Patent Publication Number: US-6209663-B1

Title: Underbalanced drill string deployment valve method and apparatus

Description:
This application claims the benefit of U.S. Provisional Application Ser. No. 60/085,893 filed May 18, 1998. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to underbalanced drilling and, more particularly, to a deployment valve method and apparatus for removing and inserting a drill string into a well bore when downhole pressures such as formation pressure are present at the surface to act on the drill string. 
     DESCRIPTION OF THE BACKGROUND 
     Underbalanced drilling has many advantages. In some cases, oil/gas well flow during underbalanced drilling of a well has been sufficient to pay for the cost of drilling the well even prior to completion of the well. Other advantages include that of avoiding formation damage for a better performing well and more accurate logging measurements of the well contents. For a discussion of advantages of underbalanced drilling including methods of controlling the well using an exemplary rotating blow out preventer, please refer to U.S. Provisional Application Ser. No. 60/083436, entitled ROTATING BOP AND METHOD, filed Apr. 29, 1998, to Hosie et. al, subsequently filed as U.S. patent application Ser. No. 09/178,006, filed Oct. 23, 1998, which applications are hereby incorporated herein by reference. Thus, there are numerous and significant advantages for underbalanced drilling of a well. 
     However, various problems exist for underbalanced drilling. These problems relate mainly to controlling the well in certain circumstances. A possible problem that may typically arise is the need to remove the drill string from the well. There are many common reasons that the drill string must be removed from the well prior to completion of drilling. It may be necessary to remove the drill string for reasons such as the need to change out the drill bit, steering tool, mud motor, and the like. Although pressure is controlled at the surface, such as with a rotating BOP, the weight of the drill string holds the drill string within the well bore. The string light point is the depth or position where the upward forces acting on the drill string become greater than the downward forces. Many factors are involved and may vary as to exactly where in a well the drill string becomes string light. Such factors include but are not necessarily limited to, or may not necessarily always include, the following: surface pressure, down hole pressure, flow rate, hole size, drill pipe size and weight, the amount of drill pipe in the hole, bit size, casing size, and fluid or gas properties. Generally assuming other factors remain constant, as the drill string is pulled further out of the hole, the downwardly acting forces decrease due to decreased drilling string weight. 
     If an attempt to remove the drill string is made with pressure at the surface, then at some point as the drill string is being removed the pressure may begin to push or accelerate the drill string out of the well bore. This is a dangerous situation that could conceivably lead to a blow out. Once the pipe begins to move upwardly there may be developed a significant momentum such that the blow out preventers may not be able to stop the upward movement. Once the heavy pipe string is moving upwardly, closing the rams may result in tearing the rams out rather than stopping the upward movement of the pipe. In this case, the rams will not be available to shut in the well after the pipe has been pushed from the well bore, assuming there is someone left at the rig site to activate the rams after the drill pipe is ejected from the well. The forces are great enough so that ejected drill pipe may be found quite far from the rig. As well, sparks produced can ignite gas to produce a hot fire that can melt a drilling rig within minutes. Blow outs can result in costly problems such as personnel injury, damage to the drilling rig, environmental damage, and loss of the hole. Presently, methods used to avoid a blow out situation are effective but have significant disadvantages. 
     While it may be possible to bleed off the surface pressure prior to reaching the point where the string becomes “string light” and begins to move upwardly, this practice is risky. For instance, a bridge in the bore hole may form in the formation that temporarily permits a bleed off to appear to occur. If the bridge should break at the wrong moment with the pipe nearly out of the hole, then significant formation pressure may be applied at the surface to result in a blow out. 
     A very effective and safe practice is to kill the well prior to removal of the drilling string. However, this practice is undesirable because the advantages of underbalanced drilling may then be lost. Once the drill string is lowered back into the well bore below the string light point it may be possible to adjust the drilling fluids so that underbalanced drilling continues. However, formation damage may have already occurred that is substantially or partially irreversible. 
     Another very effective and safe practice is that of providing a snubbing unit for removing the drilling string. However, the snubbing unit takes considerable time to rig up, requires considerable additional time while tripping the well, and then requires considerable additional time to rig down. Thus, the cost of tripping the drill string can be quite considerable due to the rig time costs and snubbing unit costs. Additional tripping of the well may also be necessary and again require the snubbing unit. This procedure then, while effective and safe, increases drilling costs considerably. 
     Consequently, an improved method and apparatus is desirable for removing drill string from a well bore that is drilled underbalanced. Such an improved method and apparatus should provide for quick, but safe, removal of the drill string from the well without the need to kill the well. The method and apparatus should be useful for repeated tripping of the drill string whenever necessary without significant time and cost increases. Those skilled in the art will appreciate the present invention that addresses these and other problems. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an elevational schematic view of an underbalanced wellbore operation wherein the drill string extends through an open deployment valve in the casing in accord with the present invention into the reservoir formation; and 
     FIG. 2 is an elevational schematic view of removal of the drill string from the well bore into the casing whereupon the deployment valve is closed. 
     FIG. 3 is a schematic view of a deployment valve of a flapper valve type; and 
     FIG. 4 is a schematic sectional view of a deployment valve that may be operated by annulus pressure and/or hydraulic lines; 
     FIG. 5 is an elevational view, in section, of an annulus operated deployment valve positioned in a liner that is cemented within the borehole; 
     FIG. 6 is an elevational view, in section, of a control line operated deployment valve positioned in a liner that is cemented within the borehole; 
     FIG. 7 is an schematic view of an upper section of a deployment valve in accord with the present invention; 
     FIG. 8 is an elevational view, in section, of a lower section of a deployment valve in accord with the present invention; 
     FIG. 9 is a cross-sectional view, in section, taken along lines  9 — 9  of FIG. 8; 
     FIG. 10 is an elevational enlarged view, in section, showing the deployment flapper valve open; 
     FIG. 11 is an elevational enlarged view, in section, showing the deployment flapper valve closed; 
     FIG. 12 is an elevational view, partially in section, showing an indexing sleeve for the deployment valve; and 
     FIG. 13 is an elevational view spread out over 360° showing an indexing slot pattern for the indexing sleeve. 
    
    
     While the present invention is disclosed in terms of a presently preferred embodiment or embodiments in accordance with patent laws, it will be understood that the present invention is not intended to be limited to the particular embodiment or embodiments shown for permitting understanding of the invention. Instead, it is desired to cover all embodiments contained within the spirit of the invention. 
     SUMMARY OF THE INVENTION 
     The present invention provides a means for more quickly removing a drilling string from a tubular string, such as a tubular string of casing, when the tubular string is exposed to downhole pressure. This might occur after drilling through a reservoir formation with the drilling string. Attempts to remove the drilling string when downhole pressure is contained within the casing may be dangerous due to the possibility of a blowout. 
     A method for underbalanced drilling of a well bore through a well bore formation is provided herein wherein the well bore has a tubular string, such as a casing string secured therein. Typically but not necessarily, the casing string is cemented within the well bore at least over a certain length if not over substantially the entire length of the casing string. There may be multiple strings of casing within a well bore. 
     The method of the present invention comprises mounting a deployment valve within or as part of a tubular string, typically the casing string, preferably adjacent the reservoir formation. However, the deployment valve should preferably be below, preferably with a good safety margin, the string light position at which point the forces acting upwardly on the drilling string are greater than the forces acting downwardly such that the drilling string may begin to move upwardly. In one embodiment, the deployment valve has an open and a closed position. The tubular string is secured within the well bore, such as by cementing or other means, such that the deployment valve is mounted within. The deployment valve may be secured to the casing and run therewith or it may be mounted within the casing such as by running a smaller tubular string, lowering, wireline methods, or other methods. A drill string is provided to be operable for drilling into the reservoir portion of the well bore formation below the tubular string. The drill string is moveable through the tubular string or casing and the deployment valve when the deployment valve is in the open position. The deployment valve may be closed when the drill string is no longer positioned within the deployment valve. 
     The drill string may be extended through the deployment valve to drill below the casing such as into the reservoir portion of the well bore formation below the tubular string. The drill string may be pulled from the reservoir portion into the tubular string, and the deployment valve closed once the last of the drill string is pulled therethrough. For this reason, it is typically desirable that the deployment valve be as close to the bottom of the casing as possible so that the deployment may be closed as soon as possible. At a minimum, the deployment valve should be located below the string light point as best determinable plus additional safety margin depth. The exact string light point may not be precisely known and could even vary. Once the drilling string is back within the casing or tubular string, it is desirable to reduce pressure in the tubular string above the deployment valve. Once reservoir pressure is bled off from the portion of the casing above the deployment valve, the drill string can be safely removed from the well bore. After the desired service is made to the drill string, the drill string may once again be introduced into the tubular string when the deployment valve is closed. 
     The casing may then be pressurized above the deployment valve to preferably equalize pressures above and below the valve. The deployment valve can then be opened to thereby extend the drill string through the deployment valve for continued drilling after servicing the drill string in some way. 
     A deployment valve system is provided for a drilling system to form a well bore through a well bore formation with the drilling string. The drilling string has a drill bit and the drill bit having an outer diameter. A deployment valve is positionable in the tubular string at a selected depth within the well bore formation preferably within the lower portion of the tubular string as discussed above. The deployment valve should be able to open such that the inner diameter is large enough to allow the drill bit to pass therethrough. While flexible sealing elements, such as expandable tubulars or bags could form portions of the valve, the valve could also take other forms as discussed below. In some embodiments of the present invention, the deployment valve body may have a bore therethrough having an inner diameter larger than the outer diameter of the drill bit. A deployment valve element is mounted within the deployment valve body for movement between an open position and a closed position. A seal surface for the deployment valve may also be provided. With the deployment valve element in the open position within the deployment valve body, the drill bit may pass through the valve. The deployment valve may be closeable with the seal surface for containing/controlling the formation reservoir pressure below the deployment valve. A rotating blowout preventer is provided for sealing around the drilling string at the surface. 
     Several options are available for the type of deployment valve used. The valve may be inflatable as by hydraulic control. The deployment valve element may include one or more flapper valve elements moveable within the deployment valve. One or more pivot connections may then be used to secure the one or more flapper valve elements to the deployment valve such that the one or more flapper valve elements are moveable between the open position and the closed position. The deployment valve could also be a ball valve or have another rotatable type of closure element. Telescoping elements may be used. Thus, the valve could be of many different constructions. The deployment valve may be hydraulically operated and have at least one hydraulic line for controlling movement of the deployment valve element. A biasing element(s), such as springs or weights, or other control lines may be used in conjunction therewith the above or subsequently described deployment valves. The deployment valve could also be annularly controlled such that an annulus is provided so that pressure acting thereon activates an annular control system to control the valve. Some type of combination of annular, hydraulic, biasing means, or other control method may be used. 
     The method for underbalanced drilling of a well bore through a reservoir formation with a drilling string operable for drilling into open hole at a well bore depth below the tubular string may also comprise positioning a deployment valve within the tubular string as described hereinbefore and below the string light position. The drilling string is moved through the tubular string and the deployment valve for drilling open hole below the tubular string. For various reasons, the drilling string may require being pulled back into the tubular from the open hole, whereupon the deployment valve is closed to thereby control pressure of the reservoir formation at a well depth below the deployment valve. The reservoir formation pressure within the tubular string at a well depth above the deployment valve is bled off. The tubular string is opened to atmospheric pressure and the drilling string is removed from the tubular string. At a later time, the drilling string may be reinserted into the tubular string. 
     An object of the present invention is to provide an improved method for underbalanced drilling. 
     Another object of the present invention is to provide a method for removing the drilling string from the well bore when the formation pressure acts on the well bore as occurs during underbalanced drilling. 
     Another object of the present invention is to provide a lower cost and faster method of removing the drill string from the bore hole for underbalanced drilling. 
     A feature of the present invention is a deployment valve preferably positioned below where the drill string becomes string light. 
     An advantage of the present invention is reduced costs and greater flexibility of an underbalanced drilling operation. 
     The above objects, features, and advantages are provided for easy review of some aspects of the invention and are not to be construed as limiting the invention in any way. It will be appreciated that other objects, features, and advantages of the present invention will become apparent in light of the drawings, claims and specification. 
     BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention represents a significant improvement in underbalanced drilling techniques and apparatus that addresses the potentially hazardous and/or costly problem of removing and reinserting a string of pipe such as the drill string from a live well. 
     Referring now to the drawings, and more particularly to FIG. 1, there is shown a schematic that illustrates basic elements of the method and apparatus of the present system  10  of the invention. In FIG. 1, drill bit  12  is shown at the bottom  14  of well bore  18  and is drilling in open hole region  16 . Drilling extends through well bore formation  19  into reservoir formation  20  below bottom  24  or shoe of casing  22 . The well is being drilled underbalanced so that downhole pressure, which can be quite substantial, may reach or flow towards the upper portion of the hole. The pressure may be controlled at the surface by various wellbore controls, such as wellbore control  26 . Wellbore control  26  may include a well head  28  with a rotating blow out preventer  30 , flow line  32  and pressurized recovery system  33  as indicated in FIG.  2 . Gas may be flared off, solids removed, oil and diesel separated off by a well bore control system such as a drilling recovery system with components such as pressure tanks, inlet manifold skid, pump skid, data acquisition, flow line, ESD valve, electrical generator, flare stack, glycol heater, heat tracing, triplex pump, and the like. The rotating blow out preventer, such as rotating blow out preventer  30 , seals around the pipe as it moves into and out of the well bore. Details of an exemplary rotating blow out preventer are described in the application referenced above to Hosie et al. that is incorporated herein by reference. So long as there is no need to remove the drill pipe, the operation can proceed by simply adding drill pipe in drill string  34  as necessary to drill deeper as indicated in FIG.  1 . 
     However, at some time it may become necessary to remove drill string  34  as indicated in FIG. 2 where deployment valve  36  is open to allow drill string  34  to move therethrough. When this occurs drill string  34  is pulled up into an outer tubular string, that may typically be casing string  22 , and may include one or more strings of casing. Once drill string  34  is in the casing string  22  as indicated in FIG. 2, deployment valve  36  closes with seal element  38  to seal off the formation pressure below deployment valve  36 . Drill string  34  can be moved upwardly to within a safe margin of it becoming string light. The pressure above deployment valve  36  can then be bled off. In fact, the casing above deployment valve  36  may be bled off at anytime after deployment valve  36  is closed. Once the pressure is bled off above deployment valve  36 , then drill string  34  can be safely and quickly removed. 
     After the desired changes or repairs to drill string  34  are made, then drill string  34  can be run back into the hole to a desired depth which may be preferably near deployment valve  36  and below the string light position. When the drill string is above the string light point, the remaining pipe in the hole is not heavy enough to hold drill string  34  in the hole and the pipe may therefore be blown out of the hole, i.e., the upwardly forces acting on drill string  34  are greater than the downwardly forces acting on the drill string  34 . 
     At some point after going below the string light position of the drill pipe when running the drill string back into the hole, casing  22  is preferably pressurized prior to opening the deployment valve  36 . This is done so as to substantially equalize pressure above and below deployment valve  36 . Deployment valve  36  is then opened, whereupon drill string  34  may be extended through deployment valve  36 , out of the casing string  22 , and back into formation  20  for continued drilling. 
     As discussed in the above referenced patent application, damage to the formation is averted because the pressure and fluids of the formation are contained therein as during drilling. It is not necessary to introduce additional pressure, fluids, or materials into the reservoir formation so as to force drilling fluid, cakes or filtrates thereof into the producing formation. 
     Deployment valve  36  may be run within casing  22  so as to be positioned surrounded by the well formation, such as near bottom end  24  of the casing. At a minimum, the position of deployment valve  36  should be below the anticipated string light point below which pipe string  34  will not be ejected from the well and preferably at a depth having a good margin of safety with respect to this position. Well bore depth may or may not be the same as vertical depth so that references to depths in the well bore such as above or below certain depths generally refer to well bore depths rather than vertical depths. 
     In some cases, well bore  18  may be considerably larger than the casing so that the diameter of deployment valve body  40  may be of larger diameter than that of casing  22 . If the deployment valve body is smaller than the diameter of casing  22  or is collapsible so as to temporarily assume a smaller diameter, then deployment valve  36  may be positioned after casing  22 , or one or more of the casing strings are in place, by some conveyance means such as wireline or tubing. 
     Deployment valve body  40  may have a flapper valve element  42 , or other type of valve closure element mounted therein. As indicated in FIG. 3, flapper valve element  42 , with hinge or pivot joint  44 , may be provided with spring control  46  or weight operated to bias close and seal against seat  48 . The weight of the drill string, and pressure above the flapper valve may be used to open flapper valve element  42 . Deployment valve  36  may be self-closing by spring and/or weight and/or seal with the valve seat due to the force of pressure below deployment valve  36 . 
     Deployment valve  36  could also be controlled by control lines such as hydraulic control lines  50  or  52  or annular pressure control through annulus  58  between outer string  54  and inner string  56  as indicated in FIG.  4 . Thus, the valve may also be a ball type of valve or other design as desired for the well conditions. Bias means, annular pressure, mechanical control, hydraulic control, other means may be used in conjunction with or used for control over the deployment valve. Valve body  40  may be inflatable or it may be comprised of a substantially inflexible material such as metal or composite materials. There may be several strings of casing in the hole whereby a portion of the valve may extend outwardly within an annulus between two strings of casing. 
     Opening diameter  60  of the valve should be sufficient to allow the drill string, including the drill bit to pass therethrough. Thus, the outer diameter of the drill bit should be smaller than the inner diameter of the deployment valve bore for some types of valve elements such as a flapper valve or ball valve. The valve element(s) should expand to allow the drill bit to pass if the valve elements are expandable or flexible. Deployment valve  36  may also be operated through interaction with the drill string or a drill string sub thereof such that vertical or rotational movement of the drill string controls deployment valve operation. More than one deployment valve may be used and if different deployment valves are used, they may be operated differently. As well, other types of valves and closure means may be used. Some leakage may be tolerable with the seal or sealing surfaces of the deployment valve so long as the leakage permits removal of the drill string. Temporary chemical plugs such as gels, cements, or the like may conceivably be used above the deployment valve. However, such uses may prevent the desirable possibility of repeated uses of the deployment valve. 
     A presently preferred embodiment of the deployment valve is shown in more detail in FIG.  5 -FIG.  13 . 
     In FIG. 5, annular operation installation  70  shows deployment valve  71  installed in casing string  88 /liner  72  for annular operation as discussed hereinafter. Liner  72  is effectively an extension of intermediate casing string  88  that goes to surface and deployment valve  71  is mounted therebetween so that deployment valve  71  is mounted within a tubular string that is partially cemented in position for installation  70 . It will be understood that depending on numerous factors such as the well program, components in the well such as additional casing strings, and the like, that many variations of annular installation operation  70  may be used. Thus, FIG. 5 is a representative basic example of how deployment valve would be installed and it will be understood that numerous factors go into each well installation such that additional components could be added/deleted from casing string  88 /liner  72 . Liner  72  has been cemented into the borehole as indicated by cement  74  positioned outside liner  72 . Liner hanger packer  76  is positioned below deployment valve  71 . Liner hanger packer  76  prevents cement  74  from proceeding upwardly within annulus  77  past liner hanger  76  during the cementing process. For this purpose, liner hanger packer  76 , or other packer means, is set or inflated prior to cementing to prevent cement from migrating into annulus  77 . Thus, annulus  77  that is around deployment valve  71  is available for use in controlling deployment valve  71  by applying pressure to annulus  77  at the surface. Annulus  77  is formed between liner  72 /casing string  88  and surface casing  86 . Moreover, deployment valve  71  is surrounded by surface casing  86 . Surface casing  86  is cemented into position as indicated by surface casing cement  90 . Surface casing  86  extends to the surface. Pressure applied to annulus  77  operates to open and close deployment valve  71  in a manner discussed hereinafter. 
     In installation  70 , between liner hanger packer  76  and deployment valve  71  are several components including casing coupling  78 , joint of casing  80 , tie back seal assembly  82 , and tie back receptacle  84 . Thus, annulus  77  extends downwardly past deployment valve  71  for some distance as might be advantageous for washing out annulus  77  as may be desired using, for instance, rotating port collar  92  or other components in the installation through which circulation may be established. It will be noted that positioned above deployment valve  71  is rotating port collar  92 , spacing nipple  93 , and locating nipple  96  that is secured to intermediate casing to surface  88 . As can be seen, the point of development in the well as per FIG. 5 is just after cementing of the liner prior to drilling out of cement float shoe  94 . Above float shoe  94  is joint of casing  96 , plug landing collar  98 , liner wiper plug  100 , and drill pipe wiper plug  102 . 
     The drill bit is normally only slightly smaller than the inner diameter  104  of liner  72 . For instance, a 7-inch liner having 7-inch connections and with an outer diameter in the range of about seven inches might have a 6-inch inner diameter, in which case the drill bit is typically in the range of 5¾ to 5⅞ inches. Thus, deployment valve  71  opens full bore or, in other words, widely enough to allow a standard bit therethrough. The inner diameter of deployment valve  71  is therefore preferably at least as large as inner diameter  104  of liner  72  or other relevant wellbore tubulars or casing strings through which drilling is effected, and may in some cases be slightly larger. Moreover, the outer diameter of deployment valve is small enough to fit in the next standard size casing string in that it is less than about 10% greater in O.D. than the size casing upon which it is installed since well bores typically have multiple casing strings therein. Thus, there is no need to have an oversized outer casing in which to use the deployment valve. This percentage may vary depending on whether the casing is heavy weight or light weight and may typically be in the range from about 5% to about 10% greater that the casing size O.D. 
     In FIG. 6, installation  110  is shown whereby deployment valve  112  is operated by control line  114  that goes to surface. Unlike installation  70 , in installation  110  annulus  116  between surface casing  118  and intermediate casing string  120  is filled with cement  121  during the cementing operation. In this way, control line  114  is rigidly secured in place. Surface casing  118  is also typically cemented in position (during a previous cement operation) as indicated by cement  123  outside of surface casing  118 . Protectors such as control line coupling protector  122  may be used to protect control line  114  intermediate casing string  120  is positioned within surface casing  118 . Casing  120  is centralized within surface casing  118  by centralizers such as casing centralizer  126 . After cementing, float shoe  124  will be drilled out as drilling proceeds. 
     In FIG.  7  and FIG. 8, deployment valve  150  is shown in two halves with actuator section  152  being shown in FIG.  7  and valve section  154  being shown in FIG.  8 . Actuator section  152  sits above valve section  154  and is used to actuate an inner slidable mandrel that is upwardly and downwardly moveable for opening and closing the flapper valve in a manner to be explained. 
     Prior to running into the borehole, nitrogen chamber  155  is precharged to a selected percentage of hydrostatic pressure as determined for the most efficient opening and closing pressures. A typical value might be about 60% of the anticipated hydrostatic pressure in the borehole where deployment valve  150  will be positioned. After charging, deployment valve  150  is run into the hole with the casing string. As the depth of the deployment valve increases, the increasing hydrostatic pressure acts on lower equalizing valve piston  156 . The hydrostatic pressure enters passage  158  which may be open to the annulus or connected to a control line. 
     Hydrostatic pressure acts on seal  160  and  162  of lower equalizing valve piston  156  creating a downward force on lower equalizing valve piston  156 . Seal  162  is preferably positioned on shoulder  164  of upper mandrel  172  for reasons to be discussed. Spring  166  biases lower equalizing valve piston  156  against shoulder  164  and seal  162  for initial sealing therebetween. Seal  168  prevents hydrostatic pressure entering bore  170 . 
     Due to increasing hydrostatic pressure, lower equalizing valve piston  156  pushes against shoulder  164  on the essentially one-piece upper mandrel  172 . Upper mandrel  172  is comprised of upper seal mandrel  174 , equalizing seal mandrel  176 , J-slot mandrel  180  and nitrogen chamber mandrel  178  screwed or otherwise connected together as a onepiece slidable mandrel  172 . Thus, upper mandrel  172  is forced downwardly relative to top body  182  of deployment valve in the direction of valve section  154  as indicated in FIG. 8 although it will be understood actual borehole orientations may vary considerably. For instance, deployment valve  150  may be used in horizontal wells and may be positioned at all borehole angles because it will be readily understood that tool orientation is dependent upon the borehole orientation direction in which the deployment valve is used. So the terms “up”, “down”, and the like are used for explanatory purposes and do not necessarily refer the position of the deployment valve in operation in the borehole. Upper mandrel  172  continues to move due to increasing hydrostatic force on lower equalizing valve piston  156  until lower face  184  of lower equalizing valve piston  156  comes into contact with shoulder  186  of housing  188 . The increasing hydrostatic pressure causes seal  162  with lower face  184  of lower equalizing valve piston to be broken as pressure acts to move upper mandrel  172  slightly until leakage may occur past seal  162  so that the hydrostatic pressure now engages nitrogen piston  190 . Any further movement of upper mandrel  172  after seal  162  with lower face  184  is broken tends to be very slight. Even a small opening provides enough space to permit pressurized flow past the broken seal to transmit the pressure to nitrogen piston  190 . Therefore, increasing hydrostatic pressure now begins to act on nitrogen piston  190  between nitrogen piston seals  192  and  194  and moves nitrogen piston  190  downwardly or away from top body  182 . Movement of nitrogen piston  190  reduces the volume of nitrogen chamber  155 , thereby increasing the pressure of the nitrogen therein. Nitrogen chamber  155  is comprised of housing  188 , nitrogen chamber mandrel  178 , nitrogen piston  190 , and center coupler  196  (shown in FIG.  8 ). Hydrostatic pressure continues to act on nitrogen piston  190  to move it further downwardly until nitrogen pressure equals hydrostatic pressure. Note that the above and continuing discussion is useful for describing how increasing/decreasing pressure can be used for operating deployment valve  150  in that the action of moving upper mandrel  172  also opens and closes deployment valve  150  as will be discussed subsequently. In some cases, this may not necessarily be the process of how the deployment valve will be run into the hole. For instance, in some cases it may be desirable to operate the valve on the surface to leave the valve in the open position while going in the hole in a manner as explained subsequently. 
     As an example of actuator section basic operation of moving upper mandrel  172  upwardly or in the opposite direction, additional pressure is applied to hydrostatic pressure either by annulus or control line. This further moves piston  190  downwardly increasing the nitrogen pressure above hydrostatic. The applied pressure is then reduced, though perhaps not to zero. Now the nitrogen pressure is greater than hydrostatic and any remaining surface applied pressure. Therefore, piston  190  moves upwardly due to the nitrogen pressure. Piston  190  applies a force on volume  200  of fluid between piston  190  and upper equalizing valve piston  198 . This force creates a pressure in volume  200  which is greater than hydrostatic pressure and any remaining surface applied pressure. The pressure acts on upper equalizing valve piston  198  between seals  202  and  204 . Upper face  206  of upper equalizing valve piston  198  acts on seal  204  that is mounted on shoulder  208  of upper seal mandrel  174 . Therefore, upper equalizing valve piston  198  pushes upper seal mandrel  174  upwardly or toward top body  182  until upper equalizing valve piston  198  contacts the lower edge  212  of equalizing spacer  210  whereupon upper equalizing valve piston  198  can move no further upwardly. Along with upper equalizing valve piston  198 , upper seal mandrel  174  and upper mandrel  172  are also forced upwardly as upper face  206  presses on shoulder  208  of upper seal mandrel  174 . Once upper equalizing valve piston  198  contacts lower edge  212  of equalizing spacer  210 , upper face  206  is separated from shoulder  208  and seal  204  so that seal  204  with upper face  206  is broken. The nitrogen pressure continues to operate on nitrogen piston  190  to move upwardly until pressure bleeds off in volume  200  past seal  204 . Any Page  17  of  30  Pages movement of upper mandrel  172  relative to upper equalizing valve piston  198  after seal  204  is broken tends to be very slight. As explained above, seal  204  was broken because upper equalizing valve piston  198  stops movement when upper face  206  thereon engages lower edge  212  but upper seal mandrel  174  continues to move slightly to open or break the seal therebetween. The end result is that nitrogen pressure is now charged to the same pressure as they hydrostatic pressure. Further operation of deployment valve is conducted by repeating the process, essentially, of applying pressure to open the valve and reducing pressure to close the valve. Some additional factors apply such as the indexing function that is explained hereinafter. The use of the nitrogen piston allows use of a single control line and/or use of hydrostatic control because the nitrogen piston provides a return force for closing the valve so that only an opening force need be applied from the surface. 
     Referring now to valve section  154  in FIGS. 8,  9 ,  10  and FIG.  11 . Upper mandrel  172  is rigidly connected to lower mandrel  220  by threads or other means and moves therewith as one piece. Lower mandrel  220  comprises upper actuator connector  222 , upper actuator extension  224 , and lower actuator extension  226 . Therefore, movement and force are transferred through upper mandrel  172  to lower mandrel  220 . Pressure is applied to the annulus or control line and, as explained hereinbefore upper mandrel  172  and hence lower mandrel  220  moves downward. Lower edge  228  (see FIG. 10 or FIG. 11) of lower actuator extension  226  applies a force to the top side of flapper  230 . The force creates a moment on flapper  230  that pivots around flapper pin  232 , rotating flapper  230  approximately ¼ turn around flapper pin  232  to the open position whereby the full bore of the casing string and/or liner is open for drilling operations. Lower actuator extension  226  is maintained in a downward position as shown for holding flapper  230  in the open position as shown in FIG.  10 . 
     When it is desired for flapper  230  to return to the closed position, the applied pressure is reduced and lower mandrel  220  moves upwardly as explained hereinbefore. Once lower actuator extension  226  is removed from the rotational path of flapper  230 , then flapper spring  234  creates a moment acting on flapper  230  pivoting around flapper pin  232  approximately ¼ turn to the closed position as shown in FIG.  1 . In the closed position, flapper  230  seals off bore  236  below flapper  230 . The seal is achieved by the upper face  238  of flapper  230  acting against lower end  240  of valve seat  242 . Seals on the valve seat may include elastomeric seals, metal seals, other types of seals or combinations thereof. FIG. 9 shows a cross-section view of flapper  230  in the closed position looking upwardly from the bottom. It will be noted that at least in this embodiment, flapper  230  is slightly eccentrically positioned such that a center line  231  of bore  236  is slightly off from a center line  233  of flapper  230 . While this flapper design is slighlty eccentric, it will be noted that this is not necessarily the case for all embodiments. Inother words, in some embodiments of the invention the flapper may be concentrically positioned with respect to the bore. Once the well is completed and valve operation is no longer desired, upper actuator extension  224  and lower actuator extension  226  can be operated for permanently leaving deployment valve  150  in the open position. 
     In a presently preferred embodiment of the invention, deployment valve is equipped with indexing sleeve  244  that allows deployment valve  150  to be held in the open position without having applied pressure acting on the tool either through the annulus or the control line. Indexing sleeve  244  is shown in FIG. 7, FIG. 12, and FIG.  13 . Indexing sleeve  244  is held on upper mandrel  172  between J-slot mandrel  180  and equalizing send mandrel  176 . Indexing sleeve  244  indexes deployment valve  150  by means of slot pattern  246  that is machined into and around the circumference thereof and by a set of J-slot pins  248  mounted to upper housing  188 . 
     Referring to FIG. 13, when pressure is applied to open deployment valve  150 , pattern  246  aligns with J-slot pins  248  so that J-slot pins  248  will maintain the valve in the open position with the loss or reduction of surface applied pressure. To close deployment valve  150 , applied pressure is reduced and then the pressure is reapplied to move slot pattern  246  to the free travel position allowing indexing sleeve  244  and upper mandrel  172  to move upwardly freely. For instance, with J-slot pin  248  at position  250  in slot pattern  246 , deployment valve  150  is closed. Pressure is applied to move J-slot pin  248  to position  252 . With J-slot pin  248  at position  252 , the deployment valve will be open as long as pressure is applied. However, upon release of pressure, J-slot pin  248  will move to position  254  whereupon deployment valve  150  is locked in the open position even with no applied pressure. To close deployment valve  150 , pressure is applied to move J-slot pin  248  to position  256  where the valve remains open so long as pressure is applied. Once pressure is now released, then J-slot pin  248  moves to position  258  and the valve is closed. Thus, indexing sleeve  244  rotates during operation with respect to housing  188 . Deployment valve  150  can be cycled through the positions of indexing sleeve as many times as necessary. 
     While the deployment valve of the present invention is highly suitable for use in underbalanced drilling as explained above, it will be appreciated that the deployment valve may find other uses including drilling even when the well is overbalanced for additional well control, or even non-drilling functions or production type uses. Since changes and modifications may be made in the disclosed embodiment without departing from the inventive concepts involved, it is the aim of this specification, drawings, and appended claims to cover all such changes and modifications falling within the spirit and the scope of the present invention.