Quantification of Shallow Water Flow Zones using Pre-stack Inversion of Seismic data
Dutta, N., De Kok, R.
ERCH Geohazard Workshop, Del Lago USA, 3-4 April 2001.
Highly porous sands, prone to flowing when drilled (shallow waterflow sands) pose a serious risk and have cost the oil industry hundreds of millions of dollars to date. Detection of these shallow waterflow prone sands is important for reducing environmental risks and commercial losses. The use of standard seismic data for detection has been explored and found successful at the Ursa site in the Mississippi Canyon area of the Gulf of Mexico. Although the data quality of the selected set is at or below average, reprocessing at 2ms yielded sufficient quality to invert the seismic data using a prestack genetic algorithm. The attributes inverted are density, pressure wave velocity and shear-wave velocity. The shear wave velocity information, obtained through the AVO characteristics, was combined with other attributes to create a reliable discriminator. Using synthetic data, it was found that the straight ratio of compressional velocity and shear-wave velocity was the most sensitive combination. Subsequently, this quantity was successfully tested on real data from a 3D survey in the Gulf of Mexico. It is recommended that the detection method described be tried prior to drilling in deepwater areas with shallow waterflow occurrences. Conventional data may be used, provided they honor large reflection angles and are processed carefully to preserve amplitudes and high frequencies.