Experimental and theoretical rock physics research with application to reservoirs, seals and fluid processes

Abstract

This paper describes a range of geophysical research activities at the Australian Resources Research Centre based around the development of an experimental capability to validate theoretical and numerical modelling predictions of geophysical properties of reservoirs and seals. Laboratory tests performed on reservoir sandstones, shales and artificial sandstones under a range of controlled triaxial stress conditions allow the full anisotropic elastic tensor to be calculated from ultrasonic measurements on a single core. The analysis of elastic properties and anisotropy in relation to varied stress, pore pressure and fluid saturation can provide significant insight for both exploration (e.g. pore pressure prediction) and production (4D seismic feasibility studies for changes in pore pressure and saturation during production). Ultrasonic data from core measurements are related to seismic response through theoretical analysis of frequency effects and the methodologies developed from this research are subsequently tested on 3D seismic data. Understanding the causes and degree of anisotropy, for example, are critical for depth conversion, imaging, fluid identification (e.g. AVO) and dynamic Poisson's ratio. Velocity hysteresis observed in shales and sandstones with different stress histories has led to an improved understanding of the concept of effective stress and its effect on seismic data. Positive correlations between effective stress and pore pressure with several instantaneous seismic attributes have been established that allow direct mapping of seismic attribute changes into absolute values of effective stress. This methodology has been tested on a 3D seismic dataset from the Northwest Shelf of Australia and shows good agreement with both the distribution and magnitude of the overpressures present. Similarly, X-ray CT images have been combined with ultrasonic measurements conducted on core samples to establish the sensitivity of instantaneous seismic attributes to various degrees of fluid saturation. These results indicate that seismic attributes can be used as an alternative approach for discrimination between pressure and saturation affects. The results from these combined research activities have improved our understanding of the impact of effective stress, anisotropy and saturation on the interpretation of geophysical data, which has implications for pore pressure prediction, 4D seismic evaluations, depth conversion and stress-saturation discrimination.