Pressure relief accumulator for thermal hydrocarbon wells
The pressure relief accumulator addresses thermal expansion-induced pressure increases by using a moveable plug to manage pressure between wellbore barriers, ensuring efficient and safe operation without costly nitrogen injection.
Patent Information
- Authority / Receiving Office
- US · United States
- Patent Type
- Applications(United States)
- Current Assignee / Owner
- CENOVUS ENERGY INC
- Filing Date
- 2026-01-05
- Publication Date
- 2026-07-09
AI Technical Summary
In thermal hydrocarbon production operations, pressure increases due to thermal expansion between adjacent plugs in a wellbore can lead to plug and casing failure, necessitating costly and time-consuming nitrogen injection procedures to displace dead fluid, which is inefficient.
A pressure relief accumulator with a moveable plug that responds to pressure differentials between an internal chamber and the wellbore, blocking or allowing fluid communication based on pressure levels to manage pressure between upper and lower barriers, using a pressurized gas to bias the plug and reduce isolation volume pressure.
Effectively manages pressure without costly nitrogen injection, ensuring plug and casing integrity by allowing pressurized fluid to enter the accumulator, thereby reducing pressure and preventing failure.
Smart Images
Figure US20260193969A1-D00000_ABST
Abstract
Description
CROSS-REFERENCE TO RELATED APPLICATION(S)
[0001] The present disclosure claims benefit of U.S. Provisional Application No. 63 / 742,338, filed on Jan. 6, 2025 (Attorney Docket No. PRO-089P), which is incorporated by reference herein in its entirety.TECHNICAL FIELD
[0002] The present disclosure relates to wellbore pressure management, and more particularly to tools and techniques for managing wellbore pressure in thermal hydrocarbon recovery operations.BACKGROUND
[0003] It is known in the oil and gas industry to deploy various tools during a wellbore life cycle. For example, where a hydrocarbon production well is underperforming, a well workover procedure may be executed in an attempt to improve production—which may involve such activities as redrilling an injection well in a thermal production operation, or repairing a failed casing string. In order to engage in the workover, however, the wellhead at surface must be opened to allow access to the wellbore itself, and this requires that the well be “killed”. A well kill fluid is pumped downhole to counter the pressure in the wellbore, which allows the placement of a tool such as a plug at a desired depth within the wellbore to allow work above the plug to take place. The plug may be set by any known technique, such as a coiled tubing string or wireline, and similarly retrieved when the work is completed.
[0004] In some higher-temperature contexts, such as thermal hydrocarbon production operations like steam-assisted gravity drainage (SAGD), one plug may not provide sufficient safety for a workover as plugs may be compromised by the extreme conditions and ultimately leak and fail. This risk may be addressed by deploying two plugs in series with a gap in-between. However, some of the kill fluid (termed “dead fluid”) may end up trapped between the plugs and be subjected to thermal expansion due to the warming of the wellbore casing. This thermal expansion can result in pressure greater than what the well is rated for, with the risk that the plugs and even the casing could fail.
[0005] A prior art solution is to inject a gas such as nitrogen downhole against the lower plug to displace the dead fluid before deploying the upper plug, but this solution is expensive and time-consuming, requiring a separate rig run.
[0006] What is needed is a means to address the risk of pressure increases from thermal expansion between adjacent plugs in a thermal hydrocarbon well, without requiring costly and time-consuming procedures involving a separate rig run.SUMMARY
[0007] According to a first broad aspect of the present disclosure, there is provided a pressure relief accumulator for reducing pressure between upper and lower barriers in a wellbore, the upper and lower barriers spaced apart and forming an isolation volume therebetween at an isolation volume pressure, the pressure relief accumulator comprising:
[0008] an internal chamber configured to be charged with a pressurized gas to a chamber pressure; and
[0009] a pressure relief passage between the internal chamber and the isolation volume, the pressure relief passage comprising:
[0010] a plug passage sealingly retaining a moveable plug, the moveable plug moveable within the plug passage between (a) a first location proximal and in fluid communication with the isolation volume and (b) a second location spaced from the first location; and
[0011] a chamber communication passage comprising (a) a first end in fluid communication with the plug passage at a connection location between the first location and the second location and (b) a second end in fluid communication with the internal chamber, the moveable plug biased toward the first location by the chamber pressure;
[0012] wherein when the chamber pressure is greater than the isolation volume pressure, the moveable plug remains biased toward the first location by the chamber pressure, thereby blocking fluid communication between the isolation volume and the internal chamber via the chamber communication passage; and
[0013] wherein when the isolation volume pressure is greater than the chamber pressure, the moveable plug is biased toward the second location by the isolation volume pressure past the connection location, thereby allowing fluid communication at the connection location between the isolation volume and the internal chamber via the chamber communication passage and allowing pressurized fluid in the isolation volume to enter the internal chamber thus reducing the isolation volume pressure.
[0014] In some exemplary embodiments of the first broad aspect, the wellbore is part of a thermal hydrocarbon recovery operation, the isolation volume contains a fluid, and the isolation volume pressure becomes greater than the chamber pressure due to thermal expansion of the fluid due to heat adjacent the wellbore from the thermal hydrocarbon recovery operation.
[0015] The upper and lower barriers may be plugs deployed for a well workover.
[0016] The pressure relief accumulator is preferably configured for connection to a downhole side of the upper barrier.
[0017] The internal chamber may comprise at least two sections in fluid communication. The pressurized gas is preferably nitrogen and the charging of the internal chamber may occur at surface via a dual check valve.
[0018] In some exemplary embodiments, the moveable plug comprises at least one collet finger configured to engage a corresponding detent in the second location to secure the moveable plug at the second location. The chamber communication passage may comprise at least two chamber communication passages connecting the plug passage and the internal chamber. The plug passage preferably comprises a peripheral shoulder at the first location against which the moveable plug is biased by the chamber pressure when the chamber pressure is greater than the isolation volume pressure.
[0019] According to a second broad aspect of the present disclosure, there is provided a method for reducing pressure between upper and lower barriers in a wellbore, the method comprising the steps of:
[0020] a. deploying and securing the lower barrier in the wellbore;
[0021] b. providing an accumulator comprising:
[0022] an internal chamber; and
[0023] a pressure relief passage comprising:
[0024] a plug passage sealingly retaining a moveable plug, the moveable plug moveable within the plug passage between a first location and a second location spaced from the first location; and
[0025] a chamber communication passage comprising a first end in fluid communication with the plug passage at a connection location between the first location and the second location and a second end in fluid communication with the internal chamber;
[0026] c. charging the internal chamber with a pressurized gas to a chamber pressure, thereby biasing the moveable plug toward the first location;
[0027] d. deploying and securing the upper barrier and the accumulator in the wellbore, the upper barrier spaced from the lower barrier to form an isolation volume therebetween at an isolation volume pressure, the first location proximal and in fluid communication with the isolation volume, the internal chamber in fluid communication with the isolation volume by means of the pressure relief passage;
[0028] e. allowing the moveable plug to remain biased toward the first location by the chamber pressure when the chamber pressure is greater than the isolation volume pressure, thereby blocking fluid communication between the isolation volume and the internal chamber via the chamber communication passage; and
[0029] f. allowing the moveable plug to be biased toward the second location by the isolation volume pressure past the connection location when the isolation volume pressure is greater than the chamber pressure, thereby allowing fluid communication at the connection location between the isolation volume and the internal chamber via the chamber communication passage and allowing pressurized fluid in the isolation volume to enter the internal chamber thus reducing the isolation volume pressure.
[0030] In some exemplary embodiments of the second broad aspect, the wellbore is part of a thermal hydrocarbon recovery operation, the isolation volume contains a fluid, and the isolation volume pressure becomes greater than the chamber pressure due to thermal expansion of the fluid due to heat adjacent the wellbore from the thermal hydrocarbon recovery operation.
[0031] The upper and lower barriers may be plugs deployed for a well workover.
[0032] In some exemplary methods, step d comprises connecting the accumulator to a downhole side of the upper barrier.
[0033] The internal chamber may comprise at least two sections in fluid communication. The pressurized gas is preferably nitrogen and step c may comprise charging of the internal chamber with the nitrogen at surface via a dual check valve.
[0034] In some exemplary embodiments, the moveable plug comprises at least one collet finger configured to engage a corresponding detent in the second location to secure the moveable plug at the second location. The chamber communication passage may comprise at least two chamber communication passages connecting the plug passage and the internal chamber. The plug passage preferably comprises a peripheral shoulder at the first location against which the moveable plug is biased by the chamber pressure when the chamber pressure is greater than the isolation volume pressure.
[0035] A detailed description of exemplary embodiments of the present disclosure is given in the following. It is to be understood, however, that the present disclosure is not to be construed as being limited to these embodiments. The exemplary embodiments are directed to particular applications of the present disclosure, while it will be clear to those skilled in the art that the present disclosure has applicability beyond the exemplary embodiments set forth herein.BRIEF DESCRIPTION OF THE DRAWINGS
[0036] In the accompanying drawings, which illustrate exemplary embodiments of the present disclosure:
[0037] FIG. 1 is a side elevation view, partially in section, of an exemplary embodiment of a pressure relief accumulator according to the present disclosure.
[0038] FIG. 2 is a side elevation view of an exemplary embodiment with a pressure relief accumulator connected to a retrievable bridge plug.
[0039] FIG. 3a is a side sectional view of an exemplary embodiment with gas charged and a plug seated in a downhole position.
[0040] FIG. 3b is a side sectional view of the exemplary embodiment of FIG. 3a with the plug moving uphole due to increased pressure in the isolation volume.
[0041] FIG. 3c is a side sectional view of the exemplary embodiment of FIG. 3a with the plug displaced fully in the uphole direction.
[0042] FIG. 4a is a side sectional view of an exemplary embodiment with gas charged and a plug seated in a downhole position.
[0043] FIG. 4b is a side sectional view of the exemplary embodiment of FIG. 4a with the plug displaced fully in the uphole direction.
[0044] FIG. 4c is a partial side sectional view illustrating an extended chamber.
[0045] Exemplary embodiments will now be described with reference to the accompanying drawings.DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
[0046] Throughout the following description, specific details are set forth in order to provide a more thorough understanding to persons skilled in the art. However, well known elements may not have been shown or described in detail to avoid unnecessarily obscuring the disclosure. The following description of examples of the present disclosure is not intended to be exhaustive or to limit the present disclosure to the precise form of any exemplary embodiment. Accordingly, the description and drawings are to be regarded in an illustrative, rather than a restrictive, sense.
[0047] The present disclosure is directed to an accumulator for relieving pressure between two barriers in casing in a wellbore. The accumulator is provided with a moveable plug that is moveable due to the pressure differential between the internal chamber within the accumulator and the pressure outside the accumulator. When the pressure in the chamber is higher, it presses the plug to a first location that thereby blocks a passage between the accumulator exterior and the internal chamber. When the pressure outside the accumulator is higher, it presses the plug toward a second position, thereby opening up the passage between the exterior and the internal chamber, allowing higher-pressure fluid to enter the internal chamber and thus reduce the pressure in the space between the two barriers.
[0048] Turning now to FIG. 1, an accumulator 10 is illustrated. This exemplary embodiment is shown partly in section to display internal components, and it is shortened at the two cut lines reducing the length of the internal chamber 14 (comprising two sections in the illustrated embodiment), which chamber 14 represents a much larger percentage of the overall length. The accumulator 10 is provided with a quick-connect component 56 (a ported tubing nipple) for connection to an upper barrier / plug 12 (as shown in FIG. 2 and described below). The accumulator 10 is shown disposed in a vertical orientation, but it will be known to those of skill in the art that the casing within which the accumulator 10 is to be deployed could be at any orientation between vertical and horizontal.
[0049] The accumulator 10 comprises an internal chamber 14 with two sections in series, the two sections connected by an accumulator crossover 58 through threaded connections, although only a single-section internal chamber could be employed in other embodiments. The use of the accumulator crossover 58 allows the connecting of a plurality of internal chamber 14 sections as desired for a particular application and pressure regime. The chamber 14 sections and intervening crossover 58 are generally cylindrical and provide a metallic casing to contain a pressurized gas 16, such as for one non-limiting example nitrogen. The pressurized gas 16 is preferably introduced to the chamber 14 by means of a removable valve manifold 62 (as shown in FIG. 3a) to the port with a dual check valve 48 which helps ensure containment of the pressurized gas 16 allowing removal of the manifold 62, although other suitable mechanisms would be known to those skilled in the art. The pressure at which the pressurized gas 16 is established and the collective length of the chamber 14 section(s) are selected based on the anticipated downhole pressure and the expected increase in volume due to thermal expansion between the upper and lower barriers / plugs which may cause failure of the barriers / plugs and casing. The initial pressurized gas 16 pressure which will need to be applied to the accumulator 10 will need to be calculated to balance the hydrostatic pressure at the true vertical setting depth plus a safety factor (to account for surge pressures in running and setting the accumulator 10 and upper barrier / plug 12), for one non-limiting example:Fluid Density×(0.00981)×Plug Setting Depth (mtvd) Plus Safety Factor of 700 Kpa
[0050] The accumulator 10 also comprises a pressure relief passage 18, which passage 18 comprises a combination of a plug passage 22 and a plurality of chamber communication passages 30. The plug passage 22, within which a moveable plug 24 is slidably disposed for movement therealong between a first location 26 (which opens external to the accumulator 10) and a second location 28. The plug 24 is forced to move within the plug passage 22 due to the pressure differential between the pressurized gas 16 in the internal chamber 14 and an external fluid pressured into the first location 26 (discussed below). The plug 24 is provided with seals (O-rings in the illustrated embodiment) to maintain a suitably tight seal against the walls of the plug passage 22 and help prevent pressurized fluid from passing around the plug 24. The plug 24 is further provided with collet fingers 50 which are configured for receipt within corresponding detents 52 in the second location 28, which collet fingers 50 become frictionally engaged in the detents 52 and secured therein. The plug passage 22 comprises a peripheral shoulder 54, against which the plug 24 abuts when forced into the first location 26 of the plug passage 22 by the pressure of the pressurized gas 16 in the internal chamber 14. The pressure of the pressurized gas 16 is selected in order to maintain the plug 24 against the peripheral shoulder 54 until the external pressure would approach unsafe levels given the ratings of the barriers / plugs and casing.
[0051] The plug passage 22 is illustrated as being parallel to the long axis of the accumulator 10, but it will be clear based on the within teaching that the plug passage 22 could be provided in other orientations, which can include without limitation a perpendicular orientation with the first location 26 disposed in a lateral surface of the accumulator 10 rather than a downhole end as shown. The skilled person would be able to determine a suitable orientation given the context and the within teaching.
[0052] The accumulator 10 further comprises a plurality of chamber communication passages 30 to connect the plug passage 22 to the internal chamber 14. There are only two chamber communication passages 30 shown in the sectional view of FIG. 1, but there are preferably a number of such passages 30 disposed around the plug passage 22 axially parallel therewith (as illustrated, for example, in a plan detail in FIG. 3b). Each of the chamber communication passages 30 connects at a first end 32 to the plug passage 22 at a connection location 34 which is disposed between the first location 26 and the second location 28. Each of the chamber communication passages 30 comprises an opposite second end 36 in fluid communication with the internal chamber 14. As can be seen, therefore, external fluids of a higher pressure than the pressurized gas 16 may pass through the plug passage 22 and the chamber communication passages 30 and into the internal chamber 14 when the plug 24 is disposed toward the second location 28 due to that higher pressure.
[0053] The accumulator 10 is also provided with a pressure release valve 60, to allow release of any pressure trapped within the accumulator 10 when the tool is brought to surface with the upper barrier / plug.
[0054] Turning now to FIG. 2, the accumulator 10 is shown as part of a tool string secured within a casing 42, which casing 42 itself is positioned within a wellbore 40. A lower barrier / plug 44 (shown in simplified form but preferably in a form similar to the illustrated upper barrier / plug 12) is secured in place within the casing 42, to provide a first barrier against downhole fluid pressure within the casing 42. Rather than inject nitrogen under pressure against the lower barrier / plug 44 to displace dead fluid as in the prior art, the upper barrier / plug 12 (which for example may be a bridge plug or double grip packer, which may for example be set by wireline, coiled tubing, or jointed pipe, and retrieved by coiled tubing or jointed pipe) is lowered into the casing 42—with the accumulator 10 connected to a downhole side 46 of the upper barrier / plug 12—and secured at a point uphole of the lower barrier / plug 44 such that the accumulator is disposed between the barriers / plugs 12, 44. The upper and lower barriers / plugs 12, 44 may be bridge plugs, packers, or any other suitable isolation tool. The accumulator 10 comprises the quick-connect module 56 for connection to a corresponding module on the downhole side 46 of the upper barrier / plug 12, and the two sections of the internal chamber 14 are connected by the accumulator crossover 58. Each of the barriers / plugs 12, 44 is sealingly engaged with the casing 42, in order to prevent fluids within the casing 42 from passing uphole. The positioning of the upper and lower barriers / plugs 12, 44 thus forms an isolation volume 20 therebetween with residual dead fluid 38 now trapped between the barriers / plugs 12, 44. As noted above, such residual fluid 38 may become heated due to process heat contained in the casing and surrounding reservoir, causing thermal expansion of the fluid 38 and increased pressure within the isolation volume 20.
[0055] The scope and utility of the present disclosure is further illustrated in another exemplary embodiment of FIG. 3a to 3c. FIG. 3a to 3c illustrate in staged fashion how movement of the plug within the plug passage due to the pressure differential can open a passageway between the isolation volume and the internal chamber to displace pressurized dead fluid into the accumulator in an effort to reduce the external pressure and protect the integrity of the plugs and casing. FIG. 3a illustrates an accumulator 90 comprising a single-section internal chamber 92, a plug passage 94, and a dual check valve arrangement 96. The internal chamber 92 is charged with pressurized gas using a gas injection valve arrangement 62, the gas injection valve arrangement 62 comprising a lateral valve connected to a pressurized gas source (not shown) to receive the pressurized gas, and a downhole-oriented valve connected to a pressure gauge (not shown). The pressurized gas in the internal chamber 92 enters the chamber communication passages 100 (which are arranged in parallel fashion around the plug passage 94 as shown in a small plan detail in FIG. 3b), and then into the plug passage 94 uphole of the plug 98. This pressure acting against the uphole side of the plug 98 forces the plug 98 against a peripheral shoulder 104 in the plug passage 94, thus blocking fluid communication between the internal chamber 14 and the surrounding external space. The accumulator 90, now charged with pressurized gas with the plug 98 in place against the peripheral shoulder 104, can be deployed downhole with the upper barrier / plug after deployment of the lower barrier / plug as described above.
[0056] As seen in FIG. 3b, the accumulator 90 is provided with a protective cap 64 after charging and before deployment downhole, which cap 64 is threaded onto the accumulator 90 at a threaded engagement section 68. Note that in FIG. 3b there is a small plan view of the chamber communication passages 100 (which correspond with the chamber communication passages 30 shown in FIG. 1) to illustrate their positioning relative to the plug passage 94. The accumulator 90 is deployed downhole and secured in place with the upper barrier / plug, creating the isolation volume with trapped dead fluid therein. Due to process heat present in the casing and the surrounding reservoir, the trapped dead fluid is heated and undergoes thermal expansion. This creates an increase in pressure in the isolation volume, and as that increased pressure exceeds the pressure of the pressurized gas in the internal chamber 92 the plug 98 is pressed away from the first location toward the second location. FIG. 3b illustrates the beginning of that plug 98 displacement, with the plug 98 beginning to release from the peripheral shoulder 104. At this stage, the plug 98 is still blocking the plug passage 94 and disallowing pressurized fluid from the isolation volume from entering the chamber communication passages 100 and thus the internal chamber 92.
[0057] Turning now to FIG. 3c, it can be seen that the plug 98 is now fully disengaged from the peripheral shoulder 104, allowing higher-pressure fluid from the isolation volume to move under pressure through the pressure relief passage (the plug passage 94 and then the chamber communication passages 100) toward the internal chamber 92, pressing against the pressurized gas therein. This has the effect of reducing the pressure within the isolation volume. FIG. 3c also illustrates what the accumulator 90 would look like at surface after its use during the workover, with a pressure release valve 102 being engaged with a tool 66 to open the valve 102 and release any residual pressure remaining within the internal chamber 92 of the accumulator 90. There are commercially available products suitable for use as the tool 66, which in the illustrated embodiment threads over the check valve assembly of the valve 102 and has a manual lead screw to force the ball of the valve 102 off the seat, allowing pressure to bleed from the internal chamber 92.
[0058] Turning now to FIG. 4a to 4c, a further exemplary embodiment is illustrated. An accumulator 72 comprises a cap 74 (which again may be threadably engaged with the accumulator 72), but the cap 74 is configured to allow external access to a dual check valve 84 (for charging an internal chamber with pressurized gas) and a pressure release valve (disposed between the dual check valve 84 and a plug passage 80), unlike other embodiments where the cap must be removed to access these components. As the dual check valve 84 is accessible with the cap 74 in place, a charging valve arrangement 76 can be used to inject gas into the accumulator 72 interior; the charging valve arrangement 76 comprises a lateral valve for connection to the pressurized gas source (not shown), and a downhole-oriented valve for connection to a pressure gauge (not shown). The pressure release valve or de-pressuring valve is also accessible with the cap 74 in place, and so a valve access point 78 can be seen (which may for one non-limiting example be a commercially-available check valve such as the “High Pressure Vented Cap Free Flow Body Grease Fitting” available from GRM Flow Products). The pressure release valve or de-pressuring valve is used with a commercially-available pressure release tool to de-pressure the accumulator 72. A plug 82 can be seen within the plug passage 80—in a downhole position in FIG. 4a due to the pressure of the pressurized gas within the accumulator 72, and in an uphole position in FIG. 4b after deployment once the external pressure exceeds the internal pressure and forces the plug 82 toward the uphole location with collet fingers engaged with the detents. Although FIGS. 4a and 4b illustrate a single-section internal chamber, FIG. 4c illustrates an accumulator crossover 86 that can be used to connect chamber sections in series to increase the tool length and therefore the ability to take in more pressurized fluids from the isolation volume between the upper and lower barriers / plugs.
[0059] The foregoing is considered as illustrative only of the principles of the present disclosure. The scope of the claims should not be limited by the exemplary embodiments set forth in the foregoing, but should be given the broadest interpretation consistent with the specification as a whole.
Claims
1. A pressure relief accumulator for reducing pressure between upper and lower barriers in a wellbore, the upper and lower barriers spaced apart and forming an isolation volume therebetween at an isolation volume pressure, the pressure relief accumulator comprising:an internal chamber configured to be charged with a pressurized gas to a chamber pressure; anda pressure relief passage between the internal chamber and the isolation volume, the pressure relief passage comprising:a plug passage sealingly retaining a moveable plug, the moveable plug moveable within the plug passage between (a) a first location proximal and in fluid communication with the isolation volume and (b) a second location spaced from the first location; anda chamber communication passage comprising (a) a first end in fluid communication with the plug passage at a connection location between the first location and the second location and (b) a second end in fluid communication with the internal chamber, the moveable plug biased toward the first location by the chamber pressure;wherein when the chamber pressure is greater than the isolation volume pressure, the moveable plug remains biased toward the first location by the chamber pressure, thereby blocking fluid communication between the isolation volume and the internal chamber via the chamber communication passage; andwherein when the isolation volume pressure is greater than the chamber pressure, the moveable plug is biased toward the second location by the isolation volume pressure past the connection location, thereby allowing fluid communication at the connection location between the isolation volume and the internal chamber via the chamber communication passage and allowing pressurized fluid in the isolation volume to enter the internal chamber thus reducing the isolation volume pressure.
2. The pressure relief accumulator of claim 1 wherein the wellbore is part of a thermal hydrocarbon recovery operation, the isolation volume contains a fluid, and the isolation volume pressure becomes greater than the chamber pressure due to thermal expansion of the fluid due to heat adjacent the wellbore from the thermal hydrocarbon recovery operation.
3. The pressure relief accumulator of claim 1 wherein the upper and lower barriers are plugs deployed for a well workover.
4. The pressure relief accumulator of claim 1 wherein the pressure relief accumulator is configured for connection to a downhole side of the upper barrier.
5. The pressure relief accumulator of claim 1 wherein the internal chamber comprises at least two sections in fluid communication.
6. The pressure relief accumulator of claim 1 wherein the pressurized gas is nitrogen and the charging of the internal chamber occurs at surface via a dual check valve.
7. The pressure relief accumulator of claim 1 wherein the moveable plug comprises at least one collet finger configured to engage a corresponding detent in the second location to secure the moveable plug at the second location.
8. The pressure relief accumulator of claim 1 wherein the chamber communication passage comprises at least two chamber communication passages connecting the plug passage and the internal chamber.
9. The pressure relief accumulator of claim 1 wherein the plug passage comprises a peripheral shoulder at the first location against which the moveable plug is biased by the chamber pressure when the chamber pressure is greater than the isolation volume pressure.
10. A method for reducing pressure between upper and lower barriers in a wellbore, the method comprising the steps of:a. deploying and securing the lower barrier in the wellbore;b. providing an accumulator comprising:an internal chamber; anda pressure relief passage comprising:a plug passage sealingly retaining a moveable plug, the moveable plug moveable within the plug passage between a first location and a second location spaced from the first location; anda chamber communication passage comprising a first end in fluid communication with the plug passage at a connection location between the first location and the second location and a second end in fluid communication with the internal chamber;c. charging the internal chamber with a pressurized gas to a chamber pressure, thereby biasing the moveable plug toward the first location;d. deploying and securing the upper barrier and the accumulator in the wellbore, the upper barrier spaced from the lower barrier to form an isolation volume therebetween at an isolation volume pressure, the first location proximal and in fluid communication with the isolation volume, the internal chamber in fluid communication with the isolation volume by means of the pressure relief passage;e. allowing the moveable plug to remain biased toward the first location by the chamber pressure when the chamber pressure is greater than the isolation volume pressure, thereby blocking fluid communication between the isolation volume and the internal chamber via the chamber communication passage; andf. allowing the moveable plug to be biased toward the second location by the isolation volume pressure past the connection location when the isolation volume pressure is greater than the chamber pressure, thereby allowing fluid communication at the connection location between the isolation volume and the internal chamber via the chamber communication passage and allowing pressurized fluid in the isolation volume to enter the internal chamber thus reducing the isolation volume pressure.
11. The method of claim 10 wherein the wellbore is part of a thermal hydrocarbon recovery operation, the isolation volume contains a fluid, and the isolation volume pressure becomes greater than the chamber pressure due to thermal expansion of the fluid due to heat adjacent the wellbore from the thermal hydrocarbon recovery operation.
12. The method of claim 10 wherein the upper and lower barriers are plugs deployed for a well workover.
13. The method of claim 10 wherein step d comprises connecting the accumulator to a downhole side of the upper barrier.
14. The method of claim 10 wherein the internal chamber comprises at least two sections in fluid communication.
15. The method of claim 10 wherein the pressurized gas is nitrogen and step c comprises charging of the internal chamber with the nitrogen at surface via a dual check valve.
16. The method of claim 10 wherein the moveable plug comprises at least one collet finger configured to engage a corresponding detent in the second location to secure the moveable plug at the second location.
17. The method of claim 10 wherein the chamber communication passage comprises at least two chamber communication passages connecting the plug passage and the internal chamber.
18. The method of claim 10 wherein the plug passage comprises a peripheral shoulder at the first location against which the moveable plug is biased by the chamber pressure when the chamber pressure is greater than the isolation volume pressure.