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.