Using Tomographic Seismic Velocities to Understand Subsalt Overpressure Drilling Risks in the Gulf of Mexico

Abstract

Abstract As interest in drilling deepwater wells below salt in overpressured areas like the northern Gulf of Mexico continues to rise, the perceived value of reliable predrill predictions of subsalt pore pressure for mitigating drilling risk is also increasing. Higher-than-normal pore pressures are a significant hazard, especially when drilling below a laterally and vertically extensive salt canopy, where salt bodies are prone to form laterally isolated chambers or compartments with potentially distinct pressure regimes. This paper describes an approach used to estimate subsalt pore pressure in the deepwater Green Canyon area of the Gulf of Mexico. The approach uses a tomographically derived 3D estimate of seismic velocity. The quality of the seismic velocities allows an accurate model of the 3D salt distribution to be defined, the precise delineation of which is essential for accurate pore pressure estimation. While the method employed is able to account for overpressuring due to clay diagenesis, analysis of formation temperatures in the study area suggests that smectite-illite transformation is unlikely to occur within the depth range of available formation pressure data. The 3D pressure model derived demonstrates that subsalt pore pressure predictions can be obtained from seismic velocities given sufficient offset well data for calibrating a velocity-effective-stress transform. Introduction Drilling for hydrocarbons in a safe and economic manner requires that wellbore pressure be maintained between the formation pore pressure and the maximum pressure a formation can withstand without fracturing. Achieving this balance in overpressured areas is much more difficult, especially when factors such as clay diagenesis increase the risk. In these situations, a quantitative predrill prediction of pore pressure becomes essential. Predrill pore pressure estimates can be obtained from seismic velocities using a velocity-effective stress transform calibrated to offset well data. In this paper, tomographic seismic velocities from the Green Canyon area of the Gulf of Mexico are used to predict pore pressure below salt. Tomographic Seismic Velocities Seismic velocities often suffer from a variety of inaccuracies and limitations which render them unsuitable for pore pressure prediction. However, velocities obtained through iterative pre-stack depth migration (PSDM) have been shown to be of much higher quality (Sayers et al. 2002). Moreover, iterative refinement of tomographically derived velocities yields progressively better results, whose accuracy can be independently assessed by comparison with checkshots and sonic logs recorded in offset wells. Improved imaging implies that an accurate 3D model can be obtained, not only of the lateral and vertical salt distribution, but also of subsalt formation velocity.