Assessment of multi-scale pore structures and pore connectivity domains of marine shales by fractal dimensions and correlation lengths

Abstract

Digital rock (DR) technology can offer three-dimensional distribution of pore networks, providing realistic pore structure information that is essential for shale reservoir evaluation. However, effective application of the DR technology in characterizing multi-scale pore size distribution (PSD) and pore connectivity has been hampered by the unresolvable conflict between the size and the detection resolution of samples being characterized, and the lack of effective characterization parameters. In this study, a novel method named FracD has been developed to obtain continuous and integrated multi-scale PSD curves of shales. The FracD method extracts two types of fractal pore structure information from pore network models at both micro- and nano- scales, respectively, to acquire two representative fractal equations. The fractal equations were then used to generate multi-scale PSD curves with pore-size ranges of ∼ 1 nm-1000 μm through a series of computations. The FracD method was used to successfully compute PSD curves for two shale samples with distinctly different pore structure characteristics from the Sichuan Basin, China. The results were verified to be realistic by microscopic pore observation and physical experiments. In addition, a percolation correlation length parameter is introduced to evaluate the pore connectivity of shales, which has the advantage of describing the size of the connected pore domains more accurately, suitable for the pore connectivity assessment of shales with locally-connected pore structure. The correlation length was effectively applied to distinguish the connectivity of different pores in the Longmaxi shales in the Sichuan Basin, China.