Using Shear and Vp/Vs to Predict Overpressure in Petroleum Basins
By
Philip D. Heppard1, Daniel Ebrom1, Michael Mueller1, Leon Thomsen1, Toby Harrold2
(1) BP Amoco, Houston, TX (2) BP, Sunbury, England
Most pressure predictions in overpressured basins are aided by deriving
interval velocity from surface seismic
and using several approaches to convert
velocity to formation pressure. These techniques are valuable and are being
continuously improved. More recently techniques have been developed to evaluate
overpressure from shear velocity and the
vertical
to shear velocity ratio, Vp/Vs.
Shear velocity and Vp/Vs measured by dipole sonic logs, four component surveys,
and wellbore
seismic
provide additional measurements to augment interpretations
based on compressional velocity. Shear velocities and Vp/Vs in some cases may be
the only reliable data when compressional velocities are difficult to obtain in
gas prone sequences. Also shear velocities are more sensitive to changes in
compaction state, rock type, and overpressure. Of immediate application only Vp/Vs
can be reasonably determined from
vertical
seismic
profiles (VSP) without an
expensive walk away acquisition as a look ahead tool for pressure prediction
while drilling. VSP techniques have long held out the promise of
ahead-of-the-bit pressure predictions, but a variety of data acquisition and
interpretation issues have limited this potentially important application. In
particular, compressional wave (P-wave) velocities ahead of the bit can not be
determined directly. However, a simple interpretation technique requiring only
the picking and registration of upward traveling, that is P-waves, and
mode-converted shear waves allows straightforward determination of Vp/Vs ratios
ahead of the bit using only zero-offset VSP data. The Vp/Vs ratios are derived
without explicit knowledge of either the interval P-wave velocities or the
interval S-wave velocities.