INNOVATIVE GEOPHYSICAL METHODOLOGY TO REDUCE THE UNCERTAINTIES IN PORE PRESSURE PREDICTION

A correct Pore Pressure prediction is a key step in safer well drilling operations. Different methods are available, but all of them generally need as input a reference well to calibrate some sort of relation between velocity and effective stress, in case only undercompaction is taken into consideration. In case overpressure is generated by other mechanism than compaction disequilibrium, the conventional PPP methods are not valid. The onset of overpressure causes a decrease of Vp, Vs and density. These variations lead to distinct effects on the P and S impedance, which in turn correspond to distinct AVO trends. In the application presented here, AVO attributes (Intercept and Gradient), generally used to assess the type of fluid saturating the rock pores, are combined to obtain an attribute that respond differently in relation to the pressure regime (i.e. hydrostatic vs overpressured). This new attribute, here called AVO4PP, depends on the P-wave and S-wave reflectivities, and has been found to be sensible to overpressures. The attribute AVO4PP has been applied to real datasets over already drilled prospects where overpressures have been encountered, showing good responses. In order to check the sensitivity of the attribute AVO4PP to overpressure, it has been used in a prospect saturated with gas and known to be characterized by a very weak overpressure. The calculated AVO4PP attribute did not show any strong response, as expected. Case histories of different subsurface settings will be presented to show the usability of the attribute for pore pressure evaluation. In conclusion, even if the presented new attribute cannot be uniquely linked to overpressures, as the same response could be due to lithology effects, its application in combination with other subsurface data analyses can contribute to qualitatively evaluate the presence of overpressure regimes and thus reduce drilling risks.