Breakage characteristics of casing and prevention and control measures in mining wells based on the evolution law of stress and crack fields in overlying strata

Accurately determining the casing breakage characteristics in mining wells and proposing corresponding preventive and control measures can provide an important guarantee for the long-term stability of the wellbore. Taking the No. 2₁ coal seam and overlying strata in the Pingdingshan mining area as the research object, theoretical calculations and numerical simulation were used to determine the stress and displacement distribution law of separation section in the overlying strata. The correlation between the height of the overlying strata, the horizontal distance from the working face and stress and displacement were analyzed, and the relationship between the height of the overlying strata, the horizontal distance from the working face and the stress and displacement were fitted. Based on the actual development experience of mining wells in the research area, the main types of casing breakage in the optimal well layout area were determined. Combined with the commonly used casing mechanical parameters, the casing breakage location and length under the “two field” evolution were characterized. The commonly used local protection technologies for mining wells was summarized and the corresponding preventive and control measures were proposed. The results shown that, in the process of working face advancement, the stress of the overlying strata shown a change pattern of from fluctuation to linear decrease, and the displacement shown a change pattern of from almost unchanged to hyperbolic type. The casing in the optimal well layout area was mainly characterized by tensile shrinkage and shear fracture, and the casing reinforcement was applied to reinforce the casing in mining well. When N80 casing was used in the third section of the ground mining well in the research area, it was necessary to install anti- tensile and shear casing reinforcement in the upper 46 m of the coal seam at the sub-critical layer and in the upper 54 m of the coal seam at the soft/hard interlayer. When P110 casing was used, it is necessary to install anti-tensile reinforcement in the upper 46 m of the coal seam at the sub-critical layer. It is recommended to use P110 casing for all three sections of the surface mining wells in the research area to increase the wellbore stability in mining well. The research achievement can provide theoretical guidance for preventing casing breakage in the Pingdingshan mine and mining wells under similar conditions.