oa Quantitative prediction methodology for differential stresses and discrimination between pressure and fluid saturation based on seismic attributes derived from experimentally recorded waveforms

A quantitative methodology for remote prediction of differential stresses is proposed, based on rock physics responses to various overpressure mechanisms. The method utilises relationships between various seismic attributes and differential pressures which were established through a series of laboratory tests. A number of ultrasonic experiments simulating normal compaction, disequilibrium compaction, fluid expansion and tectonic mechanisms of overpressure were performed on reservoir sandstone and shale core samples. The velocity changes and the effects of different stress paths on transmitted ultrasonic pulses were investigated through changes of instantaneous waveform attributes. In all measurements the dependence between seismic velocities and differential stresses coincide well with Eberhart-Phillips empirical relationship. A positive relationship between differential pressure and several instantaneous seismic attributes has been established for the first time in these experiments. Results proved that well-known empirical relationships between differential pressure and seismic velocity are also applicable for a number of seismic attributes.

Similarly, based on the combined X-ray CT images and ultrasonic measurements conducted on core samples, the sensitivity of instantaneous seismic attributes to various degree of fluid saturation has been analysed. It was found that several seismic attributes exhibit, in form, similar relationships to the known theoretical models established between seismic velocities and fluid saturation. These results indicate that seismic attributes can be used as alternative approach for discrimination between changes caused by increased pore pressure and fluid saturation.

This enables the prediction of differential stresses by using seismic attribute changes. The proposed methodology has been tested on a 3-D seismic dataset from the Northwest Shelf of Australia and shows good agreement with both the distribution of normally pressured and overpressured wells as well as the magnitude of the overpressures present.