Shear mechanical behavior and stress transfer characteristics of the bilayer casing of gob gas ventholes

Abstract

Induced by multi-seam coal extraction, the casing of gob gas venthole (GGV) is prone to shear deformation or failure, which would result in substantial reduction in the gas production of GGV. The use of bilayer casing in the GGV is conducive to reduce the probability of cemented casing failure. However, the shear mechanical characteristics of the bilayer casing are not clearly understood. In this paper, the shear experiments of the single-layer casing, the bilayer casing with flexible filler (BC-FF), and the bilayer casing with rigid filler (BC-RF) were carried out. Besides, the stress distribution and its evolution of the bilayer casing during shear deformation were numerically simulated. The results showed that the shear deformation of the bilayer casing can be divided into five stages and the BC-RF demonstrated stronger shear strength, while the BC-FF exhibited a higher resistance for shear deformation. Additionally, the stress transfer coefficient was proposed to characterize the degree of stress attenuation during the stress transferring from the outer casing to the inner casing. Based on the numerical simulation results, the stress transfer coefficient of the bilayer casing was calculated. The results showed that the stress transfer coefficient of BC-RF was higher than that of BC-FF and that the peak value of stress transfer coefficient of the bilayer casing increased with the ultimate strength and elastic modulus of the filler. The stress transfer coefficient mainly correlated with both the elastic modulus and the ultimate strength of the filler before reaching the peak. However, it was only determined primarily by the ultimate strength in the fluctuation stage after it reached the peak. The results of this paper will lay the foundation for interpreting the shear mechanical behavior of the bilayer casing and optimizing the design of GGV structure.