Size Degradation of Shale Drill Cuttings in Deepwater Discharge Conditions from Synthetic-Based Drilling Fluids

Shale drill cuttings exposed to synthetic-based drilling fluids (SBDF) processed by cuttings dryers were studied under simulated deepwater discharge conditions and found to size degrade with time. This hydration and size degradation is similar to what occurs with water-based cuttings. Lower retention on cuttings and wider dispersion of discharged cuttings results in lower organic enrichment, which reduces potential seafloor impacts from deepwater drilling operations. While this was indicated in previous studies, it had not been confirmed with laboratory studies that simulate deepwater conditions. Previous studies reported settling velocities after exposure to seawater for only a few minutes. A cutting may settle at speeds that allow exposure to seawater for 0.9 to 3 hours for each 3,280 ft (1,000 m) of water depth. A flow loop was constructed to expose cuttings to seawater for extended time periods and separate the cuttings into fractions of four different settling velocities. Changes in each fraction with increased time indicate changes in settling velocity distribution. The flow loop consists of a series of increasing larger clear vertical tubes where smaller particles carry over to larger lower velocity columns. An initial experiment with inert solids plus three different shale cuttings were tested. The concentration of synthetic on initial and final samples was measured and correlated to size. Video photography was used to document and characterize cuttings behavior. Current discharge models do not include the effect of size degradation on reduced fall velocity for deepwater discharges. Reduced fall velocity causes a wider aerial dispersion that reduces the potential for the formation of significant cuttings accumulation and reduced environmental impact. Accurate cuttings size is needed to determine a fall velocity and accurately predict the distribution of drill cuttings discharged into the ocean. Data from previous fall velocity studies and field monitoring studies will be presented and discussed.