SEJOURNE, C. and D. BEAUFORT, Elf Exploration Production, France

Abstract: Pore Pressure Prediction in Deep Waer From 1D Modeling and Seismic Velocities

Elf exploration interest has shifted strongly into the deep water environment. Classic pore and fracture pressure prognoses from seismic interval velocities have been improved and are now accurate for deep water.

The method is based upon i) 1D compaction/HC generation modelling which calculates the overburden gradient, estimates overpressure due to HC maturation, and helps in selecting the normal compaction profile. ii) the estimation of interval velocity from seismic data followed by the application of the Eaton formula which calculates pore pressure as a function of overburden gradient, formation velocity and normal compaction trend and fracture pressure as a function of pressure, overburden gradient and Poisson's ratio.

The accuracy of the results is largely dependant on the seismic velocity analysis and on the normal compaction profile selected.The interval velocity and normal compaction profile uncertainties can reach values of 5-10 % (or more) which correspond to an error of approximately + or - 0.1 emw. Great care is taken in our prognosis to reduce this uncertainty :

- The processing of seismic velocity estimation is performed in 2D. A set of Vrms is created and the velocity picking is transformed into raw interval velocities which must then be corrected for seismic bias.The main correction is related to shale anisotropy which varies from 0 to 15 %,

- The normal compaction profile is selected within a trend of known profiles i) by iteration using pressure calculations from offset wells, ii) and/or by taking into account profiles adjusted in basins with comparable sedimentation rates, thermal gradients and lithology.

- Poisson's ratio variations are calibrated using LOT data.

AAPG Search and Discovery Article #90923@1999 International Conference and Exhibition, Birmingham, England