Pore Pressure Prediction Using Seismic Velocities for Deepwater High Temperature- High Pressure Well in Offshore Krishna Godavari Basin, India

Abstract

AbstractVelocities derived from the seismic data provide indirect estimation of the formation pressure prior to drilling. The uncertainties in velocity estimation increase with the geological complexity and depth which in turn amplify the margin of error in the pore pressure prediction. Such uncertainties can be reduced by adopting suitable velocity to carry out predrill pore pressure prediction. Several advanced techniques for velocity analysis have been used in this study with varying degree of confidence for pore pressure estimation in a deep water HTHP well. The well was designed to drill to a depth of 5000 m with an overpressured Cretaceous clastic sediment column of 2000 m before reaching the reservoir (water depth 600m, maximum prognosed pressure ~11,000 psi and temperature ~190°C).In deepwater Krishna Godavari basin, the conventional seismic velocity (Handpicked and stacking velocities) based pore pressure prediction resulted in considerable uncertainties in the older Cretaceous sediments as seen in the earlier drilled wells. This called for an advanced velocity analysis (AVO based and Inversion velocities) to reduce the margin of uncertainties for this study well. Such analysis added values to our understanding of the impedance contrast, temporal and spatial variations of velocity in terms of reservoir and non reservoir inter-relationship. A definitive predrill pore pressure curve, taking into account these geological and geophysical factors was the best estimate for well planning. HTHP well drilling challenges can be constrained by depth of top of overpressure, narrow pore-frac window and large ESD-ECD variations due to high temperature gradient. The definitive pore pressure curve catered to limiting all the three critical parameters as comparison with the post drill pore pressure analysis showed a variation of ±0.5ppg. Future deepwater HTHP prospects can be planned by the similar work flow as the drilling experience of this prospect was satisfactory.