Annular pressure relief casing collapse protector

Abstract

The pressure relief assembly of the present invention is adapted to be arranged in cemented wellbore casings in high temperature wells to bleed off annular pressure on the casing due to pore pressure in cement structures around the casing or entrapped water builds up as the well heats up when used preventing collapse of the casing.

Description

[0001] ANNULAR PRESSURE RELIEF CASING COLLAPSE PROTECTOR

[0002] Field

[0003] The invention relates to a pressure relief system for casings used in high temperature wells. More specifically the present invention provides a pressure relief valve for preventing casing collapse due to high annular pressure in a cemented annulus from a temperature increase in high temperature wellbores.

[0004] Introduction

[0005] In the field of geothermal power utilization, wells are drilled to required depth for extracting geothermal media of liquid and or steam. Steel pipes (casings) are run in hole and cemented with the purpose of support against the rock formations for deeper drilling, for well control during drilling and serve as a conduit for the geothermal fluid once the well has been drilled. Wells can have several concentric casings where each new casing is deeper than the one before it. In high-temperature geothermal wells the external surface of all casings is cemented covering the casing from its casing shoe up to the surface. The cement sheath ensures sealing of different feed zones and acts as a corrosion protection for the casing. However, in some instances water can become entrapped in the annulus. The water expands when the well warms up and can generate annular pressure high enough for the casing to collapse into the wellbore.

[0006] Wells are drilled with coolant, a drilling fluid that circulates through the drill string and drill bit, that then flows up the well cleaning it of cuttings. This cooling is essential when drilling into high-temperature geothermal fields. After the well has been drilled, it is allowed to warm up for several weeks or months until it is ready for flow testing. During warm-up and flow testing the casings can warm up quickly and any excess water in the underlying cement sheath can reach high pressures near to the boiling point of confined water. Since the cement sheath is enclosed between casings or casing and rock formation, water pressure can build up to a critical level. In the worst cases these pressures can exceed the external pressure limit of the casing the so called "collapse resistance" and lead to a failure where the casing collapses. Thereby the wells cross-section becomes reduced, and the potential power output likewise is reduced. These kinds of collapses are sometimes called bulges or puckers as the collapse mode is such that the casing collapses on one side into the cross-section (Figure 1). These bulges can be severe enough to prevent entry into the well, e.g. logging tools to log temperatures and pressures in the well, or in worst cases block the flow from the well. The solution has been to mill out the damage and place and cement a less diameter casing into the failed casing. This also reduces the cross-section and power output of the well. Preventing these collapses from occurring is therefore valuable. If a cementing job is not successful and water pocket is entrapped, for instance, between two cementing jobs, the risk of casing collapse is high. In these cases where cement bond logging have indicated entrapped water, the casing is perforated by explosives, leaving around Vi inch hole in the casing. This is a solution to mitigate collapse of the casing, but in turn it leaves the casing and cement sheath exposed to the geothermal fluid that with time can further diminish the structural integrity of the well. The present invention relieves the annular pressure but closes again afterwards leading to a sound downhole structure.

[0007] Note that the likelihood of casing collapse can be affected by concurring external and internal forces, e.g., net external pressure, hoop-stress, axial tension / compression, and manufacturing irregularities, e.g., eccentricity, ovality, corrosion and wear. Collapse resistance of unsupported casings is well defined by API / ISO standards. However, casings supported by cement have been shown to have higher resistance to collapse (Figure 2), but non-uniform geometry and / or loads can severely impact the structural stability and cause premature collapse.

[0008] There exist solutions for pressure relief in well bore casings in the art, but most of those solutions relate to oil and gas wells to solve gas pressure within an uncemented casing-to- casing annulus. As mentioned above, in geothermal wells the pressure problem is from outside the casings due to boiling excess water in the cement sheath causing casing collapse from external pressure.

[0009] CN108019171 A discloses a radial pressure-relieving device for a downhole drilling of oil and gas casings having a pressure-relieving short section arranged on a circular casing wall to relief pressure from within the casing. The pressure-relieving device comprises an opening through the device with a pressure relieve valve arranged on a static plug, where the pressure relieve valve comprises a spherical seal member arranged on a suspension element and sealing the inlet opening into the pressure-relieving device.

[0010] Summary

[0011] The present invention provides a solution for cemented well bore casings to withstand casing placement, casing cementing, high cementing slurry pressures without initiating pressure relief. Once the well starts to warm up to certain temperature levels and annular pressures become high enough, the pressure relieve device and system of the invention becomes active. The solution presented herein provides a relief valve system and device to be placed in the casing wall by threaded conical connection and / or by welding. The device of the invention is designed such that it remains shut and sealed during casing installation, casing cementing under high cement slurry pressures and cement curing. When the well bore heats up from the rock formation surroundings and as high temperature media starts to flow up the casings and heat up, pore pressure in cement or pressure due to entrapped water builds up, but the relief valve system of the present invention prevents collapsing of the casing, and the element temporarily bleeds off the annular pressure into the well. The relief valve system is activated at a level approaching the collapse resistance of the casing when external pressure load is in alignment of a load preset of an internal disc spring system. Additionally, temperatures need to reach a given level for the pressure relieving mechanism to become active. When differential pressure has been reduced the relief valve system closes and seals again against internal pressure.

[0012] The relief valve system comprises a pressure relief assembly which is pressure tight against internal casing pressure. Furthermore, the seal is conical such that increased internal pressure increases the sealing pressure. The advantage of using the relief valve system of the present invention includes: i) reducing the likelihood of a casing collapse from too high annular pressure and therefore increases the structural integrity of the well, ii) prevention of casing collapse in turn ensures that the power output from the well is not reduced from reduced cross-section of the flowing media of steam and / or water, iii) an overall reduce in cost of having to drill new wells for those that are lost to severe casing failures, and iv) increasing chance of success drilling into superhot conditions where the risk of casing failures is increased due to elevated temperatures compared to conventional high- temperature geothermal wells. Since the present invention is designed to function in a cemented annulus, it remains sealed during casing placement, circulation and placement of cement, and activates only when the well warms up and annular pressures reach specific level. Temperatures typically reach 250-300 degrees centigrade but in higher enthalpy wells, so called superhot wells, the temperatures can reach 400-500 degrees. Increased temperatures result in higher annular pressures and thus higher likelihood of casing collapse.

[0013] The pressure relief assembly of the present invention is adapted to be arranged in wellbore casings in high temperature wells, to bleed off annular pressure on the casing due to pore pressure in cement structures around the casing or due to entrapped water as the well heats up, when used preventing collapse of the casing. The pressure relief assembly has a main body with inwardly tapered outer side wall to be secured into matching openings in wellbore casings and a bottom portion with an outlet opening. The inwardly tapered outer sidewall is arranged with a screw thread to be secured into matching openings in wellbore casings by welding having a mating screw thread to prevent the pressure relief assembly to be pushed into the casing as well as securing the assembly in the casing. The screw thread of the inwardly tapered outer sidewall results in the screw thread to be in a tapered or downwardly diagonal direction further aiding in securing the pressure relief assembly in the casing and preventing it to be pushed into the casing due to outside pressure. The main body is closed with a cap having an inlet and a check valve connected to a plug in the bottom of the assembly to release pressure into the casing. The functional portion of the pressure relief assembly has a press plate connected to (secured to) a plug mating with an outlet opening in the bottom portion of the main body and a suspension device is arranged between the press plate and the plug to facilitate the pressure relief of the device so that the function of the pressure relief assembly is facilitated by a single mechanism through the whole device. The plug has outwardly tapering side walls adapted to close a mating inwardly tapered outlet opening of the bottom of the main body to ensure that the plug can only be pushed in the direction into the casing relieving pressure from the outside of the casing.

[0014] The activation of the pressure relief assembly is prevented during cementing of a wellbore by a seal restriction in the inlet of the pressure relief assembly. When the well heats up during operation, the seal melts and the pressure relief assembly can be activated by sufficient pressure level.

[0015] It is an object of the present invention to overcome and / or ameliorate the aforementioned drawbacks of the prior art and to provide an improved and / or alternative and / or additional device, system or method for a pressure relief system for cemented wellbore casings in high temperature wells. It is one preferred aspect of the present invention to provide a pressure relief assembly for wellbore casings in high temperature wells. Moreover, it is a preferred aspect of the present invention to provide a method for relieving external pressure on wellbore casings in high temperature wells.

[0016] The object(s) underlying the present invention is (are) particularly solved by the features defined in the independent claims. The dependent claims relate to preferred embodiments of the present invention. Further additional and / or alternative aspects are discussed below.

[0017] In one aspect of the present invention a pressure relief assembly for wellbore casings in high temperature wells is provided. The pressure relief assembly comprises i) a main body adapted to be secured into matching openings in wellbore casings, where the main body further comprises a cavity and an opening in the bottom of the main body, ii) a cap adapted to be fitted into the main body and covering the cavity of the main body, said cap further comprising an inlet, iii) a check valve assembly, and iv) a plug adapted to close the opening in the bottom of the main body when the pressure relief assembly is in a closed position. The check valve assembly is connected to the inlet in the cap and to the plug to relief external pressure on the casing through the opening in the bottom of the main body when in use.

[0018] In one aspect of the present invention a pressure relief system for wellbore casings in high temperature wells is provided. The system comprises; i) providing a pressure relief assembly according to the invention comprising a main body adapted to be secured into matching openings in wellbore casings and where the main body further comprises a cavity and an opening in the bottom of the main body and a plug adapted to close the opening in the bottom of the main body, ii) arranging a cap according to the invention into the upper part of the main body and covering the cavity of the main body, where the cap further comprises an inlet, iii) arranging a check valve assembly according to the invention in the cavity of the main body. The system is characterised in that iv) the check valve assembly is connected to the inlet in the cap and to the plug to relief external pressure on the casing through the opening in the bottom of the main body, v) in that the pressure relief assembly is adapted to be in a closed position when no pressure force is on the check valve assembly, where the plug closes the opening in the bottom of the main body, and vi) in that the pressure relief assembly is adapted to be in an open position when pressure force is on the check valve assembly, where the plug is pushed revealing the opening in the bottom of the main body when in use.

[0019] In one aspect of the present invention a method is provided for relieving external pressure on wellbore casings in high temperature wells, where the method comprises the steps of: a) providing one or more pressure relief assemblies according to the present invention b) arranging a temporary polymer seal in the inlet(s) of the cap of the one or more pressure relief assemblies, c) securing one or more pressure relief assemblies in into matching openings in wellbore casings, wherein the check valve assembly in the one or more pressure relief assemblies is connected to the inlet in the cap and to the plug.

[0020] The method is characterized in that: i) the temporary polymer seal melts when the well warms up revealing the inlet of the cap into the pressure relief assembly, ii) wherein the pressure relief assembly is adapted to be in a closed position when no pressure force is on the check valve assembly, where the plug closes the opening in the bottom of the main body, and iii) wherein the pressure relief assembly is adapted to be in an open position when pressure force of sufficient level is on the check valve assembly, where the plug is pushed revealing the opening in the bottom of the main body.

[0021] All embodiments listed herein relate to aspects of both devices, systems and the methods of the present invention.

[0022] In an embodiment of the present invention the main body comprises inwardly tapered outer sidewalls. In an embodiment of the present invention the main body comprises an inwardly tapered annular outer sidewall.

[0023] In an embodiment of the present invention the cavity of the main body comprises an annular inner side wall and a bottom wall.

[0024] In an embodiment of the present invention the cavity of the main body comprises an annular inner side wall and an annular bottom wall.

[0025] In an embodiment of the present invention the cavity of the main body comprises an outlet in the bottom wall.

[0026] In an embodiment of the present invention the cap comprises an upper wall and an annular side wall.

[0027] In an embodiment of the present invention the cap comprises an inlet in the upper wall of the cap.

[0028] In an embodiment of the present invention an annular or peripheral groove is arranged between the mating surfaces of the cavity and the cap.

[0029] In an embodiment of the present invention the cap comprises an annular groove in the annular side wall.

[0030] In an embodiment of the present invention the cap comprises an annular groove in the lower edge of the annular inner side wall.

[0031] In an embodiment of the present invention the inwardly tapered outer sidewall is secured into matching openings in wellbore casings by welding.

[0032] In an embodiment of the present invention the inwardly tapered outer sidewall is an annular side wall further comprising a screw thread to be secured into matching openings in wellbore casings by welding having a mating screw thread.

[0033] In an embodiment of the present invention the bottom of the main body comprises an outlet opening for pressure relief into the casing.

[0034] In an embodiment of the present invention the opening in the bottom of the main body is outwardly tapered.

[0035] In an embodiment of the present invention the plug has mating inwardly tapering side walls adapted to close the opening in the bottom of the main body.

[0036] In an embodiment of the present invention the inlet of the cap comprises a structural circumference adapted to mate with a head of a screwing device. The pressure relief assembly according to claim, wherein the structural circumference of the inlet of the cap comprises a hexagonal circumference.

[0037] In an embodiment of the present invention a temporary polymer seal is arranged in the inlet of the cap.

[0038] In an embodiment of the present invention the check valve assembly further comprises a press plate and a suspension device arranged underneath the press plate.

[0039] In an embodiment of the present invention the check valve assembly further comprises a press plate attached to or secured to the plug, and a suspension device being arranged between the press plate and the plug.

[0040] In an embodiment of the present invention the press plate comprises openings to provide limited flow through the press plate and through the valve assembly.

[0041] In an embodiment of the present invention the press plate comprises a central downward extending elongated structure with means for engaging with and a central and upwardly extending orifice of the plug for securing the press plate to the plug.

[0042] In an embodiment of the present invention the central downward extending elongated structure of the press plate comprises a screw thread and wherein the central and upwardly extending orifice of the plug comprises a mating screw thread for the central downward extending elongated structure of the press plate for securing the press plate to the plug.

[0043] In an embodiment of the present invention the bottom of the plug comprises an orifice with structural circumference adapted to mate with a head of a screwing device.

[0044] In an embodiment of the present invention the structural circumference of the orifice in the bottom of the plug comprises a hexagonal circumference.

[0045] In an embodiment of the present invention the plug is secured or attached to the press plate.

[0046] In an embodiment of the present invention the plug is secured to the press plate by press fitting.

[0047] In an embodiment of the present invention the suspension device comprises a spring device.

[0048] In an embodiment of the present invention the spring device comprises disc springs.

[0049] In an embodiment of the present invention the cap comprises side structures to secure the cap to side structures in the inner side of the cavity of the main body. In an embodiment of the present invention side structures of the cap comprises a screw thread to secure the cap to a mating screw thread being the side structures in the inner side of the cavity of the main body.

[0050] In an embodiment of the present invention the upper side of the main body comprises engagement structures for engaging with a securing or screwing device.

[0051] In an embodiment of the present invention one or more pressure relief assemblies are arranged in wellbore casings, where the wellbore casings are covered by a cement sheath.

[0052] In an embodiment of the present invention one or more pressure relief assemblies are arranged in wellbore casings, where the wellbore casings are cemented metal casings.

[0053] In an embodiment of the present invention the press plate is pushed due to pressure difference and a limited flow is allowed through openings on the press plate.

[0054] In an embodiment of the present invention a temporary polymer seal is arranged in the inlet of the cap.

[0055] In an embodiment of the present invention the temporary polymer seal melts when the well warms up revealing the inlet of the cap into the pressure relief assembly.

[0056] In an embodiment of the present invention a seal ring is arranged in the annular (peripheral) groove at the lower edge of the annular inner side wall of the cap.

[0057] In an embodiment of the present invention a seal ring is a metal seal ring or a graphite seal ring.

[0058] In the present context the term "high temperature wells" refers to wells for geothermal media, gas or oil. The device, system and method of the present invention is designed to work in wells in high temperature areas of 100°C or more, where the temperature change in the casings between well completion and production differs more than 50°c, such as 100°C, 150°C,200°C, 250°C, 300°C, or even 450°C.

[0059] In the present context the terms "opening", "outlet", and "outlet opening" in relation to the orifice in the bottom of the main body all refer to an opening for relieving pressure into the casing.

[0060] In the present context the term "cap" refers to a cap closure, a lid or a cover for covering or sealing the cavity of the pressure relief assembly. Brief description of the drawings

[0061] The skilled person will understand that the drawings, described below, are for illustration purposes only. The drawings are not intended to limit the scope of the present teachings in any way.

[0062] FIG. 1 shows a cross-section of a casing collapse from one side into the casing, or so called bulges or puckers.

[0063] FIG. 2 shows a diagram for collapse resistance of unsupported casings and cement supported casings.

[0064] FIG. 3 is a schematic transection view of a pressure relief assembly in a wellbore casings.

[0065] FIG. 4 is a perspective top and bottom view of a pressure relief assembly of an embodiment of the present invention.

[0066] FIG. 5 is a cross-sectional perspective view of a pressure relief assembly of one embodiment of the present invention.

[0067] FIG. 6 is a cross-sectional perspective view of a pressure relief assembly of an embodiment of the present invention.

[0068] FIG. 7 is a cross-sectional view of a pressure relief assembly in a closed and open position.

[0069] Description of various embodiments

[0070] In the following, exemplary embodiments of the invention will be described, referring to the figures. These examples are provided to provide further understanding of the invention, without limiting its scope.

[0071] It should be appreciated that the invention is applicable for connecting casing segments in general in borehole for utilizing high temperature media, such as oil and geothermal media. Further, the connector, a system and the method according to the invention is illustrated in the embodiments that follow with a preferred embodiment of geothermal borehole, but it should be appreciated that the invention is also applicable to drilling for other purposes such as oil wells.

[0072] For the drawings below the embodiments show a connector where the sliding inner member is attached to the upper casing during drilling and assembly. The skilled person will appreciate that the connector can be used such that the sliding inner member is attached to the lower casing during drilling and assembly.

[0073] Figure 3 is a schematic transection view of a pressure relief assembly 1 of the present invention in a wellbore casings 2. The pressure relief assembly 1 has a conical shape and is threaded in the casing through screw treads 12 and 13. The pressure relief assembly 1 has an inwardly tapered outer sidewall 11, where the inwardly tapered outer sidewall 11 comprises a screw thread 12 and is secured into matching openings 4 in wellbore casings 2 by welding having a mating screw thread 13.

[0074] Figure 4 shows in shows a perspective top (A) and bottom (B) view of a pressure relief assembly 1 according to one embodiment of the present invention. The pressure relief assembly 1 has a main body 3 with an inwardly tapered outer sidewall 11 with a screw thread 12. The top view shows the cap 7 on top of the main body 3 having an inlet 8 with a structural circumference adapted to mate with a head of a screwing device which is a hexagonal circumference in the embodiment shown here. The bottom view shows the bottom of the main body 3 and the position of a plug 10 in the bottom opening 6 of the main body 3. The plug 10 comprises an orifice 23 with structural circumference adapted to mate with a head of a screwing device being a hexagonal circumference.

[0075] Figure 5 shows a cross-sections of pressure relief assemblies 1 of the present invention. In Fig. 5A, a cross-sectional perspective view of a pressure relief assembly 1 according to one embodiment is shown, where the check valve assembly 9 is indicated by a dotted line circle in the cavity 5 of the main body 3. The figure also shows a tapered outer sidewall 11 of the main body. Figure 5B shows the same cross-sectional perspective view as Fig. 5A from a different angle indication an inlet 8 in the cap 7 and the screw thread 12 of the inwardly tapered outer sidewall 11. The drawing further shows the press plate 16 of the check valve assembly being connected to the plug 10 and a suspension device 17 arranged between the press plate 16 and the plug 10, where the suspension device 17 is a disc spring device.

[0076] Figure 6 also shows a cross-sectional perspective view of a pressure relief assembly 1 according to one embodiment of the present invention. The figure further shows a hexagonal circumference of the inlet 8 in the cap 7 and a temporary polymer seal 15 which melts when the well warms up revealing the inlet of the cap into the pressure relief assembly 1. The figure further outlines the check valve assembly, where the press plate 16 comprises a central downward extending elongated structure 19 with the suspension device 17 arranged around it. The downward extending elongated structure 19 of the press plate 16 is secured to a central and upwardly extending orifice 20 of the plug 10 for connecting the press plate 16 to the plug 10. Furthermore, the main body 3 also has engagement structures 33 in the upper side of the outer sidewall 11 of the main body 3 below the screw thread 12, where the engagement structures 33 allow for arranging the pressure relief assembly in a casing with a securing or a screwing device.

[0077] Figure 7 is cross-sectional view of a pressure relief assembly 1 according to one embodiment of the present invention showing the pressure relief assembly 1 in a closed (A) and open (B) position. The figure shows the engagement structures 33 in the upper side of the main body as well as the orifice 23 in the plug 10. The inlet 8 of the cap 7 is shown as well as side structures 24 of the cap 7 to secure the cap 7 to side structures 25 in the inner side wall 26 of the cavity of the main body 3 being screw threads in this embodiment. The press plate 16 has openings 18 to provide limited flow through the press plate and through the valve assembly. Furthermore, the central downward extending elongated structure 19 of the press plate 16 has a screw thread 21 and the central and upwardly extending orifice 20 of the plug 10 has a mating screw thread 22 for the central downward extending elongated structure of the press plate 16 for securing the press plate to the plug 10. The opening 6 in the bottom of the main body 3 is outwardly tapered to mate with inwardly tapering side walls 14 of the plug 10 adapted to close the opening 6 in the bottom of the main body 3. In Fig. 7A the pressure relief assembly 1 is in a closed position as no pressure force is on the check valve assembly and the plug 10 closes the opening 6 in the bottom of the main body 3. In Fig. 7B the pressure relief assembly 1 is in an open position when pressure as force is on the check valve assembly and the plug 10 is pushed revealing the opening 6 in the bottom of the main body 3.

[0078] Figure 7C shows a pressure relief assembly 1 according to another embodiment of the present invention. The drawing shows a cap 7 fitted into the main body 3 therapy covering the cavity 5 of the main body 3, where the cavity 5 has an annular inner side wall 26 and a bottom wall 27. The cap is shown having an upper wall 28 or surface covering the cavity and an annular inner side wall 29, where in this drawing the annular inner side wall 26 of the main body 3 and the annular inner side wall 29 of the cap 7 are shown as the same line. Also the bottom wall 27 of the cavity and the edge 30 of the inner side wall 29 of the cap 7 are shown as the same line where the cap 7 meats the bottom of the cavity 5. In this embodiment the cap comprises an annular and or peripheral groove 31 (shown empty on the right) in the lower edge of the annular inner side wall and a seal ring 32 is arranged in the annular groove (shown on the right of the drawing) to isolate the pressure relief through the pressure relief assembly over the press plate.

[0079] As used herein, including in the claims, singular forms of terms are to be construed as also including the plural form and vice versa, unless the context indicates otherwise. Thus, it should be noted that as used herein, the singular forms "a," "an," and "the" include plural references unless the context clearly dictates otherwise.

[0080] Throughout the description and claims, the terms "comprise", "including", "having", and "contain" and their variations should be understood as meaning "including but not limited to", and are not intended to exclude other components. The present invention also covers the exact terms, features, values and ranges etc. in case these terms, features, values and ranges etc. are used in conjunction with terms such as about, around, generally, substantially, essentially, at least etc. (i.e., "about 3" shall also cover exactly 3 or "substantially constant" shall also cover exactly constant).

[0081] The term "at least one" should be understood as meaning "one or more", and therefore includes both embodiments that include one or multiple components. Furthermore, dependent claims that refer to independent claims that describe features with "at least one" have the same meaning, both when the feature is referred to as "the" and "the at least one".

[0082] It will be appreciated that variations to the foregoing embodiments of the invention can be made while still falling within the scope of the invention can be made while still falling within scope of the invention. Features disclosed in the specification, unless stated otherwise, can be replaced by alternative features serving the same, equivalent or similar purpose. Thus, unless stated otherwise, each feature disclosed represents one example of a generic series of equivalent or similar features.

[0083] Use of exemplary language, such as "for instance", "such as", "for example" and the like, is merely intended to better illustrate the invention and does not indicate a limitation on the scope of the invention unless so claimed. Any steps described in the specification may be performed in any order or simultaneously, unless the context clearly indicates otherwise.

[0084] All of the features and / or steps disclosed in the specification can be combined in any combination, except for combinations where at least some of the features and / or steps are mutually exclusive. In particular, preferred features of the invention are applicable to all aspects of the invention and may be used in any combination.

Claims

Claims1. A pressure relief assembly (1) for wellbore casings (2) in high temperature wells, the pressure relief assembly (1) comprising :- a main body (3) adapted to be secured into matching openings (4) in wellbore casings (2), said main body further comprising a cavity (5) and an opening (6) in the bottom of the main body (3),- a cap (7) adapted to be fitted into the main body and covering the cavity of the main body 3, said cap further comprising an inlet (8),- a check valve assembly (9), and- a plug (10) adapted to close the opening (6) in the bottom of the main body (3) when the pressure relief assembly (1) is in a closed position, characterised in that the check valve assembly (9) is connected to the inlet (8) in the cap (7) and to the plug (10) to relief external pressure on the casing through the opening (6) in the bottom of the main body (3).

2. The pressure relief assembly (1) according to claim 1, wherein the main body (3) comprises an inwardly tapered outer sidewall (11).

3. The pressure relief assembly (1) according to claim 2, wherein the inwardly tapered outer sidewall (11) is secured into matching openings (4) in wellbore casings (2) by welding or screwing.

4. The pressure relief assembly (1) according to claim 1, wherein the main body comprises an annular inner side wall (26) and a bottom wall (27).

5. The pressure relief assembly (1) according to claim 1, wherein the cap comprises an upper wall (28) and an annular side wall (29).

6. The pressure relief assembly (1) according to claim 1, wherein an annular groove (31) is arranged between the mating surfaces of the cavity (5) and the cap (7).

7. The pressure relief assembly (1) according to claim 6, wherein a seal ring (32) is arranged in the annular groove (31) between the mating surfaces of the cavity (5) and the cap (7).

8. The pressure relief assembly (1) according to claim 7, wherein the seal ring (32) is a metal seal ring or a graphite seal ring.

9. The pressure relief assembly (1) according to claim 1, wherein the opening (6) in the bottom of the main body (3) is an outlet opening (6) for pressure relief into the casing (2).

10. The pressure relief assembly (1) according to claim 1, wherein the opening (6) in the bottom of the main body (3) is outwardly tapered.

11. The pressure relief assembly (1) according to claim 10, wherein the plug (10) has mating inwardly tapering side walls (14) adapted to close the opening (6) in the bottom of the main body (3).

12. The pressure relief assembly (1) according to claim 1, wherein check valve assembly (9) further comprises a press plate (16) and a suspension device (17) arranged underneath the press plate (16).

13. The pressure relief assembly (1) according to claim 12, wherein the press plate (16) is secured to the plug (10) and the suspension device (17) is arranged between the press plate (12) and the plug (10).

14. The pressure relief assembly (1) according to claim 1, wherein the one or more pressure relief assemblies (1) are arranged in wellbore casings (2), where the wellbore casings are cemented metal casings (2).

15. A method for relieving external pressure on wellbore casings (2) in high temperature wells, the method comprising: a) providing a pressure relief assembly (1) according to claims 1 - 14, b) arranging a temporary polymer seal (15) in the inlet(s) (8) of the cap (7) of the one or more pressure relief assemblies (1), c) securing one or more pressure relief assemblies (1) in into matching openings (4) in wellbore casings (2), wherein the check valve assembly (9) is connected to the inlet (8) in the cap (7) and to the plug (10), characterized in that d) the temporary polymer seal (15) melts when the well warms up revealing the inlet of the cap into the pressure relief assembly (1), e) wherein the pressure relief assembly (1) is adapted to be in a closed position when no pressure force is on the check valve assembly (9), where the plug (10) closes the opening (6) in the bottom of the main body (3), and f) wherein the pressure relief assembly (1) is adapted to be in an open position when pressure force is on the check valve assembly (9), where the plug (10) is pushed revealing the opening (6) in the bottom of the main body (3).

16. The method according to claim 15, wherein the press plate (16) is pushed due to pressure difference and a limited flow is allowed through openings (18) on the press plate (16).

17. A pressure relief system for wellbore casings (2) in high temperature wells, the system comprising: providing a pressure relief assembly (1) comprising a main body (3) adapted to be secured into matching openings (4) in wellbore casings (2) and wherein the main body further comprises a cavity (5) and an opening (6) in the bottom of the main body (3) and a plug (10) adapted to close the opening (6) in the bottom of the main body (3),- arranging a cap (7) according to claim 1 into the upper part of the main body and covering the cavity of the main body (3), wherein the cap further comprises an inlet (8), arranging a check valve assembly (9) in the cavity of the main body, and characterised in that the check valve assembly (9) is connected to the inlet (8) in the cap (7) and to the plug (10) to relief external pressure on the casing through the opening (6) in the bottom of the main body (3), in that the pressure relief assembly (1) is adapted to be in a closed position when no pressure force is on the check valve assembly (9), where the plug (10) closes the opening (6) in the bottom of the main body (3), and in that the pressure relief assembly (1) is adapted to be in an open position when pressure force is on the check valve assembly (9), where the plug (10) is pushed revealing the opening (6) in the bottom of the main body (3).

18. The system according to claim 17, wherein a temporary polymer seal (15) is arranged in the inlet (8) of the cap (7).

19. The system according to claim 18, wherein the temporary polymer seal (15) melts when the well warms up revealing the inlet of the cap into the pressure relief assembly (1).

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