Analysis of the mechanism and influencing factors of supercritical carbon dioxide on coal permeability enhancement

Under the current carbon peaking and carbon neutrality background, supercritical carbon dioxide fracturing technology can not only improve the permeability of coal, but also geological storage of carbon dioxide, so it is increasingly valued in the coal field. However, the mechanism and main influencing factors of supercritical carbon dioxide on coal permeability enhancement are not clear, so in order to grasp its mechanism and explore its main influencing factors, it has carried out summary analysis and data processing. Firstly, it is clear that the mechanism of supercritical carbon dioxide to enhance the permeability of coal mainly includes three aspects: the transformation of coal micro-components, the swelling effect on coal matrix and the displacement of gas in coal. Then, the main factors affecting the supercritical carbon dioxide action on coal permeability enhancement are described, including the action temperature, action stress, action time and water content of coal, and the relationship between the main factors and the characteristics of coal is obtained: the porosity and permeability of coal will decrease with the increase of action temperature, but increase with the increase of action stress, action time and water content of coal, and they are in a positive exponential function with the action stress and the short-time soaking time; the mechanical strength of coal will gradually weaken with the increase of action time and water content, and the compressive strength, elastic modulus and tensile strength all conform to the negative exponential function relationship with the action time. Finally, the existing problems of supercritical carbon dioxide on coal permeability enhancement are discussed. It is pointed out that the current researches ignore the influence of soaking times on the effect of supercritical carbon dioxide on coal permeability enhancement, and lack the analysis of main controlling factors and optimal permeability improvement conditions. And the research on the soaking times of supercritical carbon dioxide can be carried out by imitating the soaking experiment of circulating liquid nitrogen, and the main controlling factors and the optimal permeability improvement conditions of supercritical carbon dioxide on coal permeability enhancement can be determined by orthogonal experiment. This has a certain guiding significance for optimizing the supercritical carbon dioxide permeability improvement technology, sequestering carbon dioxide and improving the effect of coal-bed methane extraction.