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BACKGROUND 
   The present invention relates generally to operations performed and equipment utilized in conjunction with a subterranean well and, in an embodiment described herein, more particularly provides a formation isolation valve for use in underbalanced drilling applications. 
   A formation isolation valve is typically used in underbalanced drilling operations to close off flow through a casing string while tripping a drill string, or otherwise when access to a wellbore below the valve is not required. The valve is opened when the drill string or other assembly (such as wireline tools, coiled tubing string, etc.) needs to be displaced downwardly through the valve. The valve is then closed when the assembly is displaced upwardly through the valve. 
   Some formation isolation valves are operated hydraulically using control lines which extend to the surface. Pressure applied to the control lines at the surface is used to open and close such valves. However, these long control lines have significant disadvantages. For example, long control lines are expensive to purchase and install, long control lines have increased susceptibility to damage during installation and leakage thereafter, etc. 
   Some formation isolation valves are operated by physical contact between the valve and the assembly as it is displaced through the valve. The assembly may engage and shift a sleeve or other device which causes a closure member of the valve to open. This physical contact has the disadvantage that it usually requires relatively small clearance between the valve and the assembly, which leads to a restriction in the interior of the valve. 
   Therefore, it may be seen that improvements are needed in the art. It is one of the objects of the present invention to provide such improvements. These improvements may also be useful in applications other than formation isolation valves for underbalanced drilling. 
   SUMMARY 
   In carrying out the principles of the present invention, methods and systems are provided which solve at least one problem in the art. One example is described below in which an actuator for a downhole well tool is remotely activated without the use of long control lines extending to the surface. Another example is described below in which the actuator is remotely activated without requiring any physical contact between the well tool and an assembly displaced through the well tool. 
   In one aspect of the invention, a method of operating a well tool in a well is provided. The method includes the steps of: positioning the well tool in the well, the well tool including an actuator; positioning a power source for the actuator in the well; and at a downhole position in the well remote from the actuator, causing the actuator to operate the well tool. 
   In another aspect of the invention, a well tool operating system is provided which includes a well tool with an actuator positioned downhole in a well. A device for causing the actuator to operate the well tool is also positioned downhole in the well. However, the device is positioned remote from the actuator. 
   In yet another aspect of the invention, a system for operating a formation isolation valve is provided. The system includes the formation isolation valve interconnected in a casing string and positioned downhole in a well. An assembly displaces through the casing string, such that displacement of the assembly through the casing string causes the valve to open prior to the assembly reaching the valve. 
   These and other features, advantages, benefits and objects of the present invention will become apparent to one of ordinary skill in the art upon careful consideration of the detailed description of representative embodiments of the invention hereinbelow and the accompanying drawings, in which similar elements are indicated in the various figures using the same reference numbers. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic partially cross-sectional view of a method of operating a well tool, the method embodying principles of the present invention; 
       FIG. 2  is an enlarged scale schematic cross-sectional view of a device which may be used to remotely activate an actuator of a well tool in the method of  FIG. 1 ; 
       FIG. 3  is a schematic cross-sectional view of a well tool including an actuator which may be used in the method of  FIG. 1 ; and 
       FIG. 4  is a schematic cross-sectional view of an alternate construction of the device of  FIG. 2 . 
   

   DETAILED DESCRIPTION 
   Representatively illustrated in  FIG. 1  is a method  10  which embodies principles of the present invention. In the following description of the method  10  and other apparatus and methods described herein, directional terms, such as “above”, “below”, “upper”, “lower”, etc., are used for convenience in referring to the accompanying drawings. In general, the downward direction is illustrated as being further from the earth&#39;s surface along a wellbore, and the upward direction is illustrated as being toward the surface, but it will be appreciated by those skilled in the art that, in actual practice, wellbores are seldom consistently vertical. 
   Additionally, it is to be understood that the various embodiments of the present invention described herein may be utilized in various orientations, such as inclined, inverted, horizontal, vertical, etc., and in various configurations, without departing from the principles of the present invention. The embodiments are described merely as examples of useful applications of the principles of the invention, which is not limited to any specific details of these embodiments. 
   As depicted in  FIG. 1 , an assembly  12  is being displaced downwardly through a tubular string  14 . The assembly  12  is illustrated as comprising a drill string  16  having a drill bit  18  at a lower end. The drill string  16  may also include many other elements, such as a mud motor  20 , etc. 
   The tubular string  14  is illustrated as comprising a casing string  22  which is cemented in a wellbore  24 . As used herein, the term “casing string” is used to indicate any type of tubular string which is used to form a protective lining for a wellbore, and the term can include liner strings and other types of tubular strings made of any type of material. 
   A well tool  26  is interconnected in the casing string  22 . The well tool  26  is illustrated as comprising a formation isolation valve  28 . As the drill string  16  displaces downward toward the valve  28 , the valve opens prior to the drill string reaching the valve. 
   Although the method  10  is described as including the step of displacing the drill string  16  through the casing string  22  to operate the valve  28 , it should be clearly understood that this is only one example of an application of the principles of the invention. The assembly  12  is not necessarily a drill string (for example, the assembly could be a wireline conveyed tool, a coiled tubing string, or any other type of assembly). The assembly  12  does not necessarily have to be displaced through the tubular string  14 . The tubular string  14  is not necessarily a casing string (for example, the tubular string could be a production tubing string, a coiled tubing string, or any other type of tubular string). The well tool  26  is not necessarily a formation isolation valve or any other type of valve (for example, the well tool could be a choke, a packer, a pump, a hanger, or any other type of well tool). Thus, it will be appreciated that the method  10  is but one example of a very wide variety of uses for the principles of the invention. 
   One of the important features of the method  10  is that the valve  28  is remotely operated, so that direct physical contact is not required between the valve and the drill string  16 . Another important feature is that this remote operation is accomplished in the method  10  without requiring the use of long control lines extending from the surface to the valve  28 . 
   The remote operation is accomplished in the method  10  by interconnecting a device  30  in the casing string  22  above the valve  28 . For example, the device  30  may be remotely positioned a distance L 1  above the valve  28 . As the drill string  16  displaces through the device  30 , the device causes an actuator of the valve  28  to operate the valve. In this manner, the device  30  activates the actuator (thereby causing the valve  28  to open) prior to the drill string  16  reaching the valve. 
   Preferably, the drill string  16  includes a device  32  which interacts with the device  30  to activate the actuator of the valve  28 . The device  32  may be located a distance L 2  above the lower end of the drill bit  18 , with the distance L 2  being less than the distance L 1 , so that the devices  30 ,  32  interact to activate the actuator to open the valve, prior to the drill bit reaching the valve  28  (or a closure member of the valve). 
   When the drill string  16  is displaced upwardly through the valve  28 , the devices  30 ,  32  interact to activate the actuator to close the valve. In this manner, the valve  28  closes after the drill bit  18  has passed upwardly through the valve, thereby isolating a formation intersected by the wellbore  24  below the valve. 
   The device  30  is depicted in  FIG. 1  as being connected to the valve  28  using lines  34  extending between the device and the valve external to the casing string  22 . The lines  34  are described in more detail below as including hydraulic lines, but any type of communication between the device  30  and the valve  28  could be used (for example, pneumatic lines, electrical lines, optical lines, any form of telemetry (acoustic, electromagnetic, pressure pulse, etc.)) in keeping with the principles of the invention. It also is not necessary for the lines  34  to extend external to the casing string  22 , since they could also, or alternatively, extend internal to the casing string, within a sidewall of the casing string, etc., or the lines may not be used at all if telemetry is used to communicate between the device  30  and the valve  28 . 
   Referring additionally now to  FIG. 2 , an enlarged schematic cross-sectional view of one possible construction of the device  30  is depicted with the assembly  12  being displaced through the device. In this construction of the device  30 , a magnetic coupling is created between the assembly  12  and the device  30  in order to operate a power source  36  in the device. 
   The power source  36  includes a piston  38  reciprocably received in a bore  40  formed in an outer housing assembly  76  of the device  30 . Thus, in this embodiment the power source  36  is a pump used to create a pressure differential to operate the valve  28 . However, other types of power sources (such as electrical, mechanical, thermal, optical and other types of power sources) may be used in keeping with the principles of the invention. 
   The piston  38  is on a rod  42  which is attached to a cylindrical sleeve  44 . A stack of annular shaped magnets  46  is carried on the sleeve  44 . 
   The device  32  also includes a stack of annular shaped magnets  48  carried on the assembly  12 . When the assembly  12  is displaced through the device  30 , a magnetic coupling is created between the magnets  46 ,  48 . This magnetic coupling permits a biasing force to be transmitted between the devices  30 ,  32  without requiring any physical contact. 
   When the magnetic coupling is created as depicted in  FIG. 2  and the assembly  12  is displaced downward, a biasing force is exerted on the piston  38  (via the magnets  46 , sleeve  44  and rod  42 ) to also displace the piston downward. This downward displacement of the piston  38  in the bore  40  causes a pressure differential to be created between lines  50 ,  52  connected to the device  30 . 
   Specifically, pressure in the line  52  will be increased relative to pressure in the line  50 . Of course, if the assembly  12  is displaced upwardly through the device  30 , the magnetic coupling will be used to bias the piston  38  upward and thereby increase pressure in the line  50  relative to pressure in the line  52 . 
   The lines  50 ,  52  may be included in the lines  34  depicted in  FIG. 1 . Since these lines  50 ,  52  only extend a relatively short distance (for example, approximately 20-30 meters) between the device  30  and the valve  28 , they are significantly less susceptible to damage and leakage, and less expensive to purchase and install, as compared to control lines which extend perhaps thousands of meters to the surface. 
   Another beneficial feature of the device  30  is a balance piston  54  which ensures that pressure in an internal chamber  56  of the device  30  is equalized, via an opening  62 , with pressure in an internal passage  58  through which the assembly  12  is displaced. In this manner, a wall  60  separating the magnets  46 ,  48  can be made relatively thin (since it does not have to withstand a large pressure differential), thereby increasing the biasing force which may be transmitted by the magnetic coupling. 
   Although the devices  30 ,  32  are illustrated as including magnets  46 ,  48  for transmitting a biasing force to the pump  36 , these particular elements are not necessary in keeping with the principles of the invention. A magnetic field may be produced without the use of permanent magnets, for example, by using an electric coil, magnetostrictive materials, etc. A biasing force may be transmitted using a magnetic coupling without use of permanent magnets, for example, by using magnetostrictive materials, solenoids, etc. 
   Furthermore, it is not necessary for a magnetic coupling to be used at all. A construction is illustrated in  FIG. 4  and described below in which no magnetic coupling is used. 
   Referring additionally now to  FIG. 3 , a schematic cross-sectional view of the valve  28  is representatively illustrated. The valve  28  includes an actuator  64  and a closure  66  for selectively permitting and preventing flow and access through a passage  68  formed through the valve. 
   The actuator  64  includes a sleeve  70  reciprocably and sealingly received in an outer housing assembly  74  of the valve  28 . A radially enlarged piston  72  is formed on the sleeve  70 . The lines  50 ,  52  are connected to the actuator  64  so that they communicate to below and above the piston  72 , respectively. Thus, increased pressure in the line  52  relative to pressure in the line  50  will bias the sleeve  70  downward, and increased pressure in the line  50  relative to pressure in the line  52  will bias the sleeve upward. 
   The closure  66  includes a member  80  which functions to seal off the passage  68 . The member  80  is illustrated as being a flapper, but it could be any type of sealing member, such as a ball, etc. The member  80  is preferably biased toward a closed position as shown in  FIG. 3 , for example, by use of a biasing device (such as a spring, gas charge, etc., not shown). 
   With the sleeve  70  in its upper position, the closure  66  is closed. When pressure in the line  52  is increased relative to pressure in the line  50  (by downwardly displacing the piston  38  as described above), the sleeve will displace downward. This downward displacement of the sleeve  70  will cause the closure  66  to open, for example, by pivoting the member  80  so that it no longer blocks access and flow through the passage  68 . 
   When pressure in the line  50  is increased relative to pressure in the line  52  (by upwardly displacing the piston  38  as described above), the sleeve will displace upward. This upward displacement of the sleeve  70  will cause the closure  66  to close, for example, by allowing the member  80  to pivot across the passage  68  and again block flow and access through the passage. 
   A mechanism (not shown) may be provided for releasably maintaining the sleeve  70  in its upper and/or lower position. For example, a spring or other biasing device could be used to prevent the sleeve  70  from displacing downward due to its own weight when it is desired to keep the valve  28  closed. Alternatively, or in addition, a detent mechanism (such as a snap ring, collet, spring loaded detent, etc.) could be used to releasably secure the sleeve  70  in its upper and/or lower position. 
   Referring additionally now to  FIG. 4 , a schematic cross-sectional view of an alternate construction of the device  30  is representatively illustrated. This alternate construction is similar in many respects to the construction depicted in  FIG. 2 , and so the same reference numbers are used in  FIG. 4  to indicate similar elements. 
   One significant difference between the constructions depicted in  FIGS. 2 &amp; 4  is that, instead of the wall  60 , the construction of  FIG. 4  has a sleeve  82  reciprocably and sealingly received in the housing assembly  76 . The sleeve  82  is connected to the rod  42  so that the piston  38  displaces with the sleeve. 
   Another significant difference is that no magnetic coupling is used in the construction of  FIG. 4 . Instead, the assembly  12  biases the sleeve  82  to displace via engagement with a recessed profile  84  formed in the sleeve. The device  32  includes a key, dog or other engagement member  86  for engaging the profile  84 . 
   As the assembly  12  displaces downwardly through the device  30 , the member  86  engages the profile  84 , thereby transferring a downward biasing force from the assembly to the sleeve  82 . The piston  38  displaces downward with the sleeve  82 , thereby increasing pressure in the line  52  relative to pressure in the line  50  and causing the actuator  64  to open the closure  66 . The assembly  12  can then displace downward through the open valve  28 . 
   Upward displacement of the assembly  12  through the device  30  will again cause the member  86  to engage the profile  84 , thereby transferring an upward biasing force from the assembly to the sleeve  82 . The piston  38  will displace upward with the sleeve  82 , thereby increasing pressure in the line  50  relative to pressure in the line  52  and causing the actuator  64  to close the closure  66 . The valve  28  will thus close after the assembly  12  has displaced through the valve. 
   Multiple openings  62  may be used to provide communication between the passage  58  and the balance piston  54 . Filtering may be provided for the openings  62  to prevent debris, etc. from passing through the openings. 
   The alternate constructions of  FIGS. 2 &amp; 4  demonstrate that the invention may be practiced in a variety of different forms, and with or without use of a magnetic coupling. Use of the pump  36  to transfer fluid between the device  30  and the actuator  64  is also not required. For example, the actuator  64  could instead be an electrical actuator and the device  30  could include an electrical switch, so that when the assembly  12  displaces through the device, the switch is activated and causes electrical current to flow in the actuator to operate the valve  28 . 
   If a magnetic coupling is used, the magnetic coupling could be used to activate an electrical switch or other device, instead of a pump. 
   It is not necessary for magnets to be carried on the assembly  12  if a magnetic coupling is used. For example, a sleeve which carries magnets thereon could be reciprocably mounted in the casing string  22 . The magnets on this internal sleeve could be magnetically coupled to the magnets  46  carried on the sleeve  44  on an opposite side of the wall  60  (as in the construction of the device  30  depicted in  FIG. 2 ). The assembly  12  as depicted in  FIG. 4  could then be used to shift the internal sleeve (i.e., by engaging the member  86  with a profile formed in the sleeve) to cause displacement of the piston  38  or operation of an electrical switch, etc. to activate the actuator  64 . 
   Another alternate construction could be used in which a radioactive source is carried on the assembly  12 . The device  30  could include a radiation detector (for example, a gamma ray detector) to sense the presence of the radioactive source. When the radioactive source is detected, the device  30  could cause the actuator  64  to open or close the closure  66  as appropriate. 
   Another alternate construction could be used in which the device  30  includes a density sensor for detecting density in the passage  58 . When the density sensor senses an increased density (due to the presence of the assembly  12  in the passage  58 ), the device  30  could cause the actuator  64  to open the closure  66 . When the density sensor senses a decreased density (due to an absence of the assembly  12  in the passage  58 ) the device  30  could cause the actuator  64  to close the closure  66 . 
   Of course, a person skilled in the art would, upon a careful consideration of the above description of representative embodiments of the invention, readily appreciate that many modifications, additions, substitutions, deletions, and other changes may be made to these specific embodiments, and such changes are within the scope of the principles of the present invention. Accordingly, the foregoing detailed description is to be clearly understood as being given by way of illustration and example only, the spirit and scope of the present invention being limited solely by the appended claims and their equivalents.

Summary:
A formation isolation valve for underbalanced drilling applications. A system for operating a formation isolation valve includes the valve interconnected in a casing string. An assembly displaces through the casing string, thereby causing the valve to open prior to the assembly reaching the valve. An operating system includes a well tool with an actuator positioned downhole. A device for causing the actuator to operate the well tool is also positioned downhole remote from the actuator. A method of operating a well tool includes the steps of: positioning the well tool in a well, the well tool including an actuator; positioning a power source for the actuator in the well; and at a downhole position remote from the actuator, causing the actuator to operate the well tool.