Experimental study on the flow regime transition characteristics of gas radial seepage in fractured coal bodies around extraction boreholes

The radial gas seepage flow regime of the coal body around borehole is one of important factors reflecting the seepage movement of extracted gas. In order to investigate the effects of different pore structures and gas pressure on the radial seepage motion of gas in the perimeter of borehole, five kinds of pore-bearing coal samples with different gradations were made, and the radial seepage test was carried out on the pore-bearing coal samples by using the self-designed gas radial seepage test system and the steady-state seepage method to analyze the changes of the Forchheimer number and non-Darcy error with the seepage velocity and gradation, and the flow velocity intervals and the Reynolds number ranges of different flow regimes were obtained by using the Hazen-Dupuit-Darcy equation and the Reynolds number theory. The results show that: ① Through the analysis of the Forchheimer number and non-Darcy error with the change curve of seepage velocity, the Forchheimer number increases with seepage velocity in a quadratic polynomial curve, while non-Darcy error increases linearly, the Forchheimer number and non-Darcy error are closely related to pore structure, and both become larger with the increase of gradation, indicating that seepage velocity and pore structure of the coal body around the pore are important factors for gas seepage deviating from linear seepage; ② The Hazen-Dupuit-Darcy equation was used to characterize and delineate the gas radial permeability flow regime. With the increase of seepage velocity, the gas radial seepage motion appears in order of pre-Darcy, Darcy and Forchheimer flow regimes. The small-graded porous specimens with n=0.2 and n=0.4 are more prone to the Darcy laminar flow, while the porous samples with n=0.6, n=0.8 and n=1.0 are more prone to the Forchheimer flow regime; ③ Through the analysis of the relative error curve of pressure gradient when Darcy’s law was applied, the relative error with the increase of seepage velocity shows the change law of decreasing, keeping stable, and then increasing, which is consistent with the change law of gas seepage flow pattern with seepage velocity; ④ In double logarithmic coordinates, the relationship between different flow regimes of gas seepage and Reynolds number was analyzed, and the Reynolds number intervals of different flow regimes were derived, and it was found that the radial seepage motion of gas conforms to the Reynolds number range of 0.0012～0.0025 for Darcy laminar flow. The results of the study can provide an important theoretical basis for the design of drilling parameters by determining the influence range of drilling extraction based on the flow regime of coal seepage movement around the borehole in coal seam gas pre-drainage work.