CO2 sequestration in coal seams represents an effective strategy for mitigating CO2 emissions. However, the complicated interaction of CO2-water-coal at the micro-scale may compromise the structural integrity and mechanical strength of coal, thereby adversely impacting the efficacy and safety of CO2 sequestration in coal seams. This study introduces a novel indentation testing method that reveals the scale-dependent evolution mechanisms of coal microstructures, enabling the accurate and reliable quantification level of degradation in the micromechanical properties caused by supercritical CO2-water-coal interactions. Using this method, the extent of mechanical degradation in three types of coal microstructures could be accurately evaluated under supercritical CO2-water-coal interaction. The pure organic matrix exhibited remarkable stability under all fluid treatments, with minor changes in microstructure feature and a mechanical property reduction of less than 25%. In contrast, the mineral structures were significantly altered by treatment with fluid mixed with supercritical CO2 and brine, with erosion depths and mechanical property reductions reaching 1.6 µm and 80% in granular structures, and 6.4 µm and 90% in banded structures. However, in the absence of brine or supercritical CO2, the erosion depths and mechanical property reductions of fusinite were limited.
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Fossil fuels such as coal, oil, and natural gas have been pivotal in shaping modern society, yet their use has resulted in significant emissions of greenhouse gases like carbon dioxide CO2. In pursuit of sustainable development, China has adopted strategies aimed at achieving carbon peak and carbon neutrality, prioritizing carbon reduction and energy transformation. CO2 storage is an effective strategy for mitigating the greenhouse effect, a practice that has gained global traction. The CO2-water-rock reaction plays a crucial role in CO2 geological sequestration, with the rock surface being a critical site for this reaction. Understanding the topography of rock surfaces is essential for assessing the degree of CO2-water-rock reactions and ensuring the safety of CO2 geological sequestration. Methods for characterizing rock surface topography are mainly classified into two categories. The first relies on scanning electron microscopy observations and energy-dispersive X-ray spectroscopy analysis, which, despite offering detailed surface topography, is limited by its inability to capture depth information adequately. The second category encompasses quantitative characterization methods based on surface roughness and fractal dimension derived from statistical principles. While these methods provide valuable quantitative parameters, their physical significance is not intuitively clear, nor do they directly correlate with rock erosion.
This paper introduces a new quantitative method for assessing the degree of CO2-water-rock reactions through a method based on three-dimensional point cloud computing. This method aims to quantify the reaction degree on the rock surface more intuitively by measuring erosion volume. Utilizing three-dimensional laser scanning technology, point cloud data is acquired to construct a three-dimensional model of the rock surface. This model helps define the depression volume and determine the base level of the rock surface before and after CO2-water treatment, using the root-mean-square roughness as a reference. In this paper, we propose two new methods for volume calculation, namely the concave method and the convex method. By applying these methods to calculate the volume of a regular model, their accuracy and applicability are compared. This comparison allows for the quantitative characterization of the erosion volume on the rock surface before and after CO2-water treatment. Depending on the specific conditions, either calculation method can be selected to quantify the erosion volume of the rock surface before and after CO2-water treatment. Finally, the paper uses coal samples from the Zhengtong Coal Mine, Changwu County, Xianyang City, Shaanxi Province, to validate the feasibility of the proposed erosion volume calculation method. [Results and Conclusions] These tests sought to simulate the CO2-water-rock reactions and assess the effectiveness of the method in quantifying and characterizing the reaction degree. After CO2-water treatment, a clear distinction in erosion across different areas on the rock surface was observed. Furthermore, the results revealed a significant proportional relationship between the erosion volume on the rock surface and the root-mean-square roughness.
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