Ensuring well safety during the well construction and abandonment process provided a challenge for an offshore exploration worldwide. More than ever the importance of the successful abandonment of exploration wells is critical. High quality cement plugs are required to provide a tested isolation barrier. Over the years the industry has experienced many problems in the correctly placing cement plugs. Historically, setting cement plugs in open hole at deep depths has been a difficult task. Often, several attempts are necessary to end with a proven barrier or a competent kickoff plug. Sometimes, small contaminations with synthetic drilling fluid have a detrimental effect on the physical properties of the set cement, resulting in catastrophic events. This challenging task becomes even more difficult when drilling in deepwater environments due to frontier exploration, increasing placement depth, high-pressure reservoirs, high-density drilling fluids, salt formations, and high-pressure gas reservoirs. In Brazil, the exploration campaigns are moving to pre-salt reservoirs where operators face a combination of these challenges.
An overpressured, presalt gas reservoir in the Santos basin was discovered with an equivalent pore pressure ranging from 18.5-18.7 ppg (ultra high pressure ~ 20.000 psi) at 6,136 m true vertical depth. During drilling operations in this high-pressure, high-temperature (HP/HT) deepwater well, a kick was observed after drilling a few meters below the salt. This led to the application of conventional methods of well control. Once the well was shut in, the pressure readings at the surface were stabilized at 5,600 psi, and the mud weight was increased to 19 ppg to achieve hydrostatic control of the well. Once well control was regained, the well was temporarily suspended with a sequence of cement plugs. These unexpected levels of pressures suggest a discovery. However, further research was postponed due to the current rig limitations and supplies.
A total of 392 m of 8 ½?? open hole section were abandoned using two cement plugs with a special high-density slurry of 21 ppg and high-density spacer of 20 ppg. 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 the analysis of placement techniques and practices help ensure the success of these operations in this challenging environment.
The successful practices demonstrated for these plugs will be useful for other ultrahigh pressure discoveries that may be encountered. These practices may also be useful when setting cement plugs in conventional wells or wells with high density drilling fluids.
