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CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of the filing date of PCT International Application No. PCT/US00/12329, filed May 4, 2000. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to operations performed and equipment utilized in conjunction with subterranean wells and, in an embodiment described herein, more particularly provides a system for hydraulically controlling actuation of downhole tools. 
     BACKGROUND 
     It is very advantageous to be able to independently control well tools from the earth&#39;s surface, or other remote location. For example, production from one of several zones intersected by a well may be halted due to water invasion, while production continues from the other zones. Alternatively, one zone may be in communication with a production tubing string, while the other zones are shut in. 
     In order to control multiple downhole well tools, various systems have been proposed and used. One type of system utilizes electrical signals to select from among multiple well tools for operation of the selected tool or tools. Another type of system utilizes pressure pulses on hydraulic lines, with the pulses being counted by the individual tools, to select particular tools for operation thereof. 
     Unfortunately, these systems suffer from fundamental disadvantages. The systems which use electrical communication or power to select or actuate a downhole tool typically have temperature limitations for electrical circuitry thereof or are prone to conductivity and insulation problems, particularly where integrated circuits are utilized or connectors are exposed to well fluids. The systems which use pressure pulses are typically very complex and, therefore, expensive to manufacture and difficult to maintain. 
     From the foregoing, it can be seen that it would be quite desirable to provide a well control system which does not use electricity or complex pressure pulse counting mechanisms, but which provides a reliable, simple and cost effective means of controlling downhole tools. It is accordingly an object of the present invention to provide such a well control system and associated methods of controlling well tools. 
     SUMMARY 
     In carrying out the principles of the present invention, in accordance with an embodiment thereof, a well control system is provided which permits convenient control over the actuation of well tool assemblies in a well. The system permits independent control of individual ones of the well tool assemblies. Associated methods are also provided. 
     In one aspect of the present invention, a system for selectively actuating multiple well tool assemblies is provided. Multiple hydraulic lines are connected to the multiple well tool assemblies, with each of the hydraulic lines being connected to an actuation control module of each of the well tool assemblies. Each control module includes a selecting device and a fluid metering device. 
     The selecting device compares pressure on one of the hydraulic lines to a reference pressure source. The well tool assembly associated with the selecting device is selected when the pressure on the hydraulic line is greater than the reference pressure by a predetermined amount, but differs from the reference pressure by less than another predetermined amount. The predetermined amounts may be determined by relief valves of the selecting device interconnected between the hydraulic line and the reference pressure source. 
     The fluid metering device transfers fluid from the hydraulic line to an actuator of the associated well tool assembly in response to alternating pressure increases and decreases on another one of the hydraulic lines. The fluid transferring function is only performed when the well tool assembly is selected. 
     In another aspect of the present invention, an actuation control module is provided for selectively actuating a well tool assembly in a well. At least two hydraulic lines and a reference pressure source are connected to the control module. A selecting device of the control module includes two valves interconnected in series between one of the hydraulic lines and a fluid metering device of the control module. One of the valves opens when pressure on the hydraulic line is greater than a reference pressure by a first predetermined amount, and the other valve closes when pressure on the hydraulic line is greater than the reference pressure by a second predetermined amount. 
     The fluid metering device includes two pumps. One of the pumps transfers fluid from a first hydraulic line to an actuator of the well tool assembly in response to fluctuations in pressure on a second hydraulic line, and the other pump transfers fluid from the second hydraulic line to the actuator in response to fluctuations in pressure on the first hydraulic line. 
     In each case, the fluid is transferred via a different output of the control module, so that the actuator may be operated in a chosen manner by selecting which of the pumps is to be used. Selection of the pump to use is accomplished by merely applying a greater pressure to one of the hydraulic lines as compared to the other hydraulic line after the well tool assembly has been selected. 
     Each of the pumps includes a metering chamber having a known volume. Thus, a known volume of fluid may be transferred to the actuator, in order to produce a known displacement of a piston of the actuator. 
     In yet another aspect of the present invention, a method is provided for selectively controlling actuation of multiple well tool assemblies. The method includes the steps of positioning the well tool assemblies in a well; connecting first and second hydraulic lines to each well tool assembly; selecting one of the well tool assemblies for actuation thereof by applying a predetermined pressure to the first and second hydraulic lines; and actuating the selected well tool assembly by applying another greater pressure to one of the hydraulic lines. 
    
    
     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. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view of a method of selectively controlling the actuation of downhole tools, the method embodying principles of the present invention; 
     FIG. 2 is a schematic view of a first apparatus usable in the method of FIG. 1, the first apparatus embodying principles of the present invention, and the first apparatus being shown in a configuration prior to a well tool associated with the apparatus being selected for actuation thereof; 
     FIG. 3 is a schematic view of the first apparatus shown in a configuration subsequent to the selection of the well tool for actuation thereof in a first manner; 
     FIG. 4 is a schematic view of the first apparatus shown in a configuration subsequent to the well tool being deselected; 
     FIG. 5 is a schematic view of the first apparatus shown in a configuration subsequent to the selection of the well tool for actuation thereof in a second manner; 
     FIG. 6 is a schematic view of a second apparatus usable in the method of FIG. 1, the second apparatus embodying principles of the present invention; and 
     FIG. 7 is a schematic view of a third apparatus usable in the method of FIG. 1, the third apparatus embodying principles of the present invention. 
    
    
     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 only for convenience in referring to the accompanying drawings. 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. 
     In the method  10 , multiple well tool assemblies  12 ,  14 ,  16 ,  18  are positioned in a well. As depicted in FIG. 1, each of the well tool assemblies  12 ,  14 ,  16 ,  18  includes a well tool  20 , an actuator  22  for operating the well tool (not visible in FIG. 1, see FIGS. 2-7) and an actuation control module  24 . The well tool  20  of each of the assemblies  12 ,  14 ,  16 ,  18  representatively illustrated in FIG. 1 is shown as a valve, the valves being used in the method  10  for controlling fluid flow between formations or zones  26 ,  28 ,  30 ,  32  intersected by the well and a tubular string  34  in which the tool assemblies are interconnected. However, it is to be clearly understood that other types of well tools and well tool assemblies may be utilized, without departing from the principles of the present invention, and it, is not necessary for the well tool assemblies to be interconnected in a tubular string or for the well tool assemblies to be used for controlling fluid flow. 
     Each of the tool assemblies  12 ,  14 ,  16 ,  18  is connected to hydraulic lines  36 ,  38  extending from a hydraulic control unit  40  at the earth&#39;s surface or other remote location. The hydraulic control unit  40  is of the type well known to those skilled in the art which is capable of regulating fluid pressure on the hydraulic lines  36 ,  38 . The control unit  40  may be operated manually or by computer, etc., and may perform other functions as well. 
     Preferably, the tool assemblies  12 ,  14 ,  16 ,  18  are Interval Control Valves commercially available from Halliburton Energy Services, Inc. and welt known to those skilled in the art, which are useful in regulating fluid flow rate therethrough in the manner of flow chokes. That is, the valves  20  may each variably restrict fluid flow therethrough, rather than merely permit or prevent fluid flow therethrough, so that an optimal flow rate for each of the zones  26 ,  28 ,  30 ,  32  may be independently established. To vary the restriction to fluid flow, the Interval Control Valve includes a flow choking member which is displaced by a hydraulic actuator, such as the actuator  22  depicted schematically in FIGS. 2-7. 
     Referring additionally now to FIG. 2, an actuation control module  42  embodying principles of the present invention is representatively illustrated interconnected between two hydraulic lines  44 ,  46  and the actuator  22 . The control module  42  may be used for any of the control modules  24  in the method  10 , in which case the hydraulic lines  44 ,  46  would correspond to the hydraulic lines  36 ,  38  shown in FIG. 1, and the actuator  22  would correspond to an actuator of any of the well tools  20 . However, it is to be clearly understood that the control module  42  may be used in other methods and the actuator  22  may be that of another type of well tool, without departing from the principles of the present invention. 
     The control module  42  includes a selecting device  48  and a fluid metering device  50 . The selecting device  48  senses fluid pressure on the hydraulic line  46  and determines whether the control module  42  has been selected for actuation of its corresponding actuator  22 . This determination is accomplished by comparing the pressure on the hydraulic line  46  with a reference pressure source  52 . In this embodiment, and in the case where the control module  42  is used in the method  10 , the reference pressure source  52  is an annulus in the well external to the tubular string  34 . Thus, the selecting device  48  compares the pressure on the hydraulic line  46  to hydrostatic pressure in the annulus  52  to determine whether the control module  42  is selected for operation of its corresponding actuator  22 . 
     To make this determination, the selecting device  48  includes two shuttle valves  54 ,  56  and two relief valves  58 ,  60 . The shuttle valve  54  is normally open and is biased to the open position by a spring  62 . A similar spring  64  biases the shuttle valve  56  to a normally closed position. Only when both of the shuttle valves  54 ,  56  are open is fluid flow permitted from the hydraulic line  46  to the fluid metering device  50  for operation of the actuator  22 . Thus, the control module  42  is selected for operation of its corresponding actuator  22  when both of the shuttle valves  54 ,  56  are open. 
     Fluid pressure on the hydraulic line  46  biases a shuttle  66  of the valve  56  to the left as viewed in FIG. 2, which is toward an open position of the valve. However, for the shuttle  66  to displace to the left, pressure on the hydraulic line  46  must overcome the biasing force exerted by the annulus  52  pressure and open the relief valve  60 . That is, pressure on the hydraulic line  46  must be somewhat greater than the annulus  52  pressure plus the pressure rating of the relief valve  60 . Thus, the relief valve  60  is used in the control module  42  to set a lower limit pressure by which the pressure on the hydraulic line  46  must exceed the pressure on the annulus  52  for the control module to be selected. FIG. 4 depicts the configuration of the control module  42  when pressure on the hydraulic line  46  has exceeded the annulus  52  pressure plus the pressure rating of the relief valve  60 , the shuttle  66  being displaced to the left and opening the valve  56 . 
     In a similar manner, the shuttle valve  54  includes a shuttle  68  which is displaced to the left as viewed in FIG. 2 to close the valve. Pressure on the hydraulic line  46  must exceed the pressure on the annulus  52  plus the pressure rating of the relief valve  58  for the shuttle  68  to displace to the left. Thus, the relief valve  58  is used in the control module  42  to set an upper limit pressure by which the pressure on the hydraulic line  46  must not exceed the pressure on the annulus  52  for the control module to be selected. 
     Therefore, for the control module  42  to be selected, pressure on the hydraulic line  46  must exceed the annulus  52  pressure plus the pressure rating of the relief valve  60 , and must not exceed the annulus pressure plus the pressure rating of the relief valve  58 . It will be readily appreciated that, by varying the pressure ratings of the relief valves  58 ,  60 , different control modules  42  may be configured to have different ranges of pressures at which the individual control modules are selected. For example, the control module  24  of the tool assembly  12  in the method  10  may be configured so that it is selected when the pressure on the hydraulic line  38  is between 500 and 1,000 psi greater than the annulus  52  pressure, the control module of the tool assembly  14  may be configured so that it is selected when the pressure on the hydraulic line  38  is between 1,500 and 2000 psi greater than the annulus pressure, etc. Thus, each of the well tool assemblies  12 ,  14 ,  16 ,  18  may be independently selected by merely varying the pressure on the hydraulic line  38 . 
     The fluid metering device  50  is responsive to a differential between the pressures on the hydraulic lines  44 ,  46  to shift a spool valve  70  between one configuration in which fluid is metered from the hydraulic line  46  in response to alternating fluid pressure increases and decreases on the hydraulic line  44 , and another configuration in which fluid is metered from the hydraulic line  44  in response to alternating fluid pressure increases and decreases on the hydraulic line  46 . Thus, after the control module  42  has been selected by an appropriate pressure on the hydraulic line  46 , pressure on one of the hydraulic lines  44 ,  46  is varied to transfer fluid from the other hydraulic line to the actuator  22 . The hydraulic line on which the pressure is alternately increased and decreased determines whether a piston  72  of the actuator  22  is incrementally displaced to the right or to the left as viewed in FIG.  2 . 
     Displacement of the piston  72  in increments is particularly useful where, as in the method  10 , the actuator  22  is included in a well tool assembly used to variably restrict fluid flow therethrough. That is, incremental displacement of the piston  72  may be used to incrementally vary the rate of fluid flow through any of the tool assemblies  12 ,  14 ,  16 ,  18 , so that the flow rate may be optimized for each of the associated zones  26 ,  28 ,  30 ,  32 . 
     FIG. 5 depicts the configuration of the control module  42  when the module has been selected (i.e., pressure on the hydraulic line is within the range defined by the relief valves  58 ,  60 ) and pressure on the hydraulic line  46  exceeds pressure on the hydraulic line  44 . Note that a spool  74  of the valve  70  is shifted to the left as viewed in FIG.  5 . FIG. 3 depicts the configuration of the control module  42  when the module has been selected and pressure on the hydraulic line  44  exceeds pressure on the hydraulic line  46 . Note that the spool  74  is shifted to the right as viewed in FIG.  3 . 
     Taking the configuration of the control module  42  as depicted in FIG. 3 first, note that, with the spool  74  shifted to the right, the hydraulic line  44  is in fluid communication with a fluid metering chamber  78  having a floating piston  80  therein. The metering chamber  78  is also in fluid communication with the hydraulic line  46  via a check valve  82 , which permits flow from the hydraulic line  46  to the metering chamber, but prevents flow from the metering chamber to the hydraulic line  46 . A spring  84  biases the piston  80  upward, in a direction to draw fluid into the metering chamber  78  from the hydraulic line  46 . 
     An output of the metering chamber  78  is also in fluid communication with one side of the piston  72  in the actuator  22 . It wilt be readily appreciated that, when pressure above the piston  80  overcomes pressure below the piston in the metering chamber  78  plus the biasing force of the spring  84 , the piston  80  will displace downward, and fluid in the chamber will be forced into the actuator  22 , thereby displacing the piston  72  to the right as viewed in FIG.  3 . Since the metering chamber  78  has a known volume, the amount of fluid transferred from the metering chamber to the actuator  22  is known and produces a known displacement of the piston  72 . 
     To transfer the fluid from the metering chamber  78  to the actuator  22 , pressure on the hydraulic tine  44  is increased so that it exceeds pressure on the hydraulic line  46  (thereby shifting the spool  74  to the right), and is further increased until the biasing force of the spring  84  is overcome and the piston  80  is displaced downward. To transfer further fluid, pressure on the hydraulic line  44  is decreased, thereby permitting the spring  84  to displace the piston  80  upward and drawing further fluid into the metering chamber  78  from the hydraulic line  46 . In this step, pressure on the hydraulic line  44  should not be decreased to a level where it is less than pressure on the hydraulic line  46 , or the spool  74  would shift to the left. 
     Pressure on the hydraulic line  44  is then increased again so that the biasing force of the spring  84  is overcome and the piston  80  is again displaced downward, thereby transferring the fluid into the actuator  22 . It will be readily appreciated that the metering chamber  78 , piston  80 , spring  84  and check valve  82  make up a pump responsive to pressure fluctuations on the hydraulic line  44  to transfer fluid from the hydraulic line  46  to the actuator  22 . 
     Now taking the configuration of the control module  42  as depicted in FIG. 5 (i.e., the control module  42  being selected and pressure on the hydraulic line  46  exceeding pressure on the hydraulic line  44  as described above), note that, with the spool  74  shifted to the left, the hydraulic line  46  is in fluid communication with a fluid metering chamber  76  having a floating piston  86  therein. The metering chamber  76  is also in fluid communication with the hydraulic line  44  via a check valve  88 , which permits flow from the hydraulic line  44  to the metering chamber, but prevents flow from the metering chamber to the hydraulic line  44 . A spring  90  biases the piston  86  upward, in a direction to draw fluid into the metering chamber  76  from the hydraulic line  44 . 
     An output of the metering chamber  76  is also in fluid communication with one side of the piston  72  in the actuator  22 . It will be readily appreciated that, when pressure above the piston  86  overcomes pressure below the piston in the metering chamber  76  plus the biasing force of the spring  90 , the piston  86  will displace downward, and fluid in the chamber will be forced into the actuator  22 , thereby displacing the piston  72  to the left as viewed in FIG.  5 . Since the metering chamber  76  has a known volume, the amount of fluid transferred from the metering chamber to the actuator  22  is known and produces a known displacement of the piston  72 . 
     To transfer the fluid from the metering chamber  76  to the actuator  22 , pressure on the hydraulic line  46  is increased so that it exceeds pressure on the hydraulic line  44  (thereby shifting the spool  74  to the left), and is further increased until the biasing force of the spring  90  is overcome and the piston  86  is displaced downward. In this step, pressure on the hydraulic line  46  should not be increased to a level where it is outside the control module  42  range of selection pressure determined by the selecting device  48 . 
     To transfer further fluid, pressure on the hydraulic line  46  is decreased, thereby permitting the spring  90  to displace the piston  86  upward and drawing further fluid into the metering chamber  76  from the hydraulic line  44 . In this step, pressure on the hydraulic line  46  should not be decreased to a level where it is less than pressure on the hydraulic line  44 , or the spool  74  would shift to the right, and pressure on the hydraulic line  46  should not be decreased to a level where it is outside the control module  42  range of selection pressure determined by the selecting device  48 . 
     Pressure on the hydraulic line  46  is then increased again so that the biasing force of the spring  90  is overcome and the piston  86  is again displaced downward, thereby transferring the fluid into the actuator  22 . It will be readily appreciated that the metering chamber  76 , piston  86 , spring  90  and check valve  88  make up a pump responsive to pressure fluctuations on the hydraulic line  46  to transfer fluid from the hydraulic line  44  to the actuator  22 . 
     Referring again to FIG. 1, a preferred mode of selectively actuating the well tool assemblies  12 ,  14 ,  16 ,  18  is to increase pressure on both of the hydraulic lines  36 ,  38 , until the pressure is within the selection pressure range of at least one of the control modules  24 . Note that more than one control module  24  may be selected at one time, if desired, depending upon the pressure ratings of the relief valves in the selecting devices of the control modules. In addition, note that selection of the control module(s)  24  may be accomplished using pressure applied to only one of the hydraulic lines  36 ,  38  (for example, the hydraulic line  46  of the control module  42  embodiment depicted in FIGS.  2 - 5 ), if desired. 
     Pressure on one of the hydraulic lines  36 ,  38  is then made greater than pressure on the other of the hydraulic lines to thereby determine the manner of operating the associated actuator. Pressure on the hydraulic line  36  or  38  (whichever had the greater pressure thereon to determine the manner of operating the actuator) is then alternately increased and decreased to thereby transfer known volumes of fluid incrementally from the other hydraulic line to the actuator, producing incremental displacements of a piston of the actuator. 
     Referring additionally now to FIG. 6, an alternate configuration is representatively illustrated in which the pressure reference source is an accumulator  92 , instead of the annulus  52  as depicted in FIGS. 2-5. The accumulator  92  is connected to the relief valves  58 ,  60  in place of the connection to the annulus  52 . In addition, a restrictor  94  and a check valve  96  permit fluid flow between the accumulator  92  and the hydraulic line  46 , so that the accumulator is continuously equalized with the hydrostatic pressure of the hydraulic line  46 , but pressure on the hydraulic line  46  may be increased to shift the valves  54 ,  56  if desired. For this purpose, the restrictor  94  permits only very gradual equalization of pressure between the hydraulic line  46  and the accumulator  92 . 
     Referring additionally now to FIG. 7, an alternate configuration is representatively illustrated in which the pressure reference source is a third hydraulic line  98 , instead of the annulus  52  as depicted in FIGS. 2-5. The hydraulic line  98  is connected to the relief valves  58 ,  60  in place of the connection to the annulus  52 . The hydraulic line  98  provides an additional benefit in that the pressure on the hydraulic line  98  may be varied at a remote location to thereby influence the range of pressures on the hydraulic line  46  at which the control module  42  is selected. For example, the hydraulic line  98  may be connected to the hydraulic control unit  40  in the method  10  as depicted in FIG.  1 . 
     It is to be clearly understood that other types of reference pressure sources may be used in place of the annulus  52 , the accumulator  92  and the hydraulic line  98 , without departing from the principles of the present invention. 
     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 the specific embodiments, and such changes are contemplated by 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.

Summary:
A hydraulic control system for downhole tools enables convenient selection and actuation of a well tool assembly from among multiple well tool assemblies installed in a well. Each well tool assembly includes a control module having a selecting device and a fluid metering device. A predetermined range of pressure levels on one of multiple hydraulic lines causes the well tool assembly to be selected for actuation, a differential between pressure on that hydraulic line and pressure on another hydraulic line determines a manner of actuating the selected well tool assembly, and pressure fluctuations on one of the hydraulic lines causes fluid to be transferred from another hydraulic line to an actuator of the well tool assembly.