Abstract

Premium tubing connections are not always gas tight and thereby require internal gas testing to be carried Out while running expensive completions. However to do this leak testing, the pipe dope must not obstruct the leak path and allow the passage of gas through the threads. The joint leaks can then be discovered while running the completion string and not after the pipe dope has dried out down hole and lost its temporary seal.

This paper illustrates testing carried out to determine lubrication properties of different pipe dopes, leak transmissibility, temporary sealing effects under ambient and elevated temperatures and torque correction factors.

Finally, a dry film lubricant with a liquid torque modifier is suggested as an alternative to present day usage of pipe rope to use in conjunction with string leak Detection of tubing joints.

Introduction

Thread compounds are used to lubricate the threads of tubing joints during make up and break out. These compounds are used on every threaded connection for drilling and completion strings. On non-premium connections, thread compounds are used not only as a lubricant, but as a sealant as well(l).

When dealing with premium connections, the most common cause of tailure is "Leakaqe"(2). In this case, the pipe dope appears to fail in its sealing capabilities. Premium connections rely on a metal to metal Contact as the primary seal and therefore, pipe dope need only act as a thread lubricant.

THREAD LUBRICANTS

Thread compounds are made up of various mixtures of grease, oils, and additives, with metallic and non-metallic components. API modified pipe dope is almost exclusively used to lubricate the threads and seal the connection. The specification tor API modified doper(3) lists the properties which were designed into it, two of these are worthy of note:

  • Sealing properties sufficient to prevent leakage at temperatures as high as 150 °C.

  • Sufficient inert filler to prevent leakage of API round thread casing and tubing joints under pressures as high as 69 MPa.

The sealing action arises from the dense packing of the solid parts within the dope into the small root/crest spaces in round threads. Therefore these thread compounds, apart from being an effective lubricant, also provide a high temperature and high pressure sealing compound when applied to API round threads.

Premium connections use a Buttress type of thread which has a gap between the stab flanks (see Appendix A). These stab flank gaps are typically two to three times larger in cross sectional area than the root/crest gap of eight round connections(4. It was theorized that the packing of solids between the stab flanks in premium connections is incomplete, and therefore, the dope used on premium connections has little to be sealing effect.

In order to test the above theory, a joint program was conducted in Stavanger Norway (August 1981). See Appendix B. The objective of these tests were to determine the effect of temperature and seal ability of API modified dope, and various other compounds.

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