Study and Application of a New Casing Protection Technology for Long-Term Gas Extraction in Deep Coal Seams

To address the problem of borehole instability in deep coal seams, where extraction pipes are prone to deformation and rupture under compressive stress, resulting in reduced efficiency of gas extraction, this study introduces an internal support structure to a traditional extraction pipe (TEP), developing a high-strength extraction pipe (HSEP) capable of long-term gas extraction. Its feasibility and structural parameters were determined through theoretical analysis and numerical simulation. The compressive performance and deformation characteristics of the HSEP and the TEP are analyzed by using a quasi-static compression test system. Furthermore, field industrial trials are conducted underground to validate the performance of the HSEP. The main conclusions are summarized as follows: (1) As the radial compression angle approaches 0°, the compressive strength of the HSEP gradually increases with the maximum load being 1.15 times higher than that of the TEP. (2) The two chambers formed by the internal support and the pipe wall exhibit stage-wise failure characteristics during loading, effectively prolonging the plastic deformation stage compared to the TEP. (3) Field industrial tests show that the gas extraction performance of the experimental group becomes increasingly superior over time compared to that of the control group. After 92 days of continuous extraction, the gas concentration in the control group declined to 16%, while that in the experimental group remained as high as 39%.