Failure analysis of borehole liners in soft coal seam for gas drainage

In this paper, loads acting on the liner in inserting installation, drainage lifetime and recycling operation were analyzed. Texts on axial deformation, radial compressive deformation and shear deformation properties of the liner were carried out by the universal testing machine. The correlation between the test results and the different loads was discussed. The influences of sieve opening density and diameter on the liner mechanical properties were investigated. Furthermore, the microstructures of tensile deformation, compressive deformation and shear deformation were observed by scanning electron microscope (SEM). Our results are practically useful for the structure design of production liners to improve their mechanical properties. First, in the liner installation, the application of PP liner and PE liner is a top priority in boreholes with large curvature owing to their perfect bending stiffness. Second, during the drainage lifetime, the PP liner can offer a wonderful performance on radial deformation against the borehole collapse. Third, if the liner recycling is required, it is not encouraged to apply the PE liner due to its poor performance on tensile strength and shear strength. Fourth, the increase of sieve density (from 0/m to 10/m) only has an obvious influence on the tensile strength of liner and it is feasible for the efficient recycling to decrease sieve opening density partly. Finally, it is found that the surface of all materials after torsional deformation is worse than that after compressive deformation and after tensile deformation, indicating that shear failure is the key threat to liners.