To optimize drilling efficiency, some presalt wells in the Santos basin, Brazil, had their project modified to a configuration in which the entire well is drilled, cased, and cemented in only three phases, in contrast to the usual four phases, thus saving approximately 12 to 15% on the total drilling time. The saline and production zones are cased and cemented in the same phase, requiring a cement slurry design that minimizes salt dissolution and, at the same time, has specific properties for production zone isolation.
The quality of this cement job is key for the success of the project, given that poor cementation could lead to direct exposure of the wellhead to the reservoir fluids, making the three-phases project unviable due to associated risks. It is important, then, to understand and control all aspects that can influence the cement job success.
The presalt reservoir is formed by carbonate rocks that present a variable ratio of gases such as CO2 and H2S. To avoid future risk of gas migration through the cement sheath, the slurry used in the cementing operation must be formulated with gas-migration-control additives to reduce cement matrix permeability.
The salt section is composed of layers of different salt types, with the tachyhydrite and carnallite the most soluble. The results of research conducted with salt rock cores from this basin indicated that the best salt concentration added to the cement slurry, to avoid salt dissolution, is either 15 to 20% sodium chloride or 3 to 5% potassium chloride. It is well known that the addition of salt to cement slurry impairs the slurry properties, especially free fluid and compressive strength, two important parameters to determine a successful cement evaluation. Several laboratory tests were performed until the ideal formulation was defined. The spacers were also optimized to promote superior mud removal. Cementing techniques and best practices such as effective casing centralization, mud removal, losses mitigation, and slurry density control are imperative to ensure job objectives are achieved.
To the time this paper has been written, four wells had been successfully drilled and cemented using the new configuration. Fluids design and flawless execution of the job provided an excellent cement logging evaluation, proving that it is possible to enhance the well construction to save rig time and, consequently, lower the well costs.