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 oil and gas, thermal expansion can create very high pressures (10,000 -12,000 psi or more) and cause the collapse of casing and tubing strings.1,3,11,14
Mitigation methods such as vacuum insulated tubing to limit heat transfer,5,6,13 nitrogen foam spacers to give highly compressible trapped fluids,7,8,9,10 crushable urethane foam insulation,2 etc. are somewhat successful but are either very expensive, logistically troublesome or have unacceptable failure rates. This paper covers a new approach which has created a water-based spacer fluid that will be used just ahead of the cement. The spacer contains perhaps 20–40% of emulsified 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 the target temperature can be controlled by an appropriate type and concentration of chemical initiator. Premature polymerization during spacer placement can be prevented by an appropriate type, and amount, of inhibitor.
A spacer formulation (viscosifier, emulsifiers, MMA, weighting agent, inhibitor, etc.) has been developed which covers the range of densities expected in deepwater wells. A matrix of bench top tests has determined the types and amounts of initiator and inhibitor needed to adjust the temperature of the polymerization to the range of temperatures expected in the field. These results have been confirmed in an advanced pressure-volume-temperature (PVT) cell that closely simulates downhole conditions. A successful mid-scale field trial has been conducted in a 500-ft test well using normal oilfield casing, drillpipe, pumps, etc. A method was devised to add the initiator on-the-fly as the spacer is pumped downhole. Safe handling procedures have been developed for mud plant mixing, transportation, and rigsite application. The new spacer will be tested in an onshore well in the near future, and then in several deepwater wells prior to commercialization.
Trapped annular pressure (TAP), also called Annular Pressure Build-up (APB), is due to the thermal expansion of fluids trapped in casing annuli between the top of cement and the wellhead. 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.12 The pressure can easily exceed the collapse strength of casing and production tubing. In land wells the pressure is easily relieved by bleeding off some fluid through a casing-head valve (although 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.4,5,6 A wide range of mitigation techniques have been employed, including vacuum insulated tubing (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.