Schlumberger

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

Society: OTC
Paper Number: 21546
Presentation Date: 2011
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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.

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