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CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a division of U.S. application Ser. No. 09/167,045 filed Oct. 5, 1998, now U.S. Pat. No. 6,145,595, such patent being hereby incorporated in its entirety herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to operations performed in conjunction with subterranean wells and, in an embodiment described herein, more particularly provides an annulus pressure referenced circulating valve. 
     It is well known in the art to operate a valve positioned in a subterranean well by applying fluid pressure to the valve. The fluid pressure may exist by virtue of the weight of fluid in the well, the fluid pressure may be applied to the valve by, for example, a pump at the earth&#39;s surface or in the well, and the fluid pressure may be a combination of these. When the valve is interconnected in a tubular string positioned in a wellbore of the well, the fluid pressure may exist in the tubular string, in an annulus formed between the tubular string and the wellbore, or the valve may be operated by a difference between fluid pressure in the tubular string and fluid pressure in the annulus. 
     Where a valve is operated by absolute fluid pressure in a tubular string or in an annulus exterior to the valve, the valve typically includes a chamber at atmospheric pressure or an elevated precharged pressure at the earth&#39;s surface. After positioning in the well, a fluid pressure differential (equal to the difference between the chamber pressure and the pressure in the tubular string or annulus) is generally created across a member releasably secured against displacement by, for example, one or more shear pins. When a predetermined fluid pressure differential is reached, the member is released and displaced by the differential pressure, thereby operating the valve. Unfortunately, however, it is often uncertain what pressure conditions will be experienced in the well prior to installing the valve in the tubular string, so there is a danger that the valve will be inadvertently operated due to an unexpected pressure increase in the tubular string or annulus. 
     Where the valve is operated in response to a pressure differential between the tubular string and the annulus, the member is typically released for displacement when the predetermined fluid pressure differential is created. While, strictly speaking, operation of this type of valve does not require prior knowledge of absolute fluid pressures in either the tubular string or annulus, it does requires prior knowledge of fluid pressures to be experienced in both the tubular string and the annulus, so that the fluid pressure differential may be determined and the valve may be set up to avoid inadvertent operation of the valve. 
     Solutions to the problem of inadvertent operation of pressure responsive valves have been implemented. For example, it is common for a valve to include a chamber at an elevated pressure and a member displaceable in response to a difference in pressure between the chamber and the tubular string, the annulus, or a difference between the tubular string and annulus pressures. By manipulating the tubular string pressure, the annulus pressure, or the difference between the tubular string and annulus pressures, the member is made to displace repeatedly, the member displacing sufficiently to operate the valve after a predetermined number of the pressure manipulations. The number of pressure manipulations is usually determined by a ratchet or J-slot mechanism. Unfortunately, this type of valve requires numerous pressure manipulations, and a complex and expensive ratchet or J-slot mechanism. 
     Therefore, it would be highly desirable to provide a valve responsive to fluid pressure in a well, which does not require numerous pressure manipulations or precise prior knowledge of fluid pressures to be experienced in the well, and which is relatively uncomplicated in its construction and use. 
     SUMMARY OF THE INVENTION 
     In carrying out the principles of the present invention, in accordance with an embodiment thereof, a circulating valve is provided which is annulus pressure referenced. The valve stores annulus pressure in an internal chamber as a variable reference. A subsequent relatively rapid increase in annulus pressure relative to that previously stored in the chamber causes the valve to operate. The valve is nonresponsive to fluid pressure in an axial flow passage formed therethrough. 
     In one aspect of the present invention, the valve includes a specially configured hydraulic circuit. The hydraulic circuit includes two portions interconnected in series between a fluid pressure source external to the valve, and a fluid pressure storage chamber within the valve. As fluid pressure external to the valve gradually increases and decreases, the hydraulic circuit permits the fluid pressure to be stored in the chamber. The hydraulic circuit portions permit substantially restricted fluid flow from the valve exterior to the chamber, and permit substantially unrestricted fluid flow from the chamber to the valve exterior. 
     However, when the external fluid pressure is relatively rapidly increased, one of the hydraulic circuit portions opens to permit substantially unrestricted flow therethrough from the valve exterior, while the other hydraulic circuit portion continues to substantially restrict fluid flow therethrough, thereby causing displacement of the hydraulic circuit portions relative to each other. Since one of the hydraulic circuit portions is incorporated in a housing assembly of the valve, and the other hydraulic circuit portion is incorporated in a structure displaceable relative to the housing assembly, displacement of the hydraulic circuit portions relative to each other causes displacement of the structure relative to the housing assembly. 
     In another aspect of the present invention, a structure selectively blocks and permits fluid flow through a sidewall of a housing assembly. The structure is sealingly engaged and displaceable within the housing assembly. A first hydraulic circuit portion regulates fluid flow between a fluid pressure source and a second hydraulic circuit portion across a portion of the housing assembly sealingly engaged with the structure. The second hydraulic circuit portion regulates fluid flow between the first circuit portion and a fluid pressure storage chamber across a portion of the structure sealingly engaged with the housing assembly. The second circuit portion is displaceable with the structure relative to the housing assembly. 
     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 a representative embodiment of the invention hereinbelow and the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A&amp;1B are quarter-sectional views of successive axial portions of an annulus pressure referenced circulating valve embodying principles of the present invention, the circulating valve being shown in a closed configuration thereof; 
     FIG. 2 is a schematic diagram of a hydraulic circuit of the circulating valve of FIGS. 1A&amp;1B; 
     FIGS. 3A&amp;3B are quarter-sectional views of successive axial portions of the circulating valve of FIGS. 1A&amp;1B, the circulating valve being shown in an open configuration thereof; and 
     FIG. 4 is a schematic illustration of a method of using the circulating valve of FIGS. 1A&amp;1B, the method embodying principles of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Representatively illustrated in FIGS. 1A&amp;1B is an annulus pressure referenced circulating valve  10  which embodies principles of the present invention. In the following description of the circulating valve  10  and other apparatus and methods described herein, directional terms, such as “above”, “below”, “upper”, “lower”, etc., are used for convenience in referring to the accompanying drawings. 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., without departing from the principles of the present invention. 
     The circulating valve  10  includes an outer housing assembly  12 , a generally tubular structure or sleeve  14 , and a hydraulic circuit  16 . The hydraulic circuit  16  is representatively illustrated in FIG. 2 apart from the remainder of the circulating valve  10 , and is described in more detail hereinbelow. 
     An annular chamber  18  is formed between the sleeve  14  and the housing assembly  12 . The annular chamber  18  is in fluid communication with the exterior of the valve  10  via a port  20  formed through a sidewall of the housing assembly. When the circulating valve  10  is interconnected in a tubular string and positioned within a wellbore (see FIG.  4 ), the port  20  permits fluid flow between the chamber  18  and an annulus formed between the tubular string and the wellbore. An annular piston  22  sealingly and reciprocably disposed between the housing assembly  12  and the sleeve  14  isolates wellbore fluids from the hydraulic circuit  16 , while still permitting transfer of fluid pressure from the annulus to the hydraulic circuit. For this purpose, a clean fluid, such as oil, silicone fluid, etc., is contained in the chamber  18  between the piston  22  and the hydraulic circuit  16 . 
     Another annular chamber  24  is formed between the sleeve  14  and the housing assembly  12 . The chamber  24  receives fluid flowed through the hydraulic circuit  16  from the chamber  18 . An annular piston  26  sealingly and reciprocably disposed in the chamber  24  between the sleeve  14  and the housing assembly  12  isolates the fluid flowed through the hydraulic circuit  16  from a volume of compressible fluid, such as Nitrogen, in the chamber  24  below the piston. 
     The valve  10  is representatively illustrated in FIGS. 1A&amp;1B in a configuration in which the valve is run into a well as a part of a tubular string. The piston  26  is illustrated in FIG. 1B as being downwardly spaced apart from a radially enlarged portion  28  of the sleeve  14 . This downward displacement of the piston  26  is due to fluid pressure greater than that of the compressible fluid in the chamber  24  entering the port  20 , forcing fluid from the chamber  18  through the hydraulic circuit  16  and into the chamber  24  above the piston  26 , and compressing the compressible fluid in the chamber  24 , for example, due to increased hydrostatic pressure in the annulus surrounding the valve. 
     Such transfer of fluid from the upper chamber  18  to the lower chamber  24  through the hydraulic circuit  16 , due to increasing hydrostatic pressure as the valve  10  is lowered in a well, is at a relatively low flow rate. This is because hydrostatic pressure increases very gradually as the valve  10  is lowered in the well. The hydraulic circuit  16  permits such low flow rate transfers of fluid from the upper chamber  18  to the lower chamber  24 , without causing any change in the configuration of the valve  10 . 
     In the configuration of the valve  10  depicted in FIGS. 1A&amp;1B, the sleeve  14  prevents fluid flow through openings  30  formed through a sidewall of the housing assembly  12 . If the sleeve  14  is downwardly displaced relative to the housing assembly  12 , the openings  30  will no longer be blocked by the sleeve, and fluid flow will be permitted through the openings. In this manner, fluid communication is established between the exterior of the valve  10  and an inner axial flow passage  32  formed through the valve. It will be readily appreciated by one skilled in the art that such downward displacement of the sleeve  14  relative to the housing assembly  12  will also permit fluid communication between the annulus and an axial flow passage of a tubular string, when the valve  10  is interconnected in the tubular string and positioned in a well, thereby permitting fluid circulation through the tubular string and annulus in the well. 
     The sleeve  14  is releasably retained in its position blocking fluid flow through the openings  30  by a generally C-shaped snap ring  34 . The snap ring  34  is received in an annular groove  36  formed internally in the housing assembly  12 . The snap ring  34  is also engaged with a radially reduced portion  38  formed on the sleeve  14 . It will be readily appreciated that a sufficiently large downwardly biasing force must be applied to the sleeve  14  to radially enlarge the snap ring  34  and permit the sleeve to displace downwardly. Of course, other means of releasably retaining the sleeve  14 , such as shear pins, a shear ring, a releasable latch, etc., could be utilized in place of the snap ring  34 , without departing from the principles of the present invention. 
     Another snap ring  40  is positioned in the housing assembly  12  for engagement with an annular groove  42  formed externally on the sleeve  14 . The snap ring  40  could be similar to the snap ring  34 , but is depicted in FIG. 1A as being of the conventional type which is circumferentially segmented and biased radially inward by springs encircling the segments. When the sleeve  14  is downwardly displaced relative to the housing assembly  12  to open the valve  10  and permit fluid flow through the openings  30 , the snap ring  40  radially inwardly retracts into the groove  42  and thereby prevents further displacement of the sleeve relative to the housing assembly. Thus, the valve  10  as representatively illustrated in FIGS. 1A&amp;1B is a “one-shot” valve that is actuated only once to open the valve, and the valve is not subsequently closed. However, it is to be clearly understood that principles of the present invention may be incorporated in apparatus other than a “one-shot” circulating valve. 
     Note that a portion  44  of the hydraulic circuit  16  is disposed within a threaded coupling  46  of the housing assembly  12 , and that another portion  48  of the hydraulic circuit is disposed within the radially enlarged portion  28  of the sleeve  14 . Thus, when the sleeve  14  displaces relative to the housing assembly  12 , the hydraulic circuit portion  48  also displaces relative to the other hydraulic circuit portion  44 . In addition, note that, since the sleeve  14  is sealingly engaged with the housing assembly  12  within the coupling  46  and at the radially enlarged portion  28 , the upper hydraulic circuit portion  44  regulates fluid flow between the upper chamber  18  and the lower hydraulic circuit portion  48 , and the lower hydraulic circuit portion  48  regulates fluid flow between the upper hydraulic circuit portion  44  and the lower chamber  24 . 
     Referring additionally now to FIG. 2, the hydraulic circuit  16  is schematically and representatively illustrated apart from the remainder of the valve  10 . The hydraulic circuit  16  includes the portions  44 ,  48 , the upper chamber  18  and the lower chamber  24 . A fluid pressure source  50  is shown in FIG. 2, but it may or may not be considered a part of the hydraulic circuit  16 , depending upon the configuration of the valve  10 . For example, in the embodiment of the valve  10  depicted in FIGS. 1A&amp;1B, the fluid pressure source  50  is the exterior of the valve, which is an annulus between the valve and a wellbore when the valve is positioned in the wellbore. The fluid pressure source  50  may also include a pump, such as a mud pump at the earth&#39;s surface, which may be used to apply fluid pressure to the annulus, or a downhole pump connected to the valve  10  within the well. Thus, the fluid pressure source  50  shown in FIG. 2 may be any means of introducing fluid pressure to the valve  10 . 
     As shown in FIG. 2, fluid pressure from the fluid pressure source  50  enters the chamber  18 . In the valve  10 , the fluid pressure enters the chamber  18  via the port  20 . Note that the chamber  18  is not necessary in an apparatus constructed in accordance with the principles of the present invention, since fluid pressure could be transmitted directly from the fluid pressure source  50  to the upper hydraulic circuit portion  44 . 
     Fluid flows from the chamber  18  through the upper hydraulic circuit portion  44  to the lower hydraulic circuit portion  48 , the circuit portions being interconnected in series between the chambers  18  and  24 . The upper hydraulic circuit portion  44  includes three parallel flowpaths  52 ,  54 ,  56 . Fluid flows from the upper chamber  18  to the lower hydraulic circuit portion  48  through the flowpath  54 , which includes a flow restrictor  62 , such as a choke or an orifice. 
     A check valve  58  prevents fluid flow from the chamber  18  to the lower hydraulic circuit portion  48  through the flowpath  52 . A rupture disk  60  or other releasable fluid pressure barrier prevents fluid flow from the chamber  18  to the lower hydraulic circuit portion  48  through the flowpath  56  until a predetermined fluid pressure differential is created across the upper hydraulic circuit portion  44 , at which time the rupture disk  60  ruptures, permitting substantially unrestricted fluid flow through the flowpath  56 . A screen  64  or other filtering device prevents fragments of the rupture disk  60  from entering the lower hydraulic circuit portion  48  after the rupture disk  60  ruptures. 
     The restrictor  62  and rupture disk  60  are selected so that fluid may flow  20  through the upper hydraulic circuit portion  44  from the upper chamber  18  to the lower hydraulic circuit portion  48  at a relatively low flow rate, without creating a sufficient fluid pressure differential across the upper hydraulic circuit portion  44  to cause the rupture disk  60  to rupture. This permits fluid pressure to be transmitted from the fluid pressure source  50  to the lower chamber  24 , where the fluid pressure is stored as a reference pressure. For example, when the valve  10  is conveyed into a well as a part of a tubular string, gradually increasing hydrostatic fluid pressure in an annulus between the wellbore and the valve is stored in the lower chamber  24 , without causing rupture of the rupture disk  60 . Additionally, fluid pressure in the annulus (or other fluid pressure source) may increase above hydrostatic pressure, without causing rupture of the rupture disk  60 , as long as the restrictor  62  can meter fluid flow through the flowpath  54  and prevent a sufficiently great differential pressure from being created across the upper circuit portion  44 . Or, stated differently, fluid pressure increases are transmitted from the upper chamber  18  to the lower circuit portion  48  exclusively through the flowpath  54 , until the rate of fluid pressure increase is sufficiently great to cause the predetermined pressure differential to be created across the upper circuit portion  44 , at which time the rupture disk  60  ruptures, permitting a relatively high rate of fluid flow through the flowpath  56 . 
     The lower circuit portion  48  includes two parallel flowpaths  66 ,  68 . A check valve  70  prevents fluid flow from the upper circuit portion  44  to the chamber  24  through the flowpath  66 . A flow restrictor  72  restricts fluid flow through the flowpath  68 . 
     Recall that the lower circuit portion  48  is disposed in the sleeve  14 . The restrictor  72  is sized so that when the rupture disk  60  ruptures, a fluid pressure differential is created across the lower circuit portion  48  sufficiently great to bias the sleeve  14  downwardly, radially expanding the snap ring  34  and downwardly displacing the sleeve relative to the housing assembly  12 . Thus, the restrictor  72  preferably permits fluid flow therethrough at a relatively low flow rate for storing fluid pressure in the chamber  24 , but when the rupture disk  60  ruptures, the resulting pressure differential across the lower circuit portion  48  requires a relatively high rate of fluid flow through the restrictor  72 . This differential pressure biases the sleeve  14  downward relative to the housing assembly  12 . 
     The check valves  58 ,  70  permit substantially unrestricted flow of fluid from the chamber  24  to the chamber  18  through the circuit portions  44 ,  48 . Thus, when fluid pressure of the fluid pressure source  50  decreases, the reference fluid pressure stored in the chamber  24  is also permitted to readily decrease therewith. However, it will be readily appreciated that the check valves  58 ,  70  are not necessary in the valve  10  if a pressure relief valve is used instead of a rupture disk since fluid may also flow through the restrictors  62 ,  72  from the chamber  24  to the chamber  18 . 
     It will now be fully appreciated that fluid pressure stored in the chamber  24  corresponds to fluid pressure external to the housing assembly  12 . When the valve  10  is interconnected in a tubular string positioned in a wellbore of a well, this stored fluid pressure corresponds to fluid pressure in an annulus between the valve and the wellbore. When fluid pressure in the annulus is gradually increased, due to an increase in hydrostatic pressure and/or due to fluid pressure otherwise applied to the annulus, the increased fluid pressure is transmitted through the hydraulic circuit  16  for storage in the chamber  24 . When fluid pressure in the annulus is decreased, fluid in the chamber  24  is transmitted through the hydraulic circuit  16  to the chamber  18 , thereby permitting a corresponding decrease in the stored fluid pressure. In this manner, the circulating valve  10  is annulus pressure referenced. 
     However, when fluid pressure in the annulus is relatively rapidly increased, for example, due to fluid pressure being applied to the annulus by a pump, this increased fluid pressure relative to the fluid pressure stored in the chamber  24  causes a pressure differential to be created across the upper circuit portion  44 , rupturing the rupture disk  60 . When the rupture disk  60  ruptures, a pressure differential is created across the lower circuit portion  48 , which biases the sleeve  14  downwardly to open the valve  10 . 
     Referring additionally now to FIGS. 3A&amp;3B, the valve  10  is representatively illustrated in a configuration in which it has been opened as described above. The rupture disk  60  has been ruptured and a differential pressure has been created across the lower circuit portion  48  sufficiently great to radially enlarge the snap ring  34  and downwardly displace the sleeve  14  relative to the housing assembly  12 . The openings  30  are now open to fluid flow therethrough between the flow passage  32  and the exterior of the housing assembly  12 . The snap ring  40  has radially inwardly retracted into the groove  42 , thereby substantially preventing further displacement of the sleeve  14  relative to the housing assembly  12 . 
     Note that the piston  26  has displaced further downward in the chamber  24 . Prior to running the valve  10 , the chamber  24  below the piston  26  should be charged with a compressible fluid, such as Nitrogen, at a pressure somewhat less than the expected hydrostatic pressure in the well at the depth the valve  10  is to be installed, compensated for temperature. It is preferred that the volume of the chamber  24  below the piston  26  be decreased by approximately 10% when the valve  10  is properly positioned in the well. The volume of the chamber  24  below the piston  26  should permit the sleeve  14  to displace downwardly to its position shown in FIGS. 3A&amp;3B, for example, so that a pressure differential still exists across the radially enlarged portion  28  of the sleeve (and, thus, across the lower circuit portion  48 ) when the snap ring  40  retracts into the groove  42 . It is preferred that the remaining pressure differential across the lower circuit portion  48  produces a downwardly biasing force at least about 25% greater than that needed to displace the sleeve  14  at the time the snap ring  40  retracts into the groove  42 . 
     Referring additionally now to FIG. 4, a method  80  of controlling fluid flow within a subterranean well is representatively illustrated. In the method  80 , a circulating valve  82  is interconnected in a tubular string  84 . The valve  82  may be the valve  10  described above, or it may be another differently constructed annulus pressure referenced circulating valve. The tubular string  84  may be a string of production tubing, a drill stem test string, etc. 
     An internal axial flow passage of the tubular string  84  extends axially through the valve  82 . If the valve  82  is similar to the valve  10  described above, the flow passage  32  is in fluid communication with the remainder of the flow passage in the tubular string  84 . The valve  82  initially prevents fluid communication between the flow passage of the tubular string  84  and an annulus  86  formed between a wellbore  88  of the well. 
     As the tubular string  84  is lowered into the well, hydrostatic pressure in the annulus  86  increases. The valve  82  stores this fluid pressure internally as a reference. When the valve  82  is appropriately positioned in the wellbore  88 , additional fluid pressure is applied to the annulus  86 , for example, by a pump connected to the annulus via a wellhead at the earth&#39;s surface. This additional fluid pressure is applied to the annulus  86  relatively rapidly, as compared to the increase in hydrostatic pressure due to lowering of the tubular string  84  in the wellbore  88 . 
     The relatively rapid increase in fluid pressure in the annulus  86  causes the valve  82  to open, thereby permitting fluid communication between the annulus  86  and the internal axial flow passage of the tubular string  84 . Fluid may now be circulated from the annulus  86 , in through the valve  82  and into the tubular string  84 . Of course, this fluid flow could be reversed, as well. 
     It may now be fully appreciated that the valve  10  and the method  80  permit valve actuation without requiring prior knowledge of the precise fluid pressures in the annulus  86  or tubular string  84 , or both of them. Additionally, it is not necessary for multiple fluid pressure applications to be accomplished to actuate the valve  10  or  82 . Instead, the valve  10  or  82  carries an internal fluid pressure reference, which may increase or decrease depending upon the actual fluid pressure in the annulus  86 . The valve  10  or  82  is actuated only by a relatively rapid increase in fluid pressure in the annulus  86 , and is insensitive to fluid pressure in the tubular string. 
     Of course, many modifications, additions, deletions, substitutions, and other changes may be made to the valve  10  and method  80  described above, which changes would be obvious to one skilled in the art, and these changes are contemplated by the principles of the present invention. For example, the valve  10  could be easily configured to selectively permit and prevent fluid flow through the flow passage  32  by connecting the sleeve  14  to a conventional ball valve mechanism, so that displacement of the sleeve causes actuation of the ball valve mechanism. 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 circulating valve and associated methods of using same provide control of fluid flow within a subterranean well. In a described embodiment, a circulating valve includes a fluid pressure storage chamber in fluid communication with the exterior of the valve. When positioned in a wellbore, fluid pressure in an annulus between the valve and the wellbore is stored in the storage chamber. A subsequent, relatively rapid, increase in the annulus fluid pressure causes the valve to operate.