Patent Publication Number: US-9845660-B2

Title: Pressure responsive downhole tool having a selectively activatable pressure relief valve and related methods

Description:
The present application is a U.S. National Stage patent application of International Patent Application No. PCT/US2012/071816, filed on Dec. 27, 2012, the benefit of which is claimed and the disclosure of which is incorporated herein by reference in its entirety. 
     FIELD OF THE INVENTION 
     The present invention relates generally to pressure responsive tools and, more specifically, to a pressure responsive downhole tool (e.g., drill stem tester valve) having an operating element (ball valve, for example) that is only open when a power piston pressure relief valve is activated. 
     BACKGROUND 
     Conventional tester valves, such as the Select Tester® Valve commercially offered by Halliburton Energy Services, Co., utilize a pressure relief valve in the power piston to control whether the annulus pressure application is considered to be a normal opening pressure or a Lock Open operating pressure. For example, in conditions of 12,000 psi hydrostatic pressure and 300° F., the normal operating pressure is approximately 1400 psi and the Lock Open pressure is roughly 1300 psi higher at 2700 psi. 
     Such designs can be problematic. For example, the pressure relief valve in the power piston is normally in the range of about 1250 psi. Thus, if the ball valve has high friction due to wear or high pressure differential, the pressure required to open the ball valve and unlatch the collets at the same time, could exceed the 1250 pressure relief in the piston. Therefore, instead of the ball valve opening, the tester valve will index forward into the Lock Open position. Ultimately, when the annulus pressure is bled off, the ball valve will still be closed; but, the selector will be in the Lock Open position. If this is the case, the operator will think the valve is normally closed when, actually, it never opened but, instead, has indexed to the Lock Open position. 
     Moreover, if the friction on the ball mechanism is between the 1250 psi pressure relief pressure and the applied operating pressure, the ball can be opened after the tool has indexed forward. When the annulus pressure is released, the tool will again be unexpectedly in the Locked Open position. Therefore, without ever going above the normal operating pressure, it is possible to put the tool in the Locked Open position. If the tester valve is being utilized to perform a downhole closure, it will not close. If an emergency happens and the tester valve is expected to close in the well downhole, again, it will not. It will require a pressure cycle to the high Lock Open value to return the tool to normal functioning. Such an operation may take 30 minutes minimum to perform. 
     Accordingly, in view of the foregoing, there is a need in the art for a tester valve having a pressure relief valve that is only active when the ball valve is in the open position. Therefore, if high pressure is required to get the ball open, the ball must open before the pressure relief valve can open and place the tool in the Lock open position. Such a tool would avoid inadvertent Lock Open positions whereby the operator believes that bleeding off the annulus pressure will close the ball, when in fact the tool is in the Locked Open position. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A-1I  are sectional views of an annular pressure responsive downhole tool having a selectively activatable pressure relief valve, in accordance to certain exemplary embodiments of the present invention; 
         FIG. 2  illustrates an exploded view of a power piston having a selectively activatable pressure relief valve, in accordance to certain exemplary embodiments of the present invention; and 
         FIG. 3  is an exterior view of a portion of a ratchet sleeve constructed in accordance to certain exemplary embodiments of the present invention. 
     
    
    
     DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     Illustrative embodiments and related methodologies of the present invention are described below as they might be employed in a pressure responsive downhole tool having a selectively activatable power piston pressure relief valve. In the interest of clarity, not all features of an actual implementation or methodology are described in this specification. Also, the “exemplary” embodiments described herein refer to examples of the present invention. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. Further aspects and advantages of the various embodiments and related methodologies of the invention will become apparent from consideration of the following description and drawings. 
     As described herein, exemplary embodiments of the present invention are directed to a pressure responsive downhole tool having a power piston pressure relief valve that may be selectively deactivated and activated to allow operations to be conducted using the tool. The pressure responsive downhole tool may be a variety of tools, such as, for example, a tester valve as described in U.S. Pat. No. 5,558,162, entitled “MECHANICAL LOCKOUT FOR PRESSURE RESPONSIVE DOWNHOLE TOOL,” also owned by the Assignee of the present invention, Halliburton Energy Services, Co. of Houston, Tex., the disclosure of which is hereby incorporated by reference in its entirety. As such, the inventive features described herein will be discussed in relation to a drill stem tester valve. However, those ordinarily skilled in the art having the benefit of this disclosure realize the present invention may be applied to any variety of pressure responsive tools. 
     As further described herein, exemplary embodiments of the pressure responsive tool include a power piston pressure relief valve having a vent port that vents between two annular seals positioned around the outer diameter of the power piston. In embodiments utilized within a drill stem tester valve, during downhole deployment of the tool, the ball valve assembly is in the closed position and the two seals of the power piston seal against the tool housing, thus maintaining the pressure relief valve in a deactivated position. As the ball valve is opened, annular pressure is applied to the power piston which moves the lower annular seal into a slot along the inner diameter of the tool housing, thus allowing pressure to vent around the seal to activate the pressure relief valve. Activation of the pressure relief valve can only happen if the ball valve is open. Thereafter, drill stem testing may be conducted as understood in the art. When it is desired to deactivate the pressure relief valve, the annular pressure is bled off and the power piston moves back to the deactivated position, which also actuates the ball valve back to the closed position. Accordingly, the pressure relief valve is only open when the ball valve is in the open position, thus avoiding any inadvertent Locked Open positions. 
     Referring now to  FIGS. 1A-1I , an annular pressure responsive tool  10  will now be described in accordance to one or more exemplary embodiments of the present invention. As previously described, annular pressure responsive tool  10  may be, for example, a drill stem tester valve. For example, annular pressure responsive tool  10  may be used with a formation testing string during the testing of an oil well to determine production capabilities of a subsurface formation. The testing string will be lowered into a well such that a well annulus is defined between the test string and the well bore hole. A packer associated with the annular pressure responsive tool  10  will be set in the well bore to seal the well annulus below the power port  214  of valve  10 , as hereinafter described in detail, which is then subsequently operated by varying the pressure in the well annulus. 
     Referring now to  FIGS. 1A-1I  of the present invention, the annular pressure responsive tool  10  includes a housing  12  having a central flow passage  14  disposed longitudinally therethrough. Housing  12  includes an upper adapter  16 , a valve housing section  18 , a ported nipple  20 , power housing section  22 , connector section  24 , an upper gas chamber housing section  26 , a gas filler nipple  28 , a lower gas chamber housing section  30 , a metering cartridge housing  32 , a lower oil chamber housing section  34  and a lower adapter  36 . The components just listed are connected together in the order listed from top to bottom with various conventional threaded and sealed connections. The housing  12  also includes an upper inner tubular member  38 , an inner connector  40 , and a lower inner tubular member  42 . Upper inner tubular member  38  is threadedly connected to gas filler nipple  28  at thread  44  and sealingly received within bore  46  to be affixed to inner connector  40  below. Lower gas chamber housing  30  is attached to inner connector  40  at thread  47 . O-ring seals  49  seal the connections, as understood in the art. Lower inner tubular member  42  is threadedly connected to inner connector  40  at thread  48 . Lower inner tubular member  42  is sealingly received within a bore  50  of lower adapter  36  with an O-ring seal  52  being provided therebetween. 
     An upper seat holder  54  is threadedly connected to upper adapter  16  at thread  56 . Upper seat holder  54  has a plurality of radially outward extending splines  58  which mesh with a plurality of radially inward extending splines  60  of valve housing section  18 . Upper seat holder  54  includes an annular upward facing shoulder  62  which engages lower ends  64  of splines  60  of valve housing section  18  to thereby hold valve housing section  18  in place with the lower end of upper adapter  16  received in the upper end of valve housing section  18  with a seal  66  being provided therebetween. An annular upper valve seat  68  is received in upper seat holder  54 , and a spherical ball valve (i.e., operating element)  70  engages upper seat  68 . Ball valve  70  has a bore  72  disposed therethrough. In  FIG. 1 , ball valve  70  is shown in its open position so that the bore  72  of ball valve  70  is aligned with the longitudinal flow passage  14 , or bore, of annular pressure responsive tool  10 . As will be further described below, when ball valve  70  is rotated to its closed position, the bore  72  is isolated from the central flow passage  14  of annular pressure responsive tool  10 . 
     In this exemplary embodiment, ball valve  70  is held between upper seat  68  and a lower annular seat  74 . Lower annular seat  74  is received in a lower seat holder mandrel  76 . The lower seat holder mandrel  76  is a cylindrical cage-like structure having an upper end portion  78  threadedly connected to upper seat holder  54  at thread  80  to hold the two together with the ball valve  70  and seats  68 , 74  clamped therebetween. A spring  82  (Belleville spring, for example) is located below lower seat  74  to provide the necessary resilient clamping of the ball valve  70  between seats  68  and  74 . 
     Cylindrical cage-like lower seat holder  76  has two longitudinal slots, one of which is visible in  FIG. 1B  and designated by the numeral  84 . Within each of the slots, such as  84 , there is received an actuating arm such as the one visible in  FIG. 1B  and designated as  86 . Actuating arm  86  has an actuating lug  88  disposed thereon which engages an eccentric bore  90  disposed through the side of ball valve  70  so that the ball valve  70  may be rotated to a closed position upon upward movement of actuating arm  86  relative to the housing  12 , as seen in  FIG. 1B . Although not shown, there are two such actuating arms  86  with lugs  88  engaging two such eccentric bores such as  90 . Further details regarding the operation of ball valve  70  will be understood by those ordinarily skilled in the art having the benefit of this disclosure. 
     An operating mandrel assembly  92  includes an upper operating mandrel portion  94 , and intermediate operating mandrel portion  96 , and a lower operating mandrel portion  98 . As shown in  FIG. 1B , upper operating mandrel portion  94  includes a radially outer annular groove  100  disposed therein which engages a radially inwardly extending shoulder  102  of actuating arm  86  so that actuating arm  86  reciprocates with the upper operating mandrel portion  94  within the housing  12  to move ball valve  70  between the open and closed positions. Lower seat holder mandrel  76  has an outer surface  104  closely received within an inner cylindrical bore  106  of the upper operating mandrel portion  94  with a seal being provided therebetween by annular seal  108 . An upper portion of intermediate operating mandrel portion  96  is received within a smaller bore  110  of upper operating mandrel portion  94 . Upper operating mandrel portion  94  carries a plurality of locking dogs  112  each disposed through a radial window  114  in upper operating mandrel portion  94  with a plurality of annular biasing springs  116  received about the radially outer sides of locking dogs  112  to urge them radially inward through the windows  114  against the intermediate operating mandrel portion  96 . 
     Operating mandrel assembly  92  is seen in  FIGS. 1A-1F , where annular pressure responsive tool  10  is in an initial run-in open position wherein the ball valve  70  is open as shown. However, as will also be described herein, annular pressure responsive tool  10  may also be initially run into the well with the ball valve  70  in a closed position. When in the initial run in closed position, intermediate operating mandrel portion  96  carries an annular radial outer groove  118 , which in  FIG. 1B  is shown displaced above locking dogs  112 . Intermediate operating mandrel portion  96  slides freely relative to upper operating mandrel portion  94  until locking dogs  112  are received within annular groove  118 . Thus, referring to  FIG. 1B , annular pressure responsive tool  10  could be initially assembled with upper operating mandrel portion  94  displaced upwardly relative to housing  12  and intermediate operating mandrel portion  96  from the position shown in  FIG. 1B  such that locking dogs  112  are received and locked in place in groove  118  with ball valve  70  rotated to a closed position. 
     Intermediate operating mandrel portion  96  is closely slidably received within a bore  119  of ported nipple  20  with an O-ring seal  120  being provided therebetween. Intermediate operating mandrel portion  96  includes a radially outwardly extending flange  122 . An annular mud chamber  130  is defined between ported nipple  20  and intermediate operating mandrel portion  96 . One or more power ports  132  are radially disposed through ported nipple  20  to communicate a well annulus surrounding annular pressure responsive tool  10  with mud chamber  130 . An annular oil power chamber  134  is defined between power housing section  22  and intermediate operating mandrel portion  96 . An actuating piston  136  is slidably received within annular oil power chamber  134  with an outer seal  138  sealing against power housing section  22  and an inner seal  140  sealing against intermediate operating mandrel portion  96 . Actuating piston  136  includes an upper side  133  and lower side  135 . 
     Actuating piston  136  serves to isolate well fluid (e.g., mud) entering power port  132  from hydraulic fluid (e.g., oil) contained in oil power chamber  134 . Actuating piston  136  is connected at lower threads  124  to load transfer sleeve  126  which presents four inwardly protruding load transfer shoulders proximate its lower end. One of these shoulders is shown at  128  in  FIG. 1C , which also includes upwardly facing contact surfaces  128   a . A bearing race (not shown) of slightly enlarged diameter is disposed about the inner circumference of the load transfer sleeve  126 . A bearing insertion aperture (also not shown) is disposed through the load transfer sleeve  126  proximate the bearing race. Split ring  139  and shoulder  147  fixedly surround the intermediate operating mandrel portion  96  and limit upward axial movement of the ratchet sleeve  127  with respect to the intermediate operating mandrel portion  96 . A snap ring  149  fixedly surrounds the intermediate operating mandrel portion  96  proximate the lower end of the ratchet sleeve  127  to limit downward axial movement of the ratchet sleeve  127 . 
     Referring now to  FIGS. 1C and 3 , ratchet sleeve  127  surrounds the intermediate operating mandrel portion  96  and is loosely received within load transfer sleeve  126 . Ratchet sleeve  127  is axially rotatable upon the intermediate mandrel portion  96 . The outer surface of an exemplary ratchet sleeve  127  is shown in  FIG. 3 . A milled out area  129  is located proximate the lower end and upon the outer circumference of ratchet sleeve  127 . Milled out area  129  is a section of sufficiently reduced thickness on ratchet sleeve  127  to permit load transfer shoulders  128  of the load transfer sleeve  126  to be moved freely adjacent thereto. Load bearing shoulders  131  which present downwardly facing contact surfaces  131  are provided proximate the lower end of ratchet sleeve  127 . In certain exemplary embodiments, there are four outward load bearing shoulders  131   a  disposed about the outer circumference of ratchet sleeve  127  positioned so as to be in complimentary engagement with load transfer shoulders  128  of load transfer sleeve  126 . Bearing slot grooving  133  is provided on the outer circumference of the ratchet sleeve  127  which is shaped and sized to receive a bearing. Bearing slot grooving  133  includes a first bearing stop position  133   a , a second bearing stop position  133   b , third bearing stop position  133   c  and fourth bearing stop position  133   d , as shown in the dotted lines in  FIG. 3 . 
     Bearing installation grooving  135  is provided which is deeper than the bearing slot grooving  133 . In certain exemplary embodiments, there may be two arrangements of bearing slot grooving  133  located on opposing sides of the ratchet sleeve  127 . Similarly, there would be two such milled out areas  129  with protruding load bearing shoulders  131 . While load transfer shoulders  128  are engaged with load bearing shoulders  131  of ratchet sleeve  127 , upward axial load may be transmitted to the ratchet sleeve  127 , shoulder  147  and intermediate operating mandrel portion  96  such that the ball valve  70  may be closed by an upward pressure differential upon the lower side  135  of actuating piston  136 . Upward loading on the actuating piston  136  causes the load transfer sleeve  126  to transfer its upward load through the engagement of load transfer shoulders  128  and load bearing shoulders  131  to ratchet sleeve  127 , shoulder  147  and, thereby, to operating mandrel assembly  92 . 
     Still referring to  FIGS. 1C and 3 , ratchet sleeve  127  and load transfer sleeve  126  are operatively associated as a ratchet assembly by insertion of a bearing  137  into the insertion aperture when the insertion aperture is aligned with the installation grooving  135  of the ratchet sleeve  127 . By manipulating ratchet sleeve  127 , bearing  137  is then captured and moved within the bearing race and the bearing slot grooving  133 . In operation, the arrangement functions as a selectively actuatable load transfer assembly which provides for translation of axial motion by the load transfer sleeve  126  as movement of bearing  137  along bearing slot grooving  133  rotates ratchet sleeve  127  with respect to the load transfer sleeve  126 , and selectively brings load transfer shoulders  128  of load transfer sleeve  126  into engagement with load bearing shoulders  131  of ratchet sleeve  127 . Operation of such a ratchet assembly to selectively actuate actuating arm  86  and ball valve  70  between various open positions will be readily understood by those ordinarily skilled in the art having the benefit of this disclosure. 
     Referring now to  FIG. 1D , an exemplary embodiment of an annular power piston  142  will now be described. Note that annular power piston  142  is illustrated in the activated position, whereby ball valve  70  is also in the open position. However, as will be described below, in one exemplary methodology, annular pressure responsive tool  10  is run downhole having ball valve  70  in the closed position and annular power piston  142  is the deactivated position. Nevertheless, as shown in  FIG. 1D , annular power piston  142  is fixedly attached to the operating mandrel assembly  92  and is held in place between by a sleeve  144  mounted between upper side  141  of power piston  142  and the lower end of shoulder  128 . Intermediate operating mandrel portion  96  and lower operating mandrel portion  98  are threadedly connected at thread  148  after the power piston  142  has been placed about the intermediate operating mandrel portion  96  below the sleeve  144 . 
     In addition, power piston  142  has a shoulder  145  which engages sleeve  144  positioned around intermediate operating mandrel portion  96 . Power piston  142  has an upper side  141  and a lower side  143 . Power piston  142  also carries an outer annular seal  150  which provides a sliding seal against the wall of an inner cylindrical bore  152  (i.e., power housing section  22 ) and an inner annular seal  154  which seals against the intermediate operating mandrel portion  96 . 
     Power piston  142  includes a pressure relief valve  250  and check valve  252 , both of which combine to form a fluid transfer assembly that permits fluid transfer across power piston  142 . Pressure relief valve  250  provides sufficient resistance so that it will not open to relieve pressure until the annulus has been overpressured to a second level which is above the first pressure level needed to move power piston  142  and ball valve  70  between the closed and open positions. Pressure relief valve  250  is thereby set such that it will not open during normal operation of annular pressure responsive tool  10 . Thus, if annular pressure responsive tool  10  is normally operated by increasing well annular pressure to, for example, 1,000 psi above hydrostatic well annulus pressure, pressure relief valve  250  is designed to require greater than 1,000 psi to open. 
     However, in exemplary embodiments of the present invention, pressure relief valve  250  must first be activated in order to relieve the pressure once the rated pressure level has been exceeded. As described herein, pressure relief valve is “deactivated” when, despite the rated pressure being exceeded, pressure relief valve does not function to allow relief of pressure therethrough. As a corollary, “activated” describes the state of pressure relief valve  250  whereby it is allowed to relieve the pressure once the rated level has been exceeded. 
     To further illustrate this feature of the present invention,  FIG. 1D  shows power piston  142  and pressure relief valve  250  in the activated position, while  FIG. 2  illustrates the deactivated position. As previously mentioned, in certain exemplary methodologies, annular pressure responsive tool  10  is run downhole with power piston  142  in the deactivated position and ball valve  70  in the closed position, as shown in  FIG. 2 . To achieve this objective, pressure relief valve  250  includes a vent port  260  that vents along the outer diameter of power piston  142 . In this example, vent port  260  is a one-way vent port that only allows fluid flow down out of pressure relief valve  250 . An annular fluid flow groove  262  extends around power piston  142  and communicates with vent port  260  to allow pressure communication accordingly. Vent port  260  is positioned between outer annular seal  150  (O-ring seal, for example) and another outer annular seal  264 . 
     A plurality of slots  266  are positioned along the inner diameter of power housing section  22 , which are adapted to receive seal  264 . However, in the alternative, slots  266  may instead be one continuous slot extending around power housing section  22 . As will be described herein, when power piston  142  is in the deactivated position ( FIG. 2 ), outer annular seal  264  is energized to effectively seal between power piston  142  and power housing section  22 . However, when power piston  142  is in the activated position (i.e., pressure relief valve  250  is activated) ( FIG. 1 ), outer annular seal  264  no longer seals because it has been received along slots  266  wherein vent port  260  is then allowed to communicate pressure around outer annular seal  264 . 
     In addition to activating power piston  142 , downward movement of power piston  142  relative to housing  12  due to annular pressure also results in movement of operating mandrel assembly  92 , thus moving ball valve  70  to its open position. A rapid increase in well annulus pressure will be immediately transmitted to the upper side  141  of power piston  142 , but will be delayed in being communicated with the lower side  143  of power piston  142 , so that a rapid increase in well annulus pressure will create a downward pressure differential across power piston  142  thus urging it downward within the housing  12 . Accordingly, in this exemplary embodiment, pressure relief valve  250  will not open until power piston  142  is in the activated position which also requires ball valve  70  to be in the open position, thus avoid inadvertent Locked Open tool positions. 
     To further describe this exemplary embodiment of annular pressure responsive tool  10 , lower operating mandrel portion  98  carries a radially outward extending flange  156  having a lower tapered shoulder  158  and an upper tapered shoulder  160  defined thereon. A spring collet retaining mechanism  162  has a lower end fixedly attached to connector section  24  at thread  164 . A plurality of upward extending collet fingers  166  are radially inwardly biased. Each finger  166  carries an upper collet head  168  which has the upper and lower tapered retaining shoulders  170  and  172 , respectively, defined thereon. 
     In the initial position of lower operating mandrel portion  98  as seen in  FIG. 1D , collet head  168  is located immediately below flange  156  with the upper tapered retaining shoulder  170  of collet head  168  engaging the lower tapered shoulder  158  of the flange  156  of lower operating mandrel portion  98 . This engagement prevents operating mandrel assembly  92  from moving downward relative to housing  12  until a sufficient downward force is applied thereto to cause the collet fingers  166  to be cammed radially outward and pass up over flange  156  thus allowing operating mandrel assembly  92  to move downward relative to housing  12 . Similarly, subsequent engagement of upper tapered shoulder  160  of flange  156  with lower tapered retaining shoulder  172  of collet head  168  will prevent the operating mandrel assembly  92  from moving back to its upward most position relative to housing  12  until a sufficient pressure differential is applied thereacross. In certain embodiments of the present invention, spring collet  162  is designed so that a differential pressure in the range of from 500 to 700 psi, for example, is required to move the operating mandrel assembly  92  past the spring collet  162 . Thus, spring collet  162  prevents premature movement of operating mandrel assembly  92  in response to unexpected annulus pressure changes. 
     Referring to  FIG. 1D , an irregularly shaped annular oil balancing chamber  174  is defined between power housing section  22  and lower operating mandrel portion  98  below power piston  142 . Oil balancing chamber  174  is filled with a hydraulic fluid such as oil. As shown in  FIG. 1E , an upper annular nitrogen chamber  176  is defined between upper gas chamber housing section  26  and lower operating mandrel portion  98 . An annular upper floating piston or isolation piston  178  is slidably received within nitrogen chamber  176 , as understood in the art. A plurality of longitudinal passages  180  are disposed through an upper portion of upper gas chamber housing section  26  to communicate oil balancing chamber  174  with the upper end of nitrogen chamber  176 . Floating piston  178  isolates hydraulic fluid thereabove from a compressed gas such as nitrogen located therebelow in the upper nitrogen chamber  176 . 
     An annular lower nitrogen chamber  182  is defined between lower gas chamber housing section  30  and upper inner tubular member  38 . A plurality of longitudinally extending passages  184  are disposed through gas filler nipple  28  and communicate upper nitrogen chamber  176  with lower nitrogen chamber  182 . A transversely oriented gas fill port  186  intersects passage  184  so that the upper and lower nitrogen chambers  176  and  182  can be filled with pressurized nitrogen gas in a known manner. A gas filler valve (not shown) is disposed in gas fill port  186  to control the flow of gas into the nitrogen chambers and to seal the same in place therein. The nitrogen chambers  176  and  182  serve as accumulators which store increases in annulus pressure that enter annular pressure responsive tool  10  through power ports  132  above and through equalizing port  214 . The nitrogen accumulators also function to balance the pressure increases against each other and, upon subsequent reduction of annulus pressure, to release the stored pressure to cause a reverse pressure differential within annulus pressure responsive tool  10 . 
     A lower floating piston or isolation piston  188  is slidingly disposed in the lower end of lower nitrogen chamber  182 . It carries an outer annular seal  190  which seals against an inner bore  192  of lower gas chamber housing section  30 . Piston  188  carries an annular inner seal  193  which seals against an outer cylindrical surface  195  of upper inner tubular member  38 . Lower isolation piston  188  isolates nitrogen gas in the lower nitrogen chamber  182  thereabove from a hydraulic fluid such as oil contained in the lower most portion of chamber  182  below the piston  188 . 
     Referring now to  FIG. 1H , an annular multi-range metering cartridge  194  is located longitudinally between inner tubular member connector  40  and the metering cartridge housing  32 , and is located radially between the metering cartridge housing  32  and the lower inner tubular member  42 . Multi-range metering cartridge  194  is fixed in place by the surrounding components just identified and is adjustable to meter fluid over a wide range of differential pressures. Metering cartridge  194  carries outer annular seal  196  which seals against the inner bore of metering cartridge housing  32 . Multi-range metering cartridge  194  carries an annular inner seals  198  which seal against a cylindrical outer surface  200  of lower inner tubular member  42 . An upper end of multi-range metering cartridge  194  is communicated with the lower nitrogen chamber  182  by a plurality of longitudinal passageways (not shown) cut in the radially outer portion of inner tubular member connector  40 . Operation of multi-metering cartridge  194  will not be described herein, as those ordinarily skilled in the art having the benefit of this disclosure will readily understand its function and operation. 
     Referring now to  FIG. 1I , multi-range metering cartridge  194  communicates with a lower oil filled equalizing chamber  210  via annular passage  208 . A lowermost floating piston or isolation piston  212  is slidably disposed in equalizing chamber  210  and isolates oil thereabove from well fluids such as mud which enters therebelow through an equalizing port  214  defined through the wall of lower oil chamber housing section  34 . 
     Referring to  FIGS. 1A-1I , housing  12  can be generally described as having a first pressure conducting passage  236  defined therein for communicating the well annulus with the upper side  141  of power piston  142 . In certain exemplary embodiments, the first pressure conducting passage  236  includes, for example, power port  132 , annular mud chamber  130 , and oil power chamber  134 . Housing  12  can also be generally described as having a second pressure conducting passage  238  defined therein for communicating the well annulus with the lower side  135  of actuating piston  136 . The second pressure conducting passage  238  includes oil power chamber  134 , oil balancing chamber  174 , longitudinal passage  180 , upper nitrogen chamber  176 , longitudinal passage  184 , lower nitrogen chamber  182 , longitudinal passages  202 , the flow path  204  of multi-range metering cartridge  194 , annular passage  208 , equalizing chamber  210  and equalizing port  214 . Also, as previously described, once in the activated position, pressure relief valve  250  is designed to relieve pressure from the first flow passage  236  to the second flow passage  238  when the pressure differential therebetween exceeds the pressure rating of pressure relief valve  250 . 
     As understood in the art, multi-range metering cartridge  194  and the various passages and components contained therein can generally be described as a retarding mechanism disposed in the second pressure conducting passage  238  for delaying communication of a sufficient portion of a change in well annulus pressure to the lower side  135  of actuating piston  136  for a sufficient amount of time to allow a pressure differential on the lower side  135  of actuating piston  136  to move the actuating piston  136  upwardly relative to housing  12 . Retarding mechanism also functions to maintain a sufficient portion of a change in well annulus pressure within the second pressure conducting passage and permit the differential in pressures between the first and second pressure conducting passages to balance. 
     Moreover, ball valve  70  can generally be referred to as an operating element operably associated with power piston  142  and actuating piston  136  for movement with power piston  142  between a first closed position and a second open position. However, in other exemplary embodiments, the first position may be open, while the second position may be closed. Those ordinarily skilled in the art having the benefit of this disclosure will realize that this and a variety of other alterations may be embodied within annular pressure responsive tool  10  without departing from the spirit and scope of the present invention. 
     Now that the various exemplary components of annular pressure responsive tool  10  have been described, an exemplary operation conducted using annular pressure responsive tool  10  will now be described with reference to  FIGS. 1A-1I and 2 . As will be understood by those ordinarily skilled in the art having the benefit of this disclosure, ball valve  70  may be opened and closed by increasing and decreasing the annulus pressure between hydrostatic pressure and the first level above hydrostatic. Assuming that we begin with well annulus pressure at hydrostatic levels and a closed position of ball valve  70 , annular pressure responsive tool  10  is assembled for deployment into the wellbore such that load transfer shoulders  128  are aligned with load bearing shoulders  131 . For exemplary purposes only, the first level of pressure above hydrostatic pressure may be 1000 psi above hydrostatic, a sufficient change in annulus pressure from hydrostatic to move ball valve  70  between its open and closed positions. Also by way of example, the second level of pressure above hydrostatic pressure is stated to be 2000 psi above hydrostatic. Pressure relief valve  250 , for example, may be designed to be operable at a differential pressure somewhere between those first and second levels, for example, at a pressure differential in the range of 1200 to 1400 psi. When this differential pressure is applied across relief valve  250  (after it is activated), it will open allowing hydraulic fluid to be metered slowly through the fluid restrictor from the oil power chamber  134  to the oil balancing chamber  174 , as understood in the art. 
     Nevertheless, to describe an exemplary operation in more detail, annular pressure responsive tool  10  is made up, deployed downhole and set at a desired location. During its deployment, ball valve  70  and power piston  142  are in a first closed position whereby ball valve  70  is closed and power piston  142  is in a deactivated position as shown in  FIG. 2 . After annular pressure responsive tool  10  has been set at the desired location, a pressure increase will be imposed upon the well annulus so that the pressure exterior of the housing  12  is brought to the first level above hydrostatic. Fluid pressure will be transmitted into mud chamber  130  through power port  132  and along the first pressure conducting passage  236  to exert pressure upon actuating piston  136  to move actuating piston  136  downwardly. The fluid pressure is transmitted through the fluid within the oil power chamber  134  to the power piston  142  below. At this time, pressure relief valve  250  is in the deactivated positioned because vent port  260  is sealed by seal  150  above and seal  264  below. 
     As the first level of pressure is applied to the power piston  142 , it and operating mandrel assembly  92  are moved downwardly to a second position, whereby seal  264  is de-energized, or unseals, as it moves into slot  266  thus activating pressure relief valve  250 . As a result, ball valve  70  is also actuated into an open position. Here, fluid pressure may be communicated through pressure relief valve  250  and vent port  260 . Once the fluid exits vent port  260  it may flow around flow groove  262  until it encounters slots  266  whereby the fluid may then communicate on to oil balancing chamber  174 . 
     Once pressure relief valve  250  is in the activated position as shown in  FIG. 1D , power piston  142  and pressure relief valve  250  operate as understood in the art, whereby ball valve  70  may be selectively actuated to one or more open positions (Open, Locked Open, etc., for example) in response to changes in the well annulus pressure. For example, the pressure increase within the first pressure conducting passage  236 , following downward movement of the power piston  142 , is then stored with the nitrogen chambers  176  and  182  via compression of nitrogen gas contained within. An offsetting amount of fluid pressure is then transmitted upward along the second pressure conducting passage  238  through equalization port  214  at the same time that it is transmitted downward along the first pressure conducting passage  236  through power port  132 . Ball valve  70  will still open, however, since the retarding mechanism of the multi-range metering cartridge  194  will delay the increase in well annulus pressure from being communicated from longitudinal passages  208  below to longitudinal passages  202  above. As a result of the delay, the pressure within the first pressure conducting passage  236  will be greater than that within the second pressure conducting passage  238  during the delay and permits the ball valve  70  to open. Eventually, the pressure differential between the first and second pressure conducting passages  236 , 238  will become relatively balanced after a period of time. 
     When it is desired to close ball valve  70 , annulus pressure may be reduced to hydrostatic causing a reverse pressure differential within both the first and second pressure conducting passages  236  and  238  from the stored pressure within the nitrogen chambers  176  and  182 . Metering cartridge  194  delays transmittal of the pressure differential downward within the second pressure conducting passage  238  from passages  202  to passages  208 , thereby maintaining an increased level of pressure within the upper portions of the second pressure conducting passage  238 . The pressure differential upward within first pressure conducting passage  236  urges power piston  142  and actuating piston  136  upwardly at lower side  135 . As power piston  142  moves upwardly, the lower end of power piston  142  and seal  264  are moved up out of slot  266 , thus reactivating seal  264  to seal against housing  12  and deactivating pressure relief valve  250 . Through the resulting load transfer, sleeve  126 , ratchet sleeve  127  and shoulder  147 , the upward motion is transmitted to the operating mandrel  96 , and ball valve  70  is moved back to its closed position. 
     Moreover, as previously mentioned, while pressure relief valve  250  and power piston  142  are in the activated position, annular pressure responsive assembly  10  may also be placed into a “Locked Open” position, as understood in the art. As such, ball valve  70  is retained in an open position during subsequent changes of well annulus pressure between hydrostatic and the first level above hydrostatic pressure by imposing upon the well annulus a second level of pressure which is above the first level and then reducing the pressure. Here, the well annulus pressure may be changed between hydrostatic and the first level any number of times through use of the ratchet assembly described herein. 
     Accordingly, through use of the present invention, the pressure relief valve will not open until the power piston  142  is in the activated position, which also requires ball valve  70  to be in the open position. Therefore, inadvertent Locked Open positions which persist in conventional tester valves are avoided. In addition, if the ball valve has a high differential, the operating pressure may be increased without the associated risks. Lastly, tools utilizing the inventive aspects described herein may be utilized by current field personnel, as retraining will not be necessary because the tool will operate as expected in all conditions. 
     An exemplary embodiment of the present invention provides a pressure responsive downhole tool, comprising a tool housing; a power piston slidably disposed within the tool housing for movement between a first position and a second position, the power piston comprising a first and second seal disposed around an outer diameter of the power piston to provide a seal between the power piston and the tool housing; and a pressure relief valve disposed along the power piston, the pressure relief valve comprising a vent port disposed between the first and second seals; a first pressure conducting passage for communicating a well annulus pressure with the power piston to move the power piston from the first position whereby the vent port is not allowed to vent, to the second position whereby the vent port is allowed to vent; and an operating element operably associated with the tool for movement with the power piston between the first and second positions. In another, the tool housing comprises a slot positioned along an inner diameter of the tool housing to receive the second seal when the power piston is in the second position. 
     In yet another, the operating element is a ball valve assembly that prevents fluid communication through a bore of the tool in the first position, and allows fluid communication through the bore in the second position. Another further comprises a mechanism to selectively actuate the ball valve assembly to the second position in response to changes in the well annulus pressure. Yet another further comprises a flow groove in fluid communication with the vent port, the flow groove extending around the outer diameter of the power piston. In another, the flow groove is positioned between the vent port and the second seal. Yet another further comprises a second pressure conducting passage for communicating pressure to move the power piston back to the first position. 
     An exemplary methodology of the present invention provides a method of using a pressure responsive downhole tool, the method comprising setting the tool along a desired location of a well, the tool comprising a power piston slidably disposed within a housing of the tool for movement between a first position and a second position, the power piston comprising a pressure relief valve; and an operating element operably associated with the tool for movement with the power piston between the first and second positions; applying well annulus pressure to the tool to move the power piston from the first position to the second position, thereby activating the pressure relief valve, while also moving the operating element from a closed position to an open position; and selectively actuating the operating element to one or more open positions in response to changes in the well annulus pressure. 
     In another, selective actuation of the operating element to the one or more open positions is only allowed while the power piston is in the second position. Yet another method further comprises bleeding off the well annulus pressure to allow the power piston to move back to the first position, thereby also moving the operating element back to the closed position. In another, the pressure relief valve is deactivated in the first position. 
     Another exemplary methodology of the present invention provides a method of using a pressure responsive downhole tool, the method comprising deploying the tool to a desired location within a well; applying well annulus pressure to the tool to move a power piston from a first position to a second position; activating a pressure relief valve of the power piston when the power piston is in the second position; and selectively actuating an operating element of the tool in response to changes in the well annulus pressure. In another, the pressure relief valve is deactivated while the power piston is in the first position. Yet another method further comprises moving the power piston back to the first position. In another, moving the power piston back to the first position further comprises bleeding off the well annulus pressure. 
     In yet another, activating the pressure relief valve further comprises deactivating an annular seal positioned around the power piston. In another, deactivating the annular seal comprises causing the annular seal to enter a slot along an inner diameter of a tool housing. In yet another, the operating element is in a closed position while the power piston is in the first position, the closed position preventing fluid from passing through a bore of the tool. In another, the operating element is in an open position while the power piston is in the second position, the open position allowing fluid to pass through the bore of the tool. In another, the operating element is a ball valve assembly. 
     The foregoing disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. Further, spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper” and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the apparatus in use or operation in addition to the orientation depicted in the figures. For example, if the apparatus in the figures is turned over, elements described as being “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, the exemplary term “below” can encompass both an orientation of above and below. The apparatus may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly. 
     Although various embodiments and methodologies have been shown and described, the invention is not limited to such embodiments and methodologies and will be understood to include all modifications and variations as would be apparent to one skilled in the art. Therefore, it should be understood that the invention is not intended to be limited to the particular forms disclosed. Rather, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the invention as defined by the appended claims.