In many deepwater plays around the world, salt formations overlie prolific reservoirs containing billions of barrels of oil. Drilling into these reservoirs requires the successful penetration of the challenging salt layers. Based on experiences in key deepwater basins, this paper reviews the fluids and techniques used to drill through salt formations. Salt formations are unique. Salt has little porosity and permeability. It can flow plastically through other geological rock beds under stress with "salt creep" resulting in wellbore size reduction and casing collapse. Salt can also dissolve in water necessitating the salinity of a water-based fluid be kept near or at saturation to avoid or minimize wellbore enlargement that can lead to poor cementing of the casing and deficient zonal isolation. In spite of the aforementioned issues, salt formations are drilled successfully around the world, and drilling fluids play a vital role in a successful drilling operation. A downhole simulator cell (DSC) has been found to be a key tool in assessing the effect of drilling fluids on salt formations by drilling salt cores at in-situ conditions of temperature and pressure while monitoring the core and fluid interactions. This paper combines a downhole simulation cell (DSC) testing and data from previous literature to provide a comprehensive overview of drilling fluids interactions with salt formations. This dialogue combines the experiences of drilling salt as seen from a drilling fluids perspective into one publication. Three generalized fluids are evaluated: riserless water-based fluid (WBF), high-performance water-based fluid (HPWBF), and synthetic-based fluid (SBF). Performance criteria used to evaluate fluids include rates of penetration (ROP), hole cleaning, wellbore stability and washout minimization. Environmental compliance and system strengths and limitations are outlined. Topics include evaporite mineral types and drilling challenges including exit strategies and tar beds.

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.