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