Calculation of multi-fractal dimension of coal measure sedimentary rock and analysis of influencing factors

Abstract

In order to characterize the pore structure and fractal characteristics of coal-measure sedimentary rocks, three types of typical sedimentary rocks of the Middle Jurassic shale, mudstone and sandstone were selected as the research objects. Through X-ray diffraction(XRD)analysis and nuclear magnetic resonance(NMR)experiment, the correlation between NMR fractal dimension, mineral composition and physical property parameters was discussed by using fractal theory. The results are as follows: ① Based on the relaxation time cut-off value T2C, the NMR fractal dimension of shale and mudstone can be divided into the fractal dimension of adsorption pore DA(T2 ≤3 ms)and the fractal dimension of seepage pore DS (T2 >3 ms). ② In terms of reservoir physical properties, NMR fractal dimension Df has a good linear negative correlation with porosity, permeability, and reservoir quality index, indicating that NMR fractal dimension can be used as an important indicator of rock physical properties. ③ In terms of mineral composition, the content of quartz and feldspar has a weak negative correlation with the fractal dimension Df, and clay minerals have different influences on the fractal dimension due to multiple factors such as sedimentary environment, physical and chemical properties and mineral content.

Keywords

• coal-measure sedimentary rocks
• pore structure
• nuclear magnetic resonance
• multi-fractal dimension
• permeability