Effective Use of Seismic and Well Velocities to Build a Regional Velocity Field for Depth Conversion, Geological Integration and Pressure Prediction

By

 Sherif Montasser¹, James Keggin¹, Norm Allegar¹

(1) BP-Egypt, Cairo, Egypt

 A regional velocity field calibrated to well data for the entire Nile Delta has resulted in a comprehensive velocity field and “living” database that is maintained and updated constantly with new wells and seismic data. It can be used for simple, accurate and reversible time to depth conversions, geological integration and pressure prediction.

The Nile Delta encompasses almost 250,000 square kilometers. More than 15,000 square kilometers of 3-D seismic velocities, 20,000 line kilometers of 2-D seismic velocities and 50 wells have been used. Intensive quality control for each velocity function reduced errors, allowing high quality interpretations.

Calibration of seismic velocities with wells established a correction curve at each well location. Corrections were then applied to the seismic velocities. Horizon based gridding techniques using twelve regional horizons further increased precision and defined regional trends. This gridding method accurately accounted for rapid changes of water depth or for the presence of any local fast layers.

The velocity maps and “deviation from compaction curve maps” (relative measures of shale pressure) show a strong correlation with depositional fairways. These maps appear predictive up to 4000+ meters below mud line. Finally, interval velocity data, when coupled with quantification of pore pressure from wells and test data, has been converted to pore pressure, defining favorable drilling fairways and potential drilling hazards.