Improve cement bond logs, maximize zonal isolation, and manage nonaqueous fluid (NAF) mobility to limit detrimental effects of mud channeling.
Tullow Ghana Ltd. drilled 10 wells in the deepwater Jubilee, Tweneboa, Enyenra, and Ntomme (TEN) Fields since 2018, and all experienced zonal isolation issues such as poor cement bonding and channeling in the oil-based drilling fluid. Cementing challenges include highly deviated well trajectories, a relatively narrow envelope between pore and fracture pressures, high reservoir permeability, and small distances between reservoir units. Centralization choices are also limited to avoid casing drag and enable reaching total depth.
To improve zonal isolation over the course of the project, the friction pressure hierarchy was increased, cement designs were modified to increase compressive strength and tolerance to contamination, and a preflush was added ahead of the cement spacer. Finally, the time between cementing and cement evaluation was extended up to 60 days, which did not significantly improve bond logs and detrimentally affected well economics.
Schlumberger recommended CemFIT Shield mud-sealing cement system, which interacts with any OBM that remains after the spacer train. The interaction reduces mud mobility and the likelihood of communication along channels.
CemFIT Shield system was deployed in Well 11, where target reservoir sands were separated by less than 20 m. The reservoir section was drilled to 5,492-m MD with fresh OBM after heavy losses during prior sections, and the operation was executed as expected.
Two pressure test cycles of 1,500 psi and 1,400 psi were applied to the casing within 10 days after cementing. The cement evaluation log carried out 12 days after cementing showed that the zonal isolation requirements were achieved without significant continuous channels, dry microannuli, or debonding.