Identification of SWF Sands by Elastic Inversion of Conventional 3D Seismic Data

As oil and gas exploration moves into deeper offshore areas, Shallow Water Flow (SWF) from overpressured, unconsolidated/uncemented sands becomes a high profile drilling hazard. Laboratory measurements show that SWF sands have high seismic compressional to shear velocity (Vp/Vs) ratios. Thus, elastic seismic inversion for Vp/Vs provides a quantitative identification of SWF sands, and predrill predictions of SWF. A field data example uses conventional 3-D seismic data from the Garden Banks area of the Gulf of Mexico. Two wells provide constraints for the seismic data inversion. A known SWF zone in Shell’s well GB920 #1 shows high Vp/Vs values. In Kerr-McGee's well GB877 #1, Vp/Vs anomalies match fairly well with Fugro's predrill SWF sand predictions based on geological information and seismic amplitudes. Thus, the high Vp/Vs anomalies in the seismic data, calibrated at the wells, may be used to identify potential SWF sands away from the wells. The Vp/Vs values are quantitative, and are more diagnostic than seismic amplitude alone for SWF sand detection.