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Factors such as the hydrogeological conditions, the lithological characteristics of the columns’ components, and the lithological characteristics and stress conditions of the coal seam roof and floor are interrelated and jointly affect column collapse. In this study, the disaster-causing mechanism of column collapse was studied. Based on the system theory, a collapsed column is divided into the column and the surrounding fissure zone as two subsystems for analysis. And, the permeability coefficient of the broken rock under different conditions was measured by a self-designed equipment. The variations of the permeability coefficient for rock samples with different particle diameters, different axial pressures Pa, and different seepage velocities were further studied. Through phenomena analysis and experimental data processing, it was concluded that, under the same pressure state, smaller particle diameter meant smaller permeability coefficient; with the increase of axial pressure, the permeability coefficient decreased; and the larger the water flow velocity was, the smaller the permeability coefficient became. For particle diameter Φ = 2.5–5 mm or larger, the tiny particles formed by randomly washing and breaking in the water flow blocked some of the channels. For particle diameters smaller than Φ = 2.5–5 mm, the smaller permeability coefficient was attributed to the turbulence resulting from non-Darcy flow. The study on the permeability of the fractured rock mass clarified the mechanism of water inrush from the fissure zone of the collapsed column: the collapsed column itself was impermeable, and the permeability of the fissure zone around the collapsed column was related to the lithological characteristics of the rock within the fissure zone and the sequencing of rock strata. When mining coal in areas with collapsed columns, experiments on collapsed columns and fissure zones are prerequisites. This study has a certain referential value for coal mining in this region.

The drilling fluid that is retained in the annulus may become the channeling path for oil, gas and water in the formation. Therefore, ensuring drilling fluid non-retention during cementing is an effective method to improve the annular sealing quality. In order to solve the problem that precious researches can't obtain the conditions of drilling fluid non -retention on the casing and well walls in eccentric annulus, this paper establishes a non-retention condition model through mechanism analysis of displacing fluid elements. Furthermore, several rules are obtained on the basis of the calculation results: for the retention range of drilling fluid, it becomes decreasing gradually on the casing and well walls as casing eccentricity increases, and the retention degree in the annular narrow gap will be more severe. Moreover, for the drilling fluid in the annular wide gap, the critical yield stress of drilling fluid non-retention and the difference of critical yield stress between casing and well walls are increasing. But the opposite results will obtain in the annular narrow gap. So, we can conclude that a better cementing effect can be obtained with the adjustment of displacing fluid properties and construction parameters on the basis of drilling fluid non-retention in the annular narrow gap.

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