Fate of Nonaqueous-Drilling-Fluid Cuttings Discharged From a Deepwater Well

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This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 98612, "Fate of Nonaqueous-Drilling-Fluid Cuttings Discharged From a Deepwater Exploration Well," by T.J. Nedwed, J.P. Smith, SPE, and H.R. Melton, SPE, ExxonMobil Upstream Research Co., prepared for the 2006 SPE Eighth International Conference on Health, Safety, and Environment in Oil and Gas Exploration and Production, Abu Dhabi, UAE, 2–4 April. The full-length paper describes results of a study of the fate of cuttings drilled with nonaqueous drilling fluids (NADFs) and discharged into a deepwater environment. The study characterized effects of discharges from a single-well drilling program and tested the accuracy of a discharge model that predicts seabed cuttings accumulations. Introduction Drilling programs increasingly use NADFs to drill more-challenging wells, such as those in deep water or those requiring highly deviated wellbores. Recent field studies have addressed the environmental effects of the discharge of cuttings drilled with NADF. Similar physical, chemical, and biological phenomena control the fate and effects of drilling discharges in all water depths. However, greater water depth could influence discharge fate and effects by providing increased opportunities for dispersion or other changes in cuttings properties during settling to the seafloor. The full-length paper details a study that used novel deepwater sediment traps to sample drill cuttings as they arrived at the seabed. Method Drilling Program. An exploration well was drilled to a total depth of 2430 m below the seabed in 950 m of water in the south Atlantic. Over a 9-day period, NADF was used to drill the bottom 1790-m section of the hole with a 12 1/4-in. drill bit. To separate cuttings from drilling fluid, drilling-fluid returns were processed with a conventional solids-control system comprising two shale shakers operated in series. The cuttings removed from these shakers were discharged 13 m below the sea surface. Discharge Monitoring. The volume of NADF cuttings discharged was estimated by use of the wellbore volume of the NADF-drilled length plus a 7.5% washout. Drilling records of the length of hole drilled each day were used to correlate discharge volumes and times with ocean-current measurements. Discharged Material. Drill-cuttings samples were collected at five depth ranges during drilling to determine base-fluid content and particle fall velocity. Base-fluid content was determined by dichloromethane extraction followed by total-petroleum-hydrocarbon (TPH) concentration measurement by a gas-chromatography/flame-ionization detection (GC/FID) method. GC/FID results indicated that the detected hydrocarbons in cuttings were identical to those in the NADFs. As a result, TPH concentration can be considered equivalent to base-fluid concentrations for cuttings samples. Measured cuttings-settling times in a laboratory settling column filled with seawater provided data on cuttings-particle fall-velocity distributions for use in modeling.