Abstract

An accurate knowledge of formation pore pressure is required for safe and economic drilling in overpressured areas. The use of seismic data for pore pressure prediction is well known, but the seismic velocities used have often been derived from velocities which result from a local average over the seismic aperture used in the analysis. These velocities may lead to inaccurate estimates of pore pressure in the presence of lateral variations in the velocity. Reflection tomography gives improved spatial resolution and thus allows a more reliable pre-drill pore pressure estimate to be obtained. However, seismic velocities can be influenced by changes in lithology and fluid content, as well as by changes in pore pressure. Both P- and S-wave data can be acquired in the marine environment using multi-component receivers at the seafloor. The additional information provided by the S-wave velocity may help to reduce the ambiguity between variations in pore pressure and variations in lithology and fluid content.

Introduction

A knowledge of formation pore pressure is important for the successful exploration and production of hydrocarbons. During exploration, a knowledge of formation pore pressure allows the hydraulic connectivity and fluid migration pathways in a region to be assessed. Thus an increase in pore pressure below a seismic horizon may indicate the presence of a seal, while different pore pressure either side of a fault may suggest that the fault forms a barrier to flow. In deepwater, wells are expensive, and the increased costs associated with deepwater drilling in overpressured environments demands a reliable predrill prediction of formation pore pressure be made. Indeed, some wells are drilled with zero kick tolerance. Large water depths reduce the difference between the pore pressure and fracture stress, and therefore require the pore pressure to be predicted as accurately as possible. Too low a mud weight may allow formation fluids to enter the wellbore which, in the worst case, could lead to loss of the well, whilst too high a mud weight will give too low a rate of penetration and could lead to fracturing of the formation.

The use of seismic data for pore pressure prediction is well known1-11, but the seismic interval velocities used have often been derived from stacking velocities, which locally average the velocity over the seismic aperture used in the analysis. These velocities may not be suitable for pore pressure prediction in the presence of lateral variations that can arise from the presence of dipping structures, lithology variations, salt layers of variable thickness, fault blocks, or variations in compaction and pore pressure. Reflection tomography gives improved spatial resolution and thus allows a more reliable pre-drill pore pressure estimate to be obtained. Estimates of pore pressure can be made if a suitable velocity to pore pressure transform can be established. However, seismic velocities can be influenced by changes in lithology and fluid content, as well as by changes in pore pressure. Both P- and Swave data can be acquired in the marine environment using multi-component receivers placed at the seafloor.

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