3D reconstruction of coal pore network and its application in CO₂-ECBM process simulation at laboratory scale

Abstract

Three-dimensional (3D) reconstruction of the equivalent pore network model (PNM) using X-ray computed tomography (CT) data are of significance for studying the CO₂-enhanced coalbed methane recovery (CO₂-ECBM). The docking among X-ray CT technology, MATLAB, with COMSOL software not only can realize the 3D reconstruction of PNM, but also the CO₂-ECBM process simulation. The results show that the Median filtering algorithm enabled the de-noising of the original 2D CT slices, the image segmentation of all slices was realized based on the selected threshold, and the PNM can be constructed based on the Maximum Sphere algorithm. The mathematical model of CO₂-ECBM process fully coupled the expanded Langmuir equation. At the same time for CO₂ injection, CH₄ pressure tends to decrease with the increase of CO₂ pressure, but its difference is not obvious. The CH₄ pressure in the slice center changed a lot, while at the edge it changed a little under different CO₂ pressures. The injected CO₂ was transported to matrix along the macro and micro-fractures with continuous flow. The injected CO₂ first replaced the adsorbed CH₄ by covering the inner surface of macro-pores and meso-pores to form the single molecular layer adsorption of CO₂. Then they migrated to micro-pores by Fick’s diffusion, sliding flow, and surface diffusion. Furthermore, the CO₂ replaced CH₄ adsorbed by volumetric filling in micro-pores, and formed the multi-molecular layer adsorption of CO₂. The gas pressure and migration path between CO₂ and CH₄ are opposite. This study can provide a theoretical basis for studying digital rock physics technology and enrich the development of CO₂-ECBM technology.

Graphical abstract

Keywords

CO₂-ECBM / 3D reconstruction / numerical simulation / X-ray CT / COMSOL / 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 University Synergy Innovation Program of Anhui Province (No. GXXT-2021-018), the Natural Science Research Project of Anhui University (Nos. KJ2020A0315, KJ2020A0317), the Natural Science Foundation of Anhui Province (No. 2108085MD134), the National Natural Science Foundation of China (No. 41902168), 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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