Fractal Characterization of Nanopore Structure in Shale, Tight Sandstone and Mudstone from the Ordos Basin of China Using Nitrogen Adsorption

Abstract

The characteristics of the nanopore structure in shale, tight sandstone and mudstone from the Ordos Basin of China were investigated by X-ray diffraction (XRD) analysis, porosity and permeability tests and low-pressure nitrogen adsorption experiments. Fractal dimensions D1 and D2 were determined from the low relative pressure range (0 < P/P0 < 0.4) and the high relative pressure range (0.4 < P/P0 < 1) of nitrogen adsorption data, respectively, using the Frenkel–Halsey–Hill (FHH) model. Relationships between pore structure parameters, mineral compositions and fractal dimensions were investigated. According to the International Union of Pure and Applied Chemistry (IUPAC) isotherm classification standard, the morphologies of the nitrogen adsorption curves of these 14 samples belong to the H2 and H3 types. Relationships among average pore diameter, Brunner-Emmet-Teller (BET) specific surface area, pore volume, porosity and permeability have been discussed. The heterogeneities of shale nanopore structures were verified, and nanopore size mainly concentrates under 30 nm. The average fractal dimension D1 of all the samples is 2.1187, varying from 1.1755 to 2.6122, and the average fractal dimension D2 is 2.4645, with the range from 2.2144 to 2.7362. Compared with D1, D2 has stronger relationships with pore structure parameters, and can be used for analyzing pore structure characteristics.

Microstructural characterisation of organic matter pores in coal-measure shale

“…As the TOC increases further, the color of shale turns to black, small OM regions are interconnected and form large area accumulations. This is particularly evident in carbonaceous mudstones (Li et al, 2018a(Li et al, , 2019Sheng et al, 2019).…”

“…The relationship between A and D 2 is not clear, which is possibly due to the fact that OM hydrocarbon generation is more beneficial to the development of micropores and mesopores (<10 nm), while it has a smaller contribution to development of macropores and mesopores (>10 nm). These correlations indicate that the larger the value of A is, the more developed the micropores and mesopores (<10 nm) will be, resulting in a larger D 1 (Yang et al, 2014;Li et al, 2019).…”

Recent Advances in Flow and Transport Properties of Unconventional Reservoirs

“…Base on a low-pressure nitrogen adsorption experiment and fractal theory, Li et al [4] studied the characteristics of nanopore structure in shale, tight sandstone, and mudstone, with an emphasis on the relationships between pore structure parameters, mineral compositions, and fractal dimensions. The relationships among average pore diameter, Brunner-Emmet-Teller specific surface area, pore volume, porosity, and permeability were also discussed.…”

Multifractal characteristics of shale and tight sandstone pore structures with nitrogen adsorption and nuclear magnetic resonance

“…Edited by Jie Hao and Xiu-Qiu Peng shale oil resources in the Yanchang Formation of the Ordos Basin, China (Er et al 2016;Yin et al 2020). At present, there are several experimental methods for characterizing shale and tight sandstone pore structures, including thin section analysis, scanning electron microscopy (SEM), high-pressure mercury intrusion (HPMI) (Lai et al 2018;Wang et al 2019), N 2 GA (Chen et al 2017;Li et al 2019;Singh 2016;Singh and Cai 2018), NMR (Shao et al 2017;Zhao et al 2020), micro/nano CT scanning (Peng et al 2011;Yang et al 2019) and so on. In 1980s Mandelbrot proposed the fractal theory and then it has been widely used in many fields (Mandelbrot 1983), and many scholars have studied the characteristics of pore structures in sedimentary rocks combining fractal theory with various experimental methods (Li et al 2017;Wang et al 2018).…”