
     While preferred embodiments have been shown and described, various 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 illustration and not limitation.

Summary:
A system for enhancing hydrocarbon production in long and deviated subterranean wells. Gravel is placed in the annulus between the screen liner and the borehole, together with annular isolation elements. Selective flow control is achieved. Sequential control or commingled production is achievable from multiple producing intervals of the borehole. A differential valve is incorporated in the screen liner service string to allow for gravel placement across multiple screen-liner sections, separated by annular isolation elements in a continuous one stage placement operation, thereby reducing time and complexity of such operations.