Determination of representative elementary volume of digital coal based on fractal theory with X-ray CT data and its application in fractal permeability predication model

Abstract

Representative elementary volume (REV) is the key to study the heterogeneity of digital coal and characterize its macroscopic and microscopic properties. The permeability evolution law of digital coal based on REV analysis can provide theoretical support for the application of permeability prediction model in multi-scale reservoirs. This study takes typical coal samples from Bofang and Sihe coal mines in Qinshui basin as research object. First, the nondestructive information of two samples is scanned and visualized. Secondly, the calculation methods of two-dimensional (2D) and three-dimensional (3D) fractal dimensions of pores and fractures are illustrated. Then, the determination methods of REV based on porosity and fractal dimension are compared. Finally, the distribution pattern of fractal dimension and porosity curves is studied, the relationship between 2D and 3D fractal dimension is characterized, and the application of fractal permeability model in permeability analysis of multi-scale reservoir is further discussed. The REV size varies greatly in different vertex directions of the same sample and between samples, so REV analysis can only be performed in specific directions. When the REV based on fractal dimension is determined, the porosity curve continues to maintain a downward trend and then tends to be stable. The 2D fractal dimension has a positive linear correlation with the 3D fractal dimension, and the porosity can be expressed as a linear function of the fractal dimension. The permeability through REV analysis domain is mainly affected by fractal dimension, dip angle, azimuth angle and maximum fracture length, which is of great significance for exploring permeability evolution law of coal reservoir at different scales. This study is of great significance for enriching the determination methods of REV in digital coal and exploring the permeability evolution law of multi-scale reservoirs.

Graphical abstract

Keywords

representative elementary volume / fractal dimension / permeability / digital coal / X-ray CT / Qinshui Basin

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Acknowledgment

We would like to express our gratitude to the anonymous reviewers for offering their constructive suggestions and comments which improved this manuscript in many aspects. This work was financially supported by the National Natural Science Foundation of China (Grant No. 42102217), the University Synergy Innovation Program of Anhui Province (No. GXXT-2021-018), the Natural Science Research Project of Anhui University (Nos. KJ2020A0315 and KJ2020A0317), the Natural Science Foundation of Anhui Province (No. 2108085MD134), and the Foundation of State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing (No. PRP/open-2005).

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