Fractal dimension of pore-space geometry of an Eocene sandstone formation

ABSTRACT The geometry of pore structure of reservoir rock is described by the shape, size, distribution and connection of pores, and pore throats of rocks. Pore-space properties are important for the description and characterization of fluid storage and transport in reservoir rocks. We used the fractal concept to describe the geometric structure of the pores of 24 samples from an Eocene sandstone formation in China. The fractal behavior of pore volume distribution was investigated by capillary pressure curves and nuclear magnetic resonance (NMR). Additionally, the fractal dimension of the pore surface was determined based on data of the specific surface area per unit pore volume (Spor). The comparison of fractal dimensions determined by three different methods indicates a clear differentiation into the “surface dimension” and “volume dimension.” The fractal dimension resulting from longer transverse NMR relaxation times and lower capillary pressure reflects the volume dimension of larger pores. The fractal dimension derived from the short NMR relaxation times is similar to the fractal dimension of the internal surface. The surface dimension increases with rising Spor. The average value of surface dimension was determined to be 2.31 for the set of Eocene sandstones. This value of fractal dimension was successfully applied in a model of permeability prediction that is based on formation factor and specific surface area (Spor).