Deformation and collapse patterns of gas extraction boreholes and a precise monitoring technology

This study aims to promote the precise and intelligent monitoring of gas extraction boreholes. In the engineering background of a coal mine in Shanxi Province, this study conducted simulation experiments on the plugging of gas extraction boreholes under varying particle size ratios of simulated coal samples using distributed optical fiber monitoring and the Brillouin optical time-domain analysis (BOTDA). Then, this study established a mathematical model for calculating the borehole plugging rate of the test mine, revealing the deformation and collapse patterns of gas extraction boreholes. Furthermore, this study proposed a precise in-situ monitoring technology for the gas extraction boreholes in the test mine and verified its feasibility and accuracy through field tests. Key findings are as follows: (1) There existed a linear correlation between the strain measured by fiber couplers, the mass of simulated coal samples, and the deformation and collapse of the boreholes. With an increase in the coal sample mass, the strain exhibited similar variation trends, increasing sharply, generally, and sharply in sequence. A mathematical model for calculating the borehole plugging rate was established through segmented fitting; (2) The error analysis revealed that with an increase in the strain, the maximum absolute error between the actual and theoretical borehole plugging rates manifested a trend of initial increase, followed by decrease and then increase, equaling 19.48% in the middle collapse stage. Under complete borehole plugging in the late collapse stage, the local maximum of the coal sample mass closer to the average local maximum of the mass of coal samples with different particle size ratios corresponded to a smaller error; (3) Based on calculations using the mathematical model, this study revealed the boreholes’ collapse pattern during the borehole plugging simulation. Specifically, coal blocks first accumulated in a convex shape at the bottom of a borehole, then slid toward both sides, and finally accumulated at the borehole’s top. With strain values of 0, 45.95×10^-6, and 72.19×10^-6as critical values, this study determined the early, middle, and late stages of borehole collapse, developing the precise monitoring technology for the gas extraction boreholes in the test mine. As indicated by the analysis of in-situ strain monitoring results, fractures are prone to form around the boreholes in their unstable segments under the action of factors such as stress and disturbance, with borehole collapse intensifying over time. By combining the calculated plugging rates along the boreholes, this study determined the deformation and collapse along the boreholes. Comparison with the boreholes’ inside images reveals that the results obtained using the precise monitoring technology are roughly consistent with actual observations. Therefore, the precise borehole monitoring technology based on the distributed fiber optic coupler and the BOTDA is feasible and reliable, serving as a reference for advancing the precise and intelligent monitoring of gas extraction boreholes.