As deepwater exploration and development move toward deeper water depths and reservoirs, conventional riserless drilling techniques, such as seawater sweeps, no longer provide the required equivalent density from the formation integrity test (FIT) to set the surface casing at the required depth. During the planning phase of a 26-in. hole using 20-in. casing for a well in offshore Angola, expected shallow hazards such as faults, shallow gas, saltwater influxes, and shallow sands, were identified as the key concerns before reaching the section total depth (TD). To mitigate these shallow hazard risks, the dynamic kill drilling (DKD) technique was used.
DKD is widely used in the Gulf of Mexico and recently its use has expanded to other regions in China and Mexico. The most significant challenge for its first application in deepwater Angola was the DKD fluid volume availability, as well as the logistics involved. Drilling practices and parameters that posed a challenge with regards to control included rate of penetration (ROP), flow rates, rotation (rev/min), and reaming.
The DKD 16.3-lbm/gal fluid was mixed in the Luanda liquid mud plant (LMP) 2. Approximately 10,000 bbl were mixed and then taken to the rig for storage until req uired for the 26-in. hole section. The process continued until 30,000 bbl were created and sent offshore. The 26-in. interval was successfully drilled in 30 hours. Mud density was 10.7 lbm/gal, which was achieved by combining a total of 5, 544 bbl of 16.3-lbm/gal DKD fluid mixed with 14, 335 bbl of seawater. The mix-on-the-fly (MOTF) unit was rigged up on top of the pits. A line was attached from the pits holding the 16.3-lbm/gal DKD fluid and another line was attached to the pit with the seawater. The two liquid streams were blended to achieve the required mud weight. The 16.3-lbm/gal DKD fluid was reduced to form pad mud with a density of 11.5 lbm/gal, which was pumped and placed in the openhole section until the 20-in. casing was run and cemented.
A total of 19, 879 bbl of fluid was pumped downhole without any drilling or formation issues to conclude the first DKD application in Angola's deep water.