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AAPG Asia Pacific Region GTW, Pore Pressure & Geomechanics: From Exploration to Abandonment

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Rhove Method - A New Empirical Pore Pressure Transform

Abstract

A new empirical pore pressure transform has been developed that includes many of the advanced, state-of-the-art concepts that are useful in today’s pore pressure estimation and theory. The rhob-velocity-Previous HiteffectiveNext Hit stress (Rho-V-e) method produces a model-driven, stand-alone set of “virtual” rock property relationships, which at intermediate positions are consistent with Bowers method default values for the Gulf of Mexico. The RhoVe method uses a single transform to convert both compressional sonic and bulk density to common estimates of Previous HiteffectiveNext Hit stress and pore pressure where convergence of the two transformed properties offers a robust solution. Velocity-density conversion functions are mathematically linked to a continuous series of velocity-depth normal compaction trend functions. The calculations are limited by bounding end-member curves that provide a basis for intermediate (fractional) solutions of velocity-Previous HiteffectiveNext Hit stress and density-Previous HiteffectiveNext Hit stress relationships that are applied to a well of interest. Paired “virtual” velocity-depth compaction trends were iteratively solved by using published theoretical smectite and illite porosity trends and velocity-depth normal compaction trends. By using the RhoVe-derived velocity-density functions that match the well of interest in cross-plot, normal Previous HiteffectiveNext Hit stress for each end-member and intermediate solution can be calculated. Previous HitEffectiveNext Hit stress is calculated by taking the difference between the integrated density-depth virtual overburden profile, converted from velocity-depth, and the inclusion of hydrostatic pressure. The method produces robust solutions as tested on multiple deep water Gulf of Mexico wells, and extends the predictability of high-velocity, low-Previous HiteffectiveNext Hit stress rock types such as those found in the Deepwater Gulf of Mexico Wilcox-equivalent Paleogene and older section. The velocity-Previous HiteffectiveNext Hit stress trend curves can also improve pore pressure characterization of the overlying overburden section extending to the mud line. Advantages of the RhoVe method are that it can be made interactive and fast, relative to the application of other acoustic transform methods. This paper attempts to build on previous efforts by other workers to include the role of clay type, clay volume and diagenesis on altering velocity-Previous HiteffectiveTop stress relationships and presents a technique in which the effects of clay diagenesis and other factors may be captured and utilized empirically for pore pressure analysis and prediction.