Non-Darcy percolation characteristics of cracked coal samples with different occurrences under three-bearing compression

The fracture occurrence is one of the important factors affecting the seepage state of the coal around the borehole. In order to explore the influence of different occurrences of fissures on the seepage characteristics of coal,five coal samples containing one,two and three fractures were prepared. The self-designed tri-axial seepage test system was used to observe the steady-state infiltration method. The seepage test of the coal sample was carried out,and the regres- sive Reynolds number,permeability and Forchheimer number of the fractured coal under tri-axial stress were obtained, and the evolution of non-Darcy seepage in the fractured coal was analyzed. The results show that ① During the seepage test,42% of the seepage Reynolds number is distributed in the range of 10 ~ 100. The seepage trend accords with the nonlinear seepage characteristics;it shows that the viscous drag and the flow process increase with the increase of the fracture surface area. The influence of inertial force on the flow velocity is getting larger and larger,and the possibility of non-Darcy seepage is higher. ② By analyzing the stress state of the fractured coal,the permeability k of the fractured coal sample decreases with the increase of the effective stress σ. Trend,and in line with the k = aσ-b relationship;indi- cating that as the stress level increases,the internal crack of the coal sample closes,the number and width of the seep- age channel decrease, and finally the permeability decreases sharply. ③ Through the calculation of each stage of Forchheimer the number Fo and the seepage velocity v are in accordance with the regular curve growth,and the non- Darcy seepage effect is more and more obvious with the increase of Fo . It indicates that the increase of the permeability of the fractured coal sample is the root cause of the high-speed non-Darcy seepage. Combined with the above conclu- sions,in the pre-extraction work of coal seam gas,the influence range of the extraction drilling hole can be accurately determined according to the gas flow law of the coal body around the borehole hole,which provides an important theo- retical basis for the design of the borehole pattern.