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). During a subsequence batch-set drilling program of 15 production wells, several SWF sand intervals were encountered associated with buried compartmentalized mass transport complexes (MTCs). Nonetheless, the program was completed safely, on schedule, and on budget. The key to success of the batch-set program was the well-by-well involvement of a multi-disciplinary team (geoscientists, drilling and operational staff) to assist in predicting sand bodies and the development of appropriate drilling mitigation strategies to control SWF. Drilling all the shallow sections in one batch enabled optimization of offshore SWF monitoring and reporting. The Shallow Water Flow Severity Classification Scheme, which was successfully used during the program, has recently been adopted by the US Minerals Management Service as a guideline for offshore shallow water flow monitoring at the wellhead by Remote Operated Vehicle (ROV).
The Atlantis Field, discovered in 1998, is located in Green Canyon 743, approximately 150 miles south of New Orleans, Louisiana in the Gulf of Mexico. Atlantis is located in water depths varying from 4,500 feet to greater than 6,800 feet. BP Exploration & Production Inc. operates Atlantis with a 56% working interest. BHP Billiton Petroleum (Deepwater), Inc. is the only partner with a 44% working interest.
Hydrocarbons from the Atlantis Field will be produced from a Drill Center situated at the base of the Sigsbee Escarpment within Green Canyon Block 743, Gulf of Mexico (Figure 1). The water depth at the Drill Center location is approximately 6,800 ft.
The top-hole 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 sixteenth production well (Figure 2).
The dominant bathymetric feature within the Atlantis Field Area is the Sigsbee Escarpment, which is the seafloor expression of the edge of an underlying salt canopy. Its cuspate geomorphology is the result of past slope failure events (Figure 3).1 The Drill Center lies at the base of the escarpment. The shallow geology from seabed to 3,000 ft below seabed comprises turbidites and hemipelagic clays typical of deepwater depositional systems, and compartmentalized mass transport complexes (MTCs). The stratigraphy of the area and unit nomenclature is explained in Figure 4. Stratigraphic dip at this location is gently to the south-southeast. However channeling and the uneven tops of MTCs may locally affect this trend. A north-northwest dipping thrust fault associated with the advancement of allochtonous salt underlying the Sigsbee Escarpment transects the strata below the Drill Center close to the base of an MTC identified between approximately 390 ft and 570 ft below seabed (Unit C).
Extensive shallow geohazard assessment at the Drill Center location confirmed the presence of MTCs at various depths in the top-hole section and the absence of significant shallow gas hazards (Figures 3 and 4).2