DUTTA, N.C., BP Exploration, Inc., Houston,TX
Estimation of geopressure before and during drilling is critical for various reasons: an assessment of 'Seal' integrity of a prospect, well planning and casing program design, and drilling safety. At BP, a new technique for pressure prediction has been developed. It uses a proprietary transform that relates velocity to effective stress (defined as the difference between overburden and pore-fluid pressure), temperature and lithology directly. It takes into account several sources of overpressure mechanisms in deepwater clastics basins: under-compaction, clay dehydration and alteration, and charging of fluids in dipping, permeable beds. Since a rock model is used, a normal trend analysis is not required in this approach. Thus, well-to-well calibration - often in the same play and in the same geologic environment - necessary for the conventional techniques, is not required. It predicts effective stress directly, which is the most fundamental quantity for pressure prediction.
The approach is illustrated by presenting 1-D, 2-D and 3-D applications in the Gulf of Mexico's Tertiary clastics province. Drilling experiences in the deepwater have shown that this technology can predict pressures to within 0.5 to 0.75 ppg at target depths, provided the "low frequency" trends of seismic interval velocities are of good quality and "close" to well velocities to within 5 -10%.The quantitative reliability of the method is dependent on two factors: quality and resolution of the seismic velocity data and a good understanding of the rock properties.
AAPG Search and Discovery Article #90923@1999 International Conference and Exhibition, Birmingham, England