Abstract

Pore fluid pressures encountered in the deep-water Gulf of Mexico typically exceed hydrostatic pressures shortly below the mudline. While pore pressures often become quite high at depth, the overall pressure trend usually falls within a fairly narrow window and—more importantly—the pressures display a rather consistent relationship with measurable rock properties such as velocity, resistivity, and density. The consistency of these trends follows directly from two conditions: the dominance of disequilibrium compaction as the primary source of geopressures in this basin (Osborne et al., 1997, Swarbrick et. al. 2002), and that most parts of the basin are currently at or near their maximum historical effective stress. Disequilibrium compaction leads to a direct relationship between maximum effective stress and porosity; thus, having present day effective stress near its maximum value implies that current porosities indicate current effective stresses and hence current pore fluid pressures. This generally stable behavior has allowed the development and application of a number of pressure-prediction transforms which have facilitated improvements in the safety and efficiency of deepwater drilling (e.g., Eaton, 1975, Gutierrez et al., 2006). (Effective stress is assumed to be the stress acting on the rock frame; in this work, it is taken as equal to the difference between total stress and pore-fluid pressure as proposed by Terzaghi, 1923.)

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