Abstract:

We present a new pore-pressure prediction workflow that couples seismic velocities with geomechanical modeling. Pore pressure is predicted using mean and shear stresses derived from a geomechanical model. This approach incorporates nonuniaxial loading, as well as the response of the geologic environment, and accounts for shear-induced pore pressures. We demonstrate this new workflow using data from the Mad Dog field, in the Gulf of Mexico. We use a static geomechanical model to obtain total mean stress and shear stress values around the Mad Dog salt. We use measured data along a well in front of the salt to establish a velocity – equivalent-effective-stress relationship. We use this relationship together with the shear predicted from the geomechanical model to calculate the mean effective stress across the target field. And we calculate pore pressures as the difference between the total and effective mean stresses. Overall, our new workflow incorporates the geologic history, yields the full stress tensor for the target field and can predict pore pressures ahead of the drill bit.

Introduction

Prediction of pore pressure ahead of drilling is crucial for borehole stability, planning of safe and economic well trajectories and design of casing plans (Dodson, 2004, Dutta, 2002, Zhang, 2013). It is also a key input in the exploration stage to determine the integrity of reservoir seals. Pressure prediction has become particularly important in complex geologic settings: for example, a significant percentage of deepwater well costs near salt are attributed to abnormal pore pressures (Harrison et al, 2004, York et al, 2009).

In basins, pore pressure can be greater than hydrostatic pressure because of sediment under-compaction (Bowers, 1995, Osbourne and Swarbrick, 1997). Rapid deposition of sediments may result in fluid being trapped in the sediment pores; this increases the pore fluid pressure and prevents normal compression of sediments. Because deformation in sedimentary basins is mainly uniaxial, volume changes are directly related to the vertical strain. The horizontal effective stress changes as a function of the vertical, thus the vertical effective stress is a good representation of the overall stress state. As a result, vertical-effective-stress methods have been established to calculate overpressure (Zhang, 2013). In these methods, overpressure is calculated as the difference between total and effective vertical stress, where the former is assumed as the overburden stress and the latter is obtained from the sediment compressional wave velocity (Dutta, 2002, Sayers et al, 2002).

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