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Abstract
In deepwater or other sub-sea completed wells, fluids, usually spacers or drilling fluid, are commonly trapped in casing annuli above the top-of-cement and below the wellhead. When these trapped fluids are heated by the passage of warm produced fluids, thermal expansion can create very high pressures (10,000 -12,000 psi or more) and cause the collapse of casing and tubing strings.^1,2,4,12,15

Mitigation methods such as vacuum insulated tubing to limit heat transfer,^6,7,14 nitrogen-based foam spacers to give highly compressible trapped fluids,^8,9,10,11 crushable urethane foam,³ etc. are somewhat successful but are either very expensive, logistically troublesome or have unacceptable failure rates. This paper continues the discussion of a new approach which has created a water-based spacer fluid that will be used just ahead of the cement. The spacer contains perhaps 10-30% of emulsified liquid methyl methacrylate monomer (MMA). Upon polymerization, the MMA phase shrinks by 20%, creating room for the remaining fluid to thermally expand without creating catastrophic pressure. The polymerization is triggered by heat and a chemical initiator. The target temperature can be controlled by choosing an appropriate type and concentration of chemical initiator. Premature polymerization during spacer placement can be prevented by an appropriate type, and amount, of inhibitor. The initial lab work and a mid-scale field trial of this technology were reported in detail in SPE/IADC 104698.¹

This paper covers the development and field testing (land) of all the equipment and processes necessary to apply the technology in deep water.

Introduction
Trapped annular pressure (TAP), also called Annular Pressure Build-up (APB), is due to the thermal expansion of fluids trapped in casing annuli, most commonly between the top of cement and the wellhead or seal assembly. The pressure build-up is usually due to the heat brought up by produced fluids, but can even be triggered by the circulation of hot drilling fluids while drilling an HTHP well.¹³ The pressure produced can easily exceed the collapse strength of casing and production tubing. In land wells annular pressure is easily relieved by bleeding off some fluid through a casing-head valve (although even land wells are occasionally lost through inattention). In subsea completed wells the wellheads are much less accessible and generally not fitted with the necessary valves.

One of the best documented cases involved BP’s Marlin Field, where the production casing and tubing of the first production well collapsed after only a few days of production.^5,6,7 The industry has employed a wide range of mitigation techniques, including vacuum insulated tubing (VIT - limit heat transfer), leaving the previous casing shoe uncemented (leak path to “weak” rock), burst disks in casing, nitrogen based spacers (compressible gas), crushable urethane foam, etc.