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ABSTRACT Accurate constraint of subsurface pore pressure is a critical component of safe drilling programmes. This is particularly challenging for exploratory wells where information on subsurface geology and the stress/pore pressure regime is often limited to seismic inversion. Drilling near salt structures is potentially hazardous due to the complex stress environment. Industry velocity-based pore pressure prediction workflows typically employ a Vertical Effective Stress (VES) approach (e.g. Bowers, Eaton) whereby uniaxial (vertical) stress/strain conditions are implied. In more complex geological settings (e.g. near salt structures or under tectonic compression), lateral and/or shear contributions may be important. The Mean Effective (MES) and Full Effective Stress (FES) methods extend the velocity-based approach to include fully equilibrated stress input from geomechanical modelling. This study demonstrates an early application and contrast of the VES, MES and FES methods to a region of the Sandia Field within the Tarfaya Basin. Comparison with post-drill field data confirms that consideration of the influence of the salt in terms of shear stress can provide a generally improved pore pressure prediction.
1. INTRODUCTION Accurate constraint of subsurface pore pressure is a critical component of safe drilling. In an exploration context there is often considerable uncertainty stemming from a paucity of data with which to constrain probable stress and pore pressure conditions. Offset wells may be tens or even hundreds of kilometers from the prospect and located in different tectonic and depositional settings. This is of particular relevance for salt systems at passive margins where expression of salt can differ significantly depending on the location within the basin. The influence of salt on subsurface stress and pressure distributions is well documented (Fredrich and Fossum, 2002; Koupriantchik et al., 2004; Luo et al., 2012; Nikolinakou et al., 2012). Drilling near salt structures is potentially hazardous due to the complex stress environment.
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