A New AVA Attribute Based on P-wave and S-wave Reflectivities for Overpressure Prediction

Pore pressure prediction is a key step for safe well drilling operations. This prediction is usually performed deriving a velocity-pressure relationship calibrated to a reference well. However, in the last decades, several methods commonly applied in hydrocarbon exploration have been extended to predict anomalous pressure values directly from seismic data. Among these the Amplitude Versus Angle (AVA) technique has received many attentions. In this work we show that the expected pressure effect on the elastic rock properties is very different from the gas-effect. This makes the classical AVA attributes used for lithology and fluid prediction ineffective in highlighting pressure anomalies. Therefore, we propose a new AVA attribute that can be used to evidence the decrease in P and S impedances that may be associated to overpressured formations. This attribute can be easily derived from the intercept and gradient values computed by means of the Shuey equation. To demonstrate the applicability of this new attribute for pore pressure prediction we show different applications to field datasets over already drilled prospects where overpressures have been encountered. In all cases this attribute shows anomalous responses in correspondence to overpressured layers and then it proves to be effective in highlighting anomalous fluid pressure regimes.