Poor isolation between zones in deepwater wells
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.
Cementing best practices and delayed bond logs
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.
Cement technology that interacts with oil-based mud (OBM)
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.
