Stage-Dependent Evolution of Mining-Induced Overburden Displacement Field and Its Influence on Surface Well Instability: A Comparative Analysis of Two Mining Heights

The stability of surface gas extraction boreholes in longwall-mining-disturbed zones is constrained by instability driven by mining-induced overburden movement, yet the stage-dependent evolution of overburden displacement fields under large mining height conditions and its influence on borehole failure remain insufficiently characterized. In this study, Panel 818 of Xinhu Coal Mine in the Huaibei mining area, China, was adopted as the engineering background, and response models under two mining height conditions were established with the 3-Dimensional Distinct Element Code (3DEC) to comparatively analyze the stage-dependent evolutions of vertical subsidence and horizontal displacement and their controlling effects on borehole structural stability. The results show that the vertical subsidence curve transitions progressively from an asymmetric “V” shape to a flat-bottomed “U” shape as the face advances, while the horizontal displacement field maintains a centrally symmetric pattern with pronounced interlayer shear slip at soft–hard stratum interfaces. Increasing mining height does not alter the fundamental evolutionary sequence but amplifies the maximum vertical subsidence by approximately 1.5 times, raises horizontal displacement and interlayer slip gradients by a factor of 1.4–2.0, and extends the active advance interval over which bed separation persists. Borehole instability is attributed to the concentrated transmission of incompatible overburden displacement to the wellbore; axial tensile failure and transverse shear failure are governed by strain concentration from concentrated non-uniform subsidence and by horizontal displacement gradient amplification between adjacent strata, respectively. These findings indicate that mining height is a key factor controlling the stage-dependent evolution of overburden displacement fields and the resulting mechanical loading on surface borehole. It should be noted that the present comparison is limited to two mining height conditions and that the numerical model characterizes the overburden displacement field along the wellbore trajectory without explicitly modeling the casing and cement sheath structures; the reported strain indicators therefore represent rock-mass deformation indicators of the loading imposed on the wellbore by the surrounding rock mass.