Ahead of the Bit Pore Pressure Prediction under Centroid Effect Conditions Using Express Seismic Processing Results for Exploration Drilling in the Caspian Sea

Abstract

For the offshore prospecting and appraisal well drilling optimization, an approach was proposed, included Pore Pressure prediction based on 2D processing of a seismic surface data. This approach was used for timely detection of sharp changes in Pore Pressure with depth, under expected «centroid» effect for a well construction in the waters of the Caspian Sea. At the initial stage, an acoustic inversion of 2D seismic data wascarried out. Based on the obtained acoustic impedance section and seismic velocities, after a deep migration, Pore Pressure forecast was conducted for the well area. Pore Pressure was restored for the offset well and relations between effective stresses and seismic inversions results were identified for different facies. Together with the geomechanical support, regular velocity-depth model updating process was realized, involving log and well data from drilled section. Using the velocity-depth model, updating process and continuous support, Pore Pressure forecast was successfully implemented in the intervals with sharp pressure variations. Bottom-hole updates were carried out in the critical further drilling intervals using updated formation velocities and seismic attributes, to the depth of next drilling section. In real time, the pronounced regional centroid effect was confirmed and successfully passed. Directly during the drilling, casing depths were corrected, mud weight values and equivalent circulation densities were controlled in safe ranges. In this work, regional centroid effect was predicted and verified for the Caspian Sea area. The completed project demonstrates the successful application of a process involving tomographic update of velocity-depth model and continuous Pore Pressure forecast for optimization and increasing offshore prospect well drilling safety. This approach is critical for successful well construction under centroid effect, particularly, in the Caspian Sea area.