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BACKGROUND 
       [0001]    The invention generally relates to a downhole valve. 
         [0002]    Hydrocarbon fluid (oil or gas) typically is communicated from a subterranean well using a pipe, called a “production string.” The production string extends through a wellbore that is drilled through the producing formation and may include various valves for purposes of controlling the production of the hydrocarbon fluid. One such valve is a ball valve that may be operated for purposes of controlling the flow of the hydrocarbon fluid through the central passageway of the production string. Another valve that is typically part of a production string is a circulating valve, a valve that is operated to control the flow of the hydrocarbon fluid between the central passageway and the region outside of the string, called the “annulus.” 
         [0003]    A well may be in an underbalanced state, a state in which the pressure that is exerted by the formation is greater than the hydrostatic pressure that is exerted by the fluid in the annulus. One type of circulating valve that is used in an underbalanced well has a series of check valve elements through which well fluid is circulated for purposes of opening and closing the valve. A potential challenge in using such a circulating valve is that typically, the central passageway of the production tubing string above the valve must be filled with fluid in order to properly operate the valve. 
         [0004]    Another type of conventional circulating valve is remotely operated by communicating stimuli (pressure pulses, for example) into the fluid in the annulus near the valve. A sensor (a pressure sensor, for example) of the circulating valve detects the stimuli, and electromechanics of the valve typically decode commands from the stimuli and operate the valve accordingly. Although there is no requirement that the central passageway be filled with fluid for purposes of operating this type of circulating valve, the valve typically is not suitable for use in a high pressure high temperature (HPHT) environment due to temperature limitations of the valve. 
       SUMMARY 
       [0005]    In an embodiment of the invention, a tool that is usable with a well includes a valve element, a mechanical operator, a pressure chamber and a regulator. The valve element has a first state and a second state. The mechanical operator responds to a predetermined signature in an annulus pressure relative to a baseline level of the annulus pressure to transition the valve element from the first state to the second state. The pressure chamber exerts a chamber pressure to bias the mechanical operator to transition from the second state to the first state. The baseline level is capable of varying over time, and the regulator regulates the chamber pressure based on the baseline level. 
         [0006]    In another embodiment of the invention, a tool that is usable with a well includes a valve element having a first state and a second state. The tool includes a spring, a pressure chamber and a mechanical operator. The mechanical operator responds to forces exerted in concert by the spring and the pressure chamber to bias transitioning of the valve element from the first state to the second state, and the mechanical operator responds to annulus pressure to transition the valve element from the second state to the first state. 
         [0007]    In yet another embodiment of the invention, a tool that is usable with a well includes a valve element, a first mechanical operator, a pilot valve and a second mechanical operator. The valve element has a first state and a second state. The pilot valve controls communication of an annulus pressure to the first mechanical operator; and the second mechanical operator responds to the annulus pressure to control operation of the pilot valve. The second mechanical operator is adapted to cause the pilot valve to communicate the annulus pressure to the first mechanical operator to cause the first mechanical operator to transition the valve element from the first state to the second state in response to the annulus pressure exhibiting a predetermined signature and otherwise block the communication of the annulus pressure to the first mechanical operator to cause the first mechanical operator to transition the valve element from the second state to the first state. 
         [0008]    Advantages and other features of the invention will become apparent from the following drawing, description and claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0009]      FIG. 1  is a schematic diagram of a subterranean well according to an example. 
           [0010]      FIG. 2  is a schematic diagram of a circulating valve tool according to an example. 
           [0011]      FIG. 3  is a more detailed cross-sectional view of a mechanical operator section of the tool of  FIG. 2  according to an example. 
           [0012]      FIGS. 4 and 5  are schematic diagrams of other examples of circulating valve tools. 
           [0013]      FIG. 6  is a schematic diagram of a hydraulic circuit of the circulating valve tool of  FIG. 5  when the tool is in a first state. 
           [0014]      FIG. 7  is a schematic diagram of a hydraulic circuit of the valve of  FIG. 5  when the tool is in a second state. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    In the following description, numerous details are set forth to provide an understanding of the present invention. However, it will be understood by those skilled in the art that the present invention may be practiced without these details and that numerous variations or modifications from the described embodiments are possible. 
         [0016]    As used here, the terms “above” and “below”; “up” and “down”; “upper” and “lower”; “upwardly” and “downwardly”; and other like terms indicating relative positions above or below a given point or element are used in this description to more clearly describe some embodiments of the invention. However, when applied to equipment and methods for use in wells that are deviated or horizontal, such terms may refer to a left to right, right to left, or diagonal relationship as appropriate. 
         [0017]    Referring to  FIG. 1 , in accordance with an example, a well  10  includes a wellbore  20 , which may be lined with a casing string  22  that supports the wellbore  20 . As other examples, the wellbore  20  may be only partially cased by a wellbore or may be entirely uncased. A tubular string  30  extends downhole into the wellbore  20  through one or more production or injection zones of the well  10  for purposes of facilitating the production of fluids from the well  10  and/or the injection of fluids into the well  10 . It is noted that although  FIG. 1  depicts the string  30  as being disposed in a main vertical wellbore, the wellbore  20  may be a lateral wellbore, in accordance with other examples. Furthermore, although  FIG. 1  depicts a subterranean terrestrial well, the systems, techniques, tools and systems that are described herein may likewise be applied to subsea wells. 
         [0018]    In general, the string  30  includes at least one valve assembly, such as a circulating valve tool  50  that is depicted in  FIG. 1 . For purposes of example, the tool  50  may be a multiple cycle tool, which means that the tool  50  is constructed to be opened and closed numerous times. It is noted that the string  30  may includes other types of valve assemblies (a ball valve assembly, for example), which may employ the control systems and techniques that are disclosed herein, in accordance with other examples. 
         [0019]    For the following example, it is assumed that the well  10  is an underbalanced state, although this condition is not a prerequisite for the use of the tool  50 . In the underbalanced state, the pressure that is exerted by the formation is greater than the hydrostatic pressure that is exerted by the fluid in an annulus  54 , which is the annular region of the well  10  between the borehole wall or well casing string  22  (depending on whether the well  10  is cased or uncased) and the exterior of the tool  50 . In general, the tool  50  is operated by manipulating a pressure in the annulus  54 . As examples, the annulus pressure may be manipulated using a surface-disposed pump  12 , although other systems and techniques may be used to induce pressure fluctuations in the annulus  54  for purposes of controlling the tool  50 , as can be appreciated by one of skill in the art. 
         [0020]    To operate the tool  50 , pressure stimuli may be communicated from the surface of the well  10  downhole into the annulus  54  for purposes of delivering a command to the tool  50 , such as a command to open fluid communication through radial ports  100  of the tool  50  or a command to close the fluid communication through the radial ports  100  to isolate the annulus  54  from the central passageway of the string  30 , as non-limiting examples. As more specific examples, the communication of the pressure stimuli may involve momentarily increasing the pressure in the annulus  54  above a baseline annulus pressure level; momentarily decreasing the annulus pressure below the annulus baseline pressure level; a series of annulus pressure increases or decreases; etc. 
         [0021]    In one control scheme, a sequence of pressurization cycles may be applied to the annulus  54  to operate the tool  50 . The pressurization cycles may include cycles (called “up cycles”) in which the annulus pressure is increased and cycles (called “down cycles”) in which the annulus pressure is relaxed or decreased back to the annulus baseline level. In this manner, a particular number of up and down pressurization cycles may be used for purposes of transitioning the tool  50  from its closed state to its open state, and vice versa. 
         [0022]    As described herein, the tool  50  includes a mechanical operator  130 , which responds to the fluid pressure in the annulus  54 . Unlike conventional arrangements, the actuation of the mechanical operator  130  does not depend on whether a full column of fluid exists in the central passageway of the string  30 , and the operation of the mechanical operator does not involve circulating well fluid through the tool  50 . Instead, as described herein, the tool  50  communicates the annulus pressure to the mechanical operator  130  for purposes of transitioning the tool  50  from a first state (an open or closed state, as non-limiting examples) to a different, second state (an open or closed state, as non-limiting examples). 
         [0023]    As further described herein, a gas chamber  134  of the tool  50  exerts a force to counter the force that is produced by the annulus pressure (e.g., to bias the tool  50  to remain in the first state or return to the first state from the second state). The tool  50  has features to compensate the force that is exerted by the gas chamber  134  for purposes of causing this force to track the baseline pressure level of the annulus. In this way, the gas chamber accommodates downhole pressure and temperature fluctuations, which may otherwise adversely affect the operation of the tool  50 . 
         [0024]      FIG. 2  depicts a partial cross-sectional view of the tool  50 , in accordance with a non-limiting example. Although  FIG. 2  depicts a simplified, right-hand cross-sectional view of the tool  50  (on the right hand side of a longitudinal axis  51  of the tool  50 ), as can be appreciated by one of skill in the art, the tool  50  is generally symmetrical about the longitudinal axis  51 , with the corresponding mirroring left-hand cross-section generally not being depicted in  FIG. 2 . 
         [0025]    Referring to  FIG. 2  in conjunction with  FIG. 1 , the tool  50  includes a generally tubular outer housing  99 , which is generally coaxial with the longitudinal axis  51  and is designed to connect in line with the string  30 . The outer housing  99  includes a central passageway  90  that is in fluid communication with the corresponding central passageways of the string sections above and below the valve assembly  50 . The tool  50  includes a circulating valve element  107 , which includes the radially-disposed flow ports  100 , which are formed in the housing  99 . 
         [0026]    In the open state of the circulating valve element  107  (and tool  50 ), fluid communication is established between the annulus  54  (see  FIG. 1 ) and the central passageway  90  through the flow ports  100 . In this open state, an internal sleeve  104  of the circulating valve element  107  is in its downward position of travel (as depicted in  FIG. 2 ), which means that the flow ports  100  are above the highest o-ring  106  on the sleeve  104  (i.e., the sleeve  104  and its associated o-rings do not block the radial flow). 
         [0027]    For the closed state (not depicted in  FIG. 2 ) of the valve element  107  (and tool  50 ), the sleeve  104  is near or at the uppermost point of travel such that the flow ports  100  are disposed between the o-rings  106  to therefore block fluid communication between the central passageway  90  and the annulus  54 . 
         [0028]    The up and down travel of the sleeve  104  is controlled by the mechanical operator  130  of the tool  50 . In general, the operator  130  includes a piston head  140 , which is connected through a mandrel  105  to the sleeve  106 . In general, the piston head  140  is concentric with the sleeve  104  and has a central passageway to form part of the central passageway  90  of the tool  50 . The piston head  140  moves up and down in response to a pressure differential between upper and lower gas chambers: the gas chamber  134  (called the “upper chamber  134 ” below), which exerts a downward force on an upper surface of the piston head  140  and a gas chamber  135  (called the “lower chamber  135 ” below), which exerts an upward force on a lower surface of the piston head  140 . The upper  134  and lower  135  chambers reside inside a corresponding annular recess of the housing  99 . 
         [0029]    The volumes of the upper  134  and lower  135  gas chambers are variable in that the volume of the upper chamber  134  is maximized and the volume of the lower chamber  135  is minimized (as depicted in  FIG. 2 ) in the open state of the tool  50 ; and the volume of the upper chamber  134  is minimized, and the volume of the lower chamber  135  is maximized in the closed state of the valve  50 . The upper  134  and lower  135  chambers contain an inert gas (Nitrogen, for example); and the differential pressure between the upper  134  and lower  135  chambers control the upward and downward movement of the piston head  140 , and thus, control the upper and downward movement of the sleeve  104 . The lower chamber  135  is in fluid communication with another gas chamber  146  via a gas passageway  147 . 
         [0030]    The gas chamber  146  is part of a compensator  150 , which transfers the annulus pressure to the gas chamber  146  while isolating the gas chamber  146  from the well fluid in the annulus  54 . More specifically, the compensator  150  includes a floating compensating piston  148 , which resides in an annular recess of the housing  99  to form the gas chamber  146  above the piston  148  and a chamber  149  below the piston  148 , which receives annulus fluid communicated from one or more radially-disposed ports  160  (one port being shown in  FIG. 2 ) that are formed in the outer housing  99 . Thus, in general, via the ports  160 , well fluid enters the chamber  149  and exerts upward pressure on the compensating piston  148 . In response to this pressure, the compensating piston  148  pressurizes the gas in the gas chamber  146 , which in turn, produces an upward force on the piston head  140 . 
         [0031]    As described in more detail below, a valve control network is built into the piston head  140  to allow equalization of pressures between the upper  134  and lower  135  gas chambers. However, the equalization occurs at a controlled rate for purposes of permitting pressure differentials to develop to act on the piston head  140 . More specifically, the flow rate between the gas chambers  134  and  135  is initially limited when the annulus pressure first changes with respect to its steady state baseline pressure level. This limited flow rate, in turn, produces a set upward or downward force on the piston head  140 . 
         [0032]    For example, in response to an increase in annulus pressure, the pressure in the chamber  135  exceeds the pressure in the chamber  134  to cause an upward force on the piston head  140 . As the piston head  140  moves upwardly, the pressures between the chambers  134  and  135  equalize to create a balanced condition after the piston head  140  is shifted to an upper position. 
         [0033]    When the annulus pressure subsequently decreases, a downward force is initially produced on the piston head  140  due to the momentary differential pressure. Due to the valve system in the piston head  140 , the pressures generally equalize so that when the piston head  140  reaches a point near its lowermost position of travel (as depicted in  FIG. 2 ), a balanced condition once again rises. Due to the above-described pressure balancing, the gas pressure in the tool  50  adjusts to the baseline annulus pressure level; and as such, the gas charge is compensated for shrinkage or expansion due to thermal changes and changes in the annulus pressure. 
         [0034]    Among the other features of the tool  50 , in accordance with some examples, the tool  50  includes an indexer  110  to control the sequence of annulus pressurization cycles for purposes of causing the tool  50  to change states. As a non-limiting example, the indexer  110  may a J-slot mechanism, in which a pin on the operator mandrel  105  traverses a J-slot that has a predetermined pattern that restricts the travel of the operator mandrel  105  until the end of the pattern is reached. In other words, the J-slot establishes a predetermined number up/down pressurization cycles that must occur before the tool  50  transitions from a closed state to an open state. Once at the end of the pattern, the indexer  110  may be reset by releasing pressure on the annulus to move the operator mandrel  105  back to its lowermost point of travel to close the tool  50 . 
         [0035]    The tool  50  may include a mechanism  120  to restrict all motion of the operator mandrel  105  until a predetermined force on the piston head  140  (and operator mandrel  105 ) builds up. This allows the pressure differential across the piston head  140  to increase to a predetermined threshold before the operator mandrel  105  shifts for purposes of increasing the tool shifting speed to avoid leaving the tool  50  in an undesirable mid state (never fully opened or fully closed, for example). In accordance with some examples, the mechanism  120  may be a collet, which includes a plurality of fingers that engage corresponding features on the operator mandrel  105  to secure the operator mandrel  105  in place until the predetermined force threshold is reached. The fingers on the collet hold the operator mandrel  105  in its original position until the pressure differential across the piston head  140  is sufficiently high to overcome the grasp of the collet fingers and quickly shift the operator mandrel  105  all the way to the end position. 
         [0036]    Referring to  FIG. 3 , the piston head  140  may include an embedded valve system, which includes a first flow path  190  for purposes of communicating gas pressure from the lower chamber  135  to the upper chamber  134 . This flow path includes a flow restrictor  210  and a check valve  200 . In this arrangement, when the pressure in the lower chamber  135  exceeds the pressure in the upper chamber  134 , the check valve  200  opens to permit a bleed flow between the chambers  134  and  135 . The flow restrictor  210  ensures that the flow rate is relatively small to create a pressure differential to produce an upward force on the piston head  140 . After the piston head  140  has traveled upwardly by a sufficient distance, a radial crosshole  204 , which is in communication with the above-described communication path bypasses a seal that is created by an upper o-ring  212  to bypass the flow restrictor  210  and allow relatively fast equalization of the pressure between the upper  134  and lower  135  chambers. 
         [0037]    In a similar arrangement, a metered flow path  191  is disposed in the piston head  140  for purposes of equalizing pressures in the chambers  134  and  135  for the scenario in which the lower chamber  135  is de-pressurized due to a decrease in the annulus pressure. This flow path  191  includes a flow restrictor  208  and a check valve  206 , which is constructed to open to allow communication through the flow restrictor  208  between the chambers  134  and  135  when the pressure in the upper chamber  134  is greater than the pressure in the lower chamber  135 . Due to the metering by the flow restrictor  208 , a downward force is created while the pressures in the chambers  134  and  135  are being equalized. After the piston head  130  has traveled downwardly by a sufficient distance, a cross hole  207 , which is in communication with the passageway travels past the seal created by a lower o-ring  214  to therefore bypass the flow restrictor  208  to allow relatively rapid equalization of the chamber pressures. 
         [0038]    Thus, due to the above-described valve system in the piston head  140 , the pressure in the upper chamber  134  tracks the baseline pressure level in the annulus  54  to compensate its gas pressure for shrinkage or expansion due to thermal changes and changes in the annulus pressure. 
         [0039]      FIG. 4  depicts a circulating valve tool  250  in accordance with other another example. Similar to the tool  50 , the tool  250  includes a mechanical operator that responds to pressure changes in the annulus  54 , without requiring a full column of fluid in the tubing string and without requiring circulation of well fluid through the tool  250 . However, unlike the tool  50 , the tool  250  does not use a gas chamber that equalizes its pressure with the baseline annulus pressure. Instead, the tool  250  includes a gas chamber  264  that has a fill port to store a predetermined charge of inert gas (Nitrogen gas, for example), which is used for purposes of operating a circulating valve element  252  of the tool  250 . 
         [0040]    More specifically, the combination of pressure from the gas chamber  264  and a spring  260  (a Belleville spring or bellows spring, as non-limiting examples) produces an upward force on a power piston head  258 . The power piston head  258 , in turn, is connected by way of an operator mandrel  254  to the circulating valve element  252 . As also shown in  FIG. 4 , the tool  250  may include an indexer  270  to establish a predefined up and down transition cycle in order to change the state of the circulating valve  252 . The upper surface of the piston  258  is exposed through radial ports  256  to the annulus pressure. Therefore, the piston  258  moves downwardly in response to increasing pressure in the pressure stimuli, and when the pressure relaxes, the upward force provided by the compressed spring  260  and the gas pressure exerted by the gas chamber  264  produce a force in concert to move the piston  258  in an upward direction. 
         [0041]    Other variations are contemplated and are within the scope of the appended claims. For example, the valve assembly  250  may include a retention mechanism, such as the above-described collet, for purposes of storing energy and ensuring a fast valve opening, which avoids half states and overcomes the effects of erosion. 
         [0042]      FIG. 5  depicts a circulating valve tool  300  in accordance with another example. The tool  300  has a similar design, in some aspects, relative to the tool  50 , in that the tool  300  has upper  320  and lower  326  gas chambers, an operator piston  324  and indexer  314 , similar in design to the upper  134  and lower  135  gas chambers, piston  130  and indexer  110 , respectively, of the tool  50 . In this regard, the lower gas chamber  326  has pressure that is derived by a compensator from the annulus pressure (not depicted in  FIG. 5 ). However, unlike the tool  50 , the valve assembly  300  does not use the gas pressure to drive an operator mandrel for purposes of opening and closing a circulating valve element  302  of the tool  300 . Instead, the tool  300  uses the annulus pressure for purposes of operating the circulating valve element  302 . 
         [0043]    More specifically, the piston  324  may be connected to operator a pilot valve  312 , which controls the application of annulus pressure to a power piston  304 , which, in turn, operates the circulating valve  302 . As shown in  FIG. 5 , the system to control the power piston  304  includes a pilot valve  312  (connected to the piston  320 ), a hydrostatic chamber  308  and a dump chamber  306 . 
         [0044]    Operation of the tool  300  may be better understood with reference to  FIGS. 6  (depicting the power piston  304  at its uppermost position of travel) and  7  (depicting the power piston  304  at its lowermost position of travel). Referring to  FIG. 6 , annulus pressure is always applied to an upper chamber that is communication with an upper face of the power piston  304 . The lower face of the piston  304 , in turn, is connected either to the dump chamber  306  or to the hydrostatic chamber  308 , as depicted in  FIG. 6 . When an operator section  322  (that contains the piston  320 ) configures the pilot valve  312  to connect the lower chamber to the hydrostatic chamber  308 , the power piston  304  moves upwardly, as depicted in  FIG. 6 . As depicted in  FIG. 7 , when the operator section  322  configures the pilot valve  312  to connect the lower chamber to the dump chamber  306 , then the power piston  304  moves to the lower position as depicted in  FIG. 7 . It is noted that the number of up and down cycles to effect a transition of the power piston  304  is controlled by the capacity of the dump chamber  306 . 
         [0045]    While the present invention has been described with respect to a limited number of embodiments, those skilled in the art, having the benefit of this disclosure, will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.

Summary:
A tool that is usable with a well includes a valve element, a mechanical operator, a pressure chamber and a regulator. The valve element has a first state and a second state. The mechanical operator responds to a predetermined signature in an annulus pressure relative to a baseline level of the annulus pressure to transition the valve element from the first state to the second state. The pressure chamber exerts a chamber pressure to bias the mechanical operator to transition from the second state to the first state. The baseline level is capable of varying over time, and the regulator regulates the chamber pressure based on the baseline level.