Study on the influencing factors of radial non-Darcy seepage of gas in CO2 fracturing boreholes

CO2 fracturing, as a key technology to improve the gas extraction efficiency of low-permeability coal seams, has a core mechanism of injecting high-pressure CO2 into coal seams to induce the generation of a fracture network within the coal body, thus greatly improving the permeability of the coal seams. After the implementation of downhole CO2 fracturing technology, a large number of macroscopic fissure extensions were formed in the coal body around the fractured holes, forming a special ‘ring-like’ pore/fracture structure, and these pore/fracture networks constituted the main channels for the flow of gas around the holes. In order to study the gas seepage characteristics of this kind of ‘annular’ pore/fracture structure, the LFTD1812-3 gas radial seepage test system was designed and constructed, and a series of gas seepage experiments were carried out with different combinations of ‘annular’ coal samples. The test results show that The test results show that: ① At the initial stage, the permeability of coal around the hole decreased with the increase of gas pressure, and then gradually stabilized. In this process, the permeability in the fractured area was generally higher than that in the fractured area, and with the deepening of coal damage, the fluid flow channels in the coal body continued to increase and improve, further verifying the effectiveness of CO2 fracturing technology. ② There is a significant correlation between the non-Darcy flow factor β and the permeability of the coal body. As the β value increases, the permeability decreases, which is consistent with the positive correlation between the non-Darcy flow factor β and the structural complexity of the coal body pores/fractures. The reason for this is that, with a small change in porosity (φ≤0.01), the non-Darcy flow factor of coal samples consisting of smaller grain sizes (d≤0.4 mm) decreases from 1.28×105 to 1.4×104 eventually, whereas the non-Darcy flow factor of coal samples consisting of larger grain sizes (0.6≤d≤1.0 mm) decreases from 2.6×104 to 7.5×103, which indicates that the non-Darcy flow phenomenon is more significant in the samples composed of smaller particle sizes. ③ The combination of sample particle sizes has a significant effect on the permeability, the larger the particle size leads to the larger the pore space, and the larger the permeability of the combined samples. In the ‘ring’ combination of specimens, when the outer ring particle size remains unchanged and the inner ring particle size increases, the permeability tends to increase. Compared with the outer ring composed of smaller particle size (0−0.2 mm) and larger particle size (0.4−0.6 mm), the permeability of the latter is significantly larger than that of the former, and the growth trend is more significant. ④ With the increase of effective stress, the permeability of the coal body around the drilling hole shows a decreasing trend and follows the law of the negative exponent, which can be expressed as \begin{document}$k = {a_1}{\sigma ^{ - {b_1}}} $\end{document}, and the permeability of the crushed area of the drilling hole increases with the increase of the effective stress, with the increase of the effective stress. effective stress increases, the decreasing trend of permeability is relatively moderate. In addition, the change of effective stress will affect the stress balance state inside the coal body at the same time, which will promote the expansion of coal body fissures around the hole, and the permeability of the coal seam will be further improved. Based on the above conclusions, during the implementation of CO2 fracturing technology, the fracturing radius of the drill holes can be accurately determined based on the characteristic parameters of the gas seepage in the coal body around the fracturing drill holes, and accordingly provide theoretical guidance for the reasonable arrangement of the spacing of the CO2 fracturing drill holes, so as to realise the highly efficient gas extraction from the low-permeability coal seams.