Atlantis, the Quest for Stray Shallow-Water-Flow Sands

Abstract

This article, written by Technology Editor Dennis Denney, contains highlights of paper OTC 17658, "Atlantis, the Quest for its Stray Shallow-Water-Flow Sands," by H. Mannaerts, J. Oldroyd, M. Williams, R. Fisher, E. Radjef, R. Day, J. Faulkerson, A. Pere, R. Sanders, and J. Thomson, BP plc, and K. Mix, BHP Billiton Petroleum , prepared for the 2005 Offshore Technology Conference, Houston, 2-5 May. The presence of shallow overpressured sand bodies in the Atlantis field led to the abandonment of one of the appraisal wells and created challenges to predicting, monitoring, and controlling shallow water flow (SWF). Drilling the shallow sections of all wells in one batch enabled optimization of offshore SWF monitoring and reporting. The SWF severity-classification scheme was used successfully during the program, and it has been adopted by the U.S. Minerals Management Service (MMS) as a guideline for offshore SWF monitoring at the wellhead by a remotely operated vehicle (ROV). Introduction The Atlantis field, discovered in 1998, is in U.S. Gulf of Mexico (GOM) Green Canyon Block 743, approximately 150 miles south of New Orleans. Atlantis is in water depths varying from 4,500 ft to greater than 6,800 ft. Hydrocarbons from the field will be produced from a drill center at the base of the Sigsbee escarpment in Green Canyon Block 743. The water depth at the drill center is approximately 6,800 ft. The tophole sections of 15 of the 16 production wells were batch-set (drilled) over a period of 3 months during the first half of 2004. A previously existing well constitutes the 16th production well. Appraisal Drilling Extensive shallow-geohazard assessment at the drill center confirmed the presence of mass-transport complexes (MTCs) at various depths in the tophole section and the absence of significant shallow gas hazards. Exploration-well drilling in the Atlantis area did not encounter SWF, suggesting that the area had an overall low potential for encountering SWF despite the presence of sandy turbidites and MTCs. The shallow-geohazard assessment for appraisal Well GC 743-4, drilled 2 years before the batch-set, was favorable. However, while the 20-in.-casing interval was being drilled, an unexpected 100-ft-thick sand was encountered within an MTC between 1,365 and 1,665 ft below mudline (Unit F). The sand flowed undetected, creating a ledge in the wellbore. Drilling continued to the proposed casing depth of 3,615 ft below mudline. As a result of borehole instability associated with the SWF sand, the 20-in. casing encountered a ledge while running to bottom, buckled, and failed. Consequently, the well was abandoned. The appraisal well was respudded 200 ft southwest of the original location. Over-burden conditions for this well were expected to be similar, if not identical, to those of the abandoned well. However, the SWF sand associated with the MTC in Unit F, which caused the previous appraisal well to fail, was absent.