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.