Multifractal characteristics of shale and tight sandstone pore structures with nitrogen adsorption and nuclear magnetic
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ORIGINAL PAPER
Multifractal characteristics of shale and tight sandstone pore structures with nitrogen adsorption and nuclear magnetic resonance Fu‑Yong Wang1 · Kun Yang1 · Yun Zai1 Received: 13 November 2019 © The Author(s) 2020
Abstract Based on the experiments of nitrogen gas adsorption ( N2GA) and nuclear magnetic resonance (NMR), the multifractal characteristics of pore structures in shale and tight sandstone from the Chang 7 member of Triassic Yanchang Formation in Ordos Basin, NW China, are investigated. The multifractal spectra obtained from N 2GA and NMR are analyzed with pore throat structure parameters. The results show that the pore size distributions obtained from N 2GA and NMR are different, and the obtained multifractal characteristics vary from each other. The specific surface and total pore volume obtained by N2GA experiment have correlations with multifractal characteristics. For the core samples with the similar specific surface, the value of the deviation of multifractal spectra Rd increases with the increase in the proportion of large pores. When the proportion of macropores is small, the Rd value will increase with the increase in specific surface. The multifractal characteristics of pore structures are influenced by specific surface area, average pore size and adsorption volume measured from N2GA experiment. The multifractal characteristic parameters of tight sandstone measured from NMR spectra are larger than those of shale, which may be caused by the differences in pore size distribution and porosity of shale and tight sandstone. Keywords Shale · Tight sandstone · Nitrogen adsorption · Nuclear magnetic resonance · Multifractal · Pore structure List of symbols Dq Generalized fractal dimension D1 The generalized fractal dimension when q=1 f (𝛼) Multifractal spectra f (𝛼)max The maximum values of f (𝛼) f (𝛼)min The minimum values of f (𝛼) k Positive integer N(𝜀) The number of sections divided N𝛼 (𝜀) The number of boxes with singularity index between 𝛼 and 𝛼 + d𝛼 Pi (𝜀) Probability distribution function q Exponent Rd Deviation of the multifractal spectra 𝛼 Singularity index 𝛼0 (q) Singularity index corresponding to f (𝛼)max 𝛼(q+ ) The smallest value of 𝛼0 (q) when q > 0 Edited by Jie Hao and Xiu-Qiu Peng * Fu‑Yong Wang [email protected] 1
Unconventional Petroleum Research Institute, China University of Petroleum (Beijing), Changping, Beijing 102249, China
𝛼(q− ) The largest value of 𝛼0 (q) when q < 0 𝛼1 (q) Width to the left of the multifractal spectra 𝛼2 (q) Width to the right of the multifractal spectra Δ𝛼(q) The width of the multifractal spectra 𝜒(q, 𝜀) Partition function 𝜀 The scale of box 𝜇i (𝜀) The pore volume for the ith interval 𝜏(q) Mass exponent
1 Introduction Shale oil resources are abundant in China and it is another hot spot in current exploration and development (Hu et al. 2017). The Chang 7 member of the Yanchang Formation in the Ordos Basin of China is rich in unconventional petroleum resources. In particular, the exploration and develop
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