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A test plug tool for use in testing a pressure integrity of a pressure control stack mounted to a wellhead, including a joint between a casing and a casing support in the wellhead. The test plug tool includes a test plug of an appropriate diameter used to pressure test the pressure control stack as

A test plug tool for use in testing a pressure integrity of a pressure control stack mounted to a wellhead, including a joint between a casing and a casing support in the wellhead. The test plug tool includes a test plug of an appropriate diameter used to pressure test the pressure control stack as well as a joint between any one of a surface casing and the wellhead, an intermediate casing and an intermediate casing mandrel, and a production casing and a production casing mandrel. The pressure integrity of the wellhead is ensured at each stage of well drilling and well completion, and safety is improved. Optionally, a backpressure valve permits pressurized fluid that leaks below the test plug tool to flow upwardly through a central bore in a landing tool that is secured to the test plug tool to permit detection of the leak.

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We claim: 1. A test plug tool for use in testing a pressure integrity of a pressure control stack mounted to a wellhead, including testing the pressure integrity of a joint between a casing and a casing support that secures the casing to a wellhead stack assembly, the test plug tool providing a hig

We claim: 1. A test plug tool for use in testing a pressure integrity of a pressure control stack mounted to a wellhead, including testing the pressure integrity of a joint between a casing and a casing support that secures the casing to a wellhead stack assembly, the test plug tool providing a high pressure seal with the casing below the joint between the casing and the casing support, the test plug tool comprising: a test plug hanger and a test plug, the test plug hanger including a hanger flange at a top end thereof and a test plug support leg that depends from the hanger flange and includes a bottom end for supporting the test plug in the casing; an axial passageway that extends through a central portion of the test plug hanger, the test plug support leg and the test plug, the axial passageway permitting pressurized fluid that may have leaked past the test plug to flow upwardly through the axial passageway; and a backpressure valve in fluid communication with the axial passageway, the backpressure valve throttling the pressurized fluid flowing upwardly through the axial passageway. 2. The test plug tool as claimed in claim 1 further comprising a box threaded socket used to connect a landing tool to an upper portion of the test plug hanger, the landing tool defining a central bore through which the pressurized fluid can flow upwardly after being throttled through the backpressure valve. 3. The test plug tool as claimed in claim 1 wherein the backpressure valve is threadedly connected to an upper portion of the test plug hanger. 4. The test plug tool as claimed in claim 1 wherein the backpressure valve comprises a spring-loaded ball valve having a spring exerting a downward force on a ball for obstructing an aperture of the backpressure valve. 5. The test plug tool as claimed in claim 4 wherein the backpressure valve further comprises an annular body having a lower annular shoulder defining the aperture, the lower annular shoulder supporting a gasket against which the ball is forced by the spring. 6. The test plug tool as claimed in claim 5 wherein the test plug hanger has an annular groove for housing a seal for providing a fluid-tight seal between the backpressure valve and the test plug hanger. 7. The test plug tool as claimed in claim 1 wherein the test plug comprises a cup tool and the cup tool comprises a cup sleeve that terminates in a bull nose for guiding the test plug through the wellhead stack assembly. 8. A method for testing a pressure integrity of a wellhead stack assembly, comprising: using a landing tool to insert a test plug tool into the wellhead stack assembly and testing the pressure integrity of a joint between a casing and a casing support that secures the casing to the wellhead stack assembly using the test plug tool, which provides a high pressure seal with the casing below the joint between the casing and the casing support; landing a test plug of the test plug tool in the casing in a position beneath the joint between the casing and the casing support; locking the test plug tool in the position in which the test plug is beneath the joint between the casing and the casing support; pressurizing the wellhead stack assembly; and monitoring a top end of the landing tool for pressurized fluid flowing upwardly through a backpressure valve of the test plug tool and into a central bore of the landing tool to determine whether pressurized fluid is leaking past the test plug. 9. The method as claimed in claim 8 wherein if it is determined that pressurized fluid is not leaking past the test plug, the method further comprises: detaching the landing tool from the test plug tool; removing the landing tool from the wellhead stack assembly; injecting a test fluid to pressurize the wellhead stack assembly to at least an estimated operating pressure; and inspecting seals and joints of the wellhead stack assembly, including a joint between the casing and the casing support, to determine whether any test fluid is leaking from the seals and the joints. 10. A test plug tool for use in testing a pressure integrity of a wellhead stack assembly, including testing the pressure integrity of a joint between a casing and a casing support that secures the casing to the wellhead stack assembly, the test plug tool comprising: a test plug hanger for suspending the test plug tool in the wellhead stack assembly, the test plug hanger including a fluid passage to permit test fluid to pass there through; a test plug leg connected to an underside of the test plug hanger; a test plug connected to a bottom end of the test plug leg, the test plug having a sealing element for providing a high-pressure fluid seal with the casing when the test fluid is injected into the wellhead stack assembly; an axial passageway through a central portion of the test plug hanger, the axial passageway permitting pressurized fluid that may have leaked past the test plug to flow upwardly through the axial passageway; and a backpressure valve in fluid communication with the axial passageway, the backpressure valve throttling the pressurized fluid flowing upwardly through the axial passageway. 11. The test plug tool as claimed in claim 10 wherein the test plug hanger comprises a hanger flange that extends laterally from a hanger socket and has a beveled top edge that is locked in place in the wellhead stack assembly by locking pins of the wellhead stack assembly after the test plug tool is inserted into the wellhead stack assembly, so that the test plug tool is restrained from upward movement. 12. The test plug tool as claimed in claim 10 wherein the fluid passage comprises a fluid passage through the hanger flange. 13. The test plug tool as claimed in claim 10 wherein the back pressure valve comprises a spring-loaded ball valve having a spring exerting a downward force on a ball for obstructing an aperture of the backpressure valve, and further comprising an annular body having a lower annular shoulder defining a lower aperture, the lower annular shoulder supporting a gasket against which the ball is forced by the spring. 14. The test plug tool as claimed in claim 10 wherein the test plug hanger comprises a hanger flange with beveled shoulders dimensioned to fit within a bore of a tubing head spool and further having a lower shoulder machined to rest against a bowl-shaped abutment in the tubing head spool to prevent the test plug from descending further into the wellhead stack assembly. 15. A test plug tool used to test a pressure integrity of a wellhead stack assembly, the test plug tool providing a high pressure fluid seal with a casing below a joint between the casing and a casing support of the wellhead stack assembly, the test plug tool comprising: a test plug hanger that supports a test plug, the test plug hanger including a hanger flange and a test plug support leg that depends from the hanger flange and supports the test plug; an axial passageway that extends through the test plug hanger, the test plug support leg and the test plug, through which fluid may flow; and a backpressure valve that seals the axial passageway until fluid pressure builds beneath the test plug. 16. The test plug tool as claimed in claim 15 wherein the test plug comprises a bull nose at a bottom end thereof and a cup sleeve with an outer diameter less than that of the bull nose, a top surface of the bull nose forming an annular shoulder that supports a metal gauge ring dimensioned to support an elastomeric sealing element that seals against the casing when the test plug is exposed to fluid pressure injected into the wellhead stack assembly and the elastomeric sealing element is compressed by an elastomeric cup dimensioned to form a fluid seal against the casing and slidably supported on the cup sleeve above the elastomeric sealing element. 17. The test plug tool as claimed in claim 16 wherein the elastomeric cup is bonded to a steel ring that slides over the cup sleeve, the steel ring comprising at least one annular groove that receives an O-ring that provides a fluid seal between the steel ring and the cup sleeve. 18. The test plug tool as claimed in claim 15 wherein a top end of the test plug tool comprises a landing joint connector, which is a box threaded socket to which is connected one of a pin threaded landing joint, a drill pipe, and a production tubing to land the test plug tool in the wellhead stack assembly. 19. The test plug tool as claimed in claim 15 wherein the hanger flange comprises at least one fluid passage that extends through the hanger flange to permit test fluid to migrate through the hanger flange so that seals above the hanger flange in the wellhead stack assembly can be pressure tested. 20. The test plug tool as claimed in claim 19 wherein the backpressure valve comprises a ball urged by a spring against a gasket surrounding an axial passage through the backpressure valve.