Mitigating The Shallow Water Flow Risk At Mississippi Canyon 849: A Team Approach

Abstract

Abstract Wells drilled near the Mars, Ursa and Europa Basins within the Mississippi Canyon Area, Gulf of Mexico, have experienced varying degrees of shallow water flow problems. Two wells drilled in Mississippi Canyon Block 849 (MC 849) penetrated overpressured sands within the shallow section causing catastrophic shallow water flow (SWF). Both wells were subsequently abandoned. A team of geohazards specialists, geologists, geophysicists and engineers worked together to identify and mitigate this SWF, and successfully drilled the exploration well (Slam Dunk) in MC 849. Figure 1: Seafloor Rendering Showing Mississippi Canyon and MC Block 849.(AVAILABLE IN FULL PAPER) Introduction Mississippi Canyon Block 849 is located 145 miles southeast of New Orleans, LA, in about 3600 feet of water (Figure1). The Pliocene target of the two wells previously drilled on the block, the Norcen #1 (Norcen) and UPRC #2 (UPRC), is a well-defined series of vertically stacked amplitudes within a thickening wedge of seismic reflectors. These amplitudes are stratigraphically trapped on the flank of a large salt ridge, which separates the prospect from the Mars basin to the east. While attempting to drill this prospect both wells were lost due to SWF. A seafloor amplitude map generated from a subsequent TGS 3D geophysical survey over MC 849 showed areas of high seafloor amplitude around the Norcen and UPRC wells, which were interpreted to represent sand "mined" by the SWF. These areas are not visible in the original 3D seismic data acquired by Western Geophysical (Figures 2 and 3). The SWF encountered at the Norcen and UPRC wells was so severe that during the planning of the Figure 2: Seafloor Amplitudes Showing AmplitudeAnomalies at the Norcen and UPRC Wellsites Interpreted as â??Minedâ?? Sand from SWF. Figure 3: Seafloor Amplitude Map Prior to Drilling of Norcen and UPRC Wells.(AVAILABLE IN FULL IN PAPER) Slam Dunk prospect, it was debated if drilling a successful well was feasible. SWF is defined as water or, sand and water flowing within and around the outside of structural well casing to the seafloor (Alberty and others, 1997). Isolating the specific stratigraphic interval where SWF was previously reported, and delineating the extent and distribution of the potential SWF sands were key factors in reducing the risk of encountering SWF at the Slam Dunk prospect. Data Sets The geohazards investigation was based on interpretation of reprocessed 3D seismic exploration data acquired by TGS. The 3D data cube contains 2-millisecond sample rate data to a record length of 3.5 seconds TWT below sea surface (BSS). The exploration data has been reprocessed with spectral whitening to enhance the higher frequencies in the tophole section. The dominant frequency of the 3D seismic data set is estimated to be about 50 Hz in the shallow section, which corresponds to a limit of separability of approximately 27.5 ft, assuming an average velocity of 5,500 ft/sec for the shallow sediments.