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Technical Paper: Successful High-Density Cement Plugs in an HP/HT Presalt Deepwater Well in Brazil’s Santos Basin

Society: OTC
Paper Number: 23908
Presentation Date: 2013
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Abstract

Ensuring well safety during well construction and abandonment presents challenges for offshore exploration worldwide. High-quality cement plugs are required to provide a tested isolation barrier, but there have been problems in placing cement plugs. Setting cement plugs in deep open holes is difficult, and often, several attempts are necessary to end with a proven barrier or a competent kickoff plug. Minor contamination with synthetic drilling fluids has a detrimental effect on the physical properties of the set cement. Setting cement plugs is more difficult in deepwater environments because of the nature of frontier exploration, increasing placement depth, high-pressure reservoirs, high-density drilling fluids, salt formations, and high-pressure gas reservoirs. In Brazil, exploration campaigns are moving to presalt reservoirs.

An overpressured presalt gas reservoir in the Santos basin was discovered with an equivalent pore pressure ranging from 18.5 to 18.7 lbm/galUS (ultrahigh pressure ~ 20,000 psi) at 6,136-m true vertical depth. During drilling in the high-pressure, high-temperature (HPHT) deepwater well, a kick was observed. Conventional methods of well control were applied. Once the well was shut in, the pressure readings at the surface stabilized at 5,600 psi, and mud weight was increased to 19 lbm/galUS to achieve hydrostatic control of the well. Once well control was regained, the well was temporarily suspended with a sequence of cement plugs.

A total of 392 m of 8½-in openhole section was abandoned using two cement plugs with a high-density slurry of 21 lbm/galUS and high-density spacer of 20 lbm/galUS. A third cement plug was set in the previous casing string. The design included a special chemistry for formations with gas and salt. Software simulations and analysis of placement techniques and practices supported operations in this environment.

These practices will be used for other ultrahigh-pressure discoveries and in conventional wells or wells with high-density drilling fluids.

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