ABSTRACT:

Geophysical tools have been in common use since a long time for assessing shallow hazards before positioning the drilling rig. Deep water E&P is producing some changes in the methodologies employed for the shallow hazards detection; the key point is to adopt the E&P 3D surveys to scan the near sea bottom geology and define safe well trajectories, thus saving the cost of a conventional well site survey; the last will be used only if deeper investigation is needed. The resolution of usual 3D seismic cubes, is often inadequate for detecting the features generating hazards and so a special reprocessing is needed for increasing the overall frequency content and consequently the vertical detail (approximately 3 m); also spatial resolution is improved using bin interpolation. Once achieved the necessary quality in the seismic data, the following 3D cubes are produced:

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    Migrated seismic volume: it is used for detailed seismic stratigraphy and shallow layering definition; potentially dangerous levels or objects are here identified as function of amplitude, geometry and structural features. Water bottom bathymetry is also produced.

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    Continuity cube: it is necessary for a very detailed geometrical characterisation of the potentially hazardous features (i.e. small faults). From the continuity cube the geomorphology of the sea bottom is usually extracted.

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    Acoustic inversion cube: it is used for lithology and fluid prediction in the investigated levels.

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    Velocity cube: it is used to for the depth conversion of the previously defined layers and for pore pressure prediction.

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    Pressure cube: it is calculated from the above velocities and gives the pore pressure development along well trajectory.

The final result of the studies performed on a E&P 3D survey is the production of a tophole prognosis chart that identify both the hazard percentage and the pressure development as a function of depth.

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