Abstract
The sealing depth of a gas-drainage borehole is critically important as it directly affects the efficiency of the whole drainage system. In order to determine the shortest reasonable sealing depth, in this paper, a theoretical drainage model using different sealing depths was proposed. Based on theoretical analysis presented, two parts of the fractures system surrounding the drainage borehole were proposed, i.e. the fractures induced by roadway excavation and the fractures induced by borehole drilling. A series of geological in-situ tests and simulations research were conducted to determine the stress and fracture distributions in the surrounding rock of the borehole. The depths of crushing zones, plastic zones and stress concentration zones were determined as 5 m, 2 m and 12 m, respectively. Meanwhile, stress simulation shows that the depth of the stress concentration zone was 12 m from the roadway wall and the stress peak was located at the depth of 8 m, which can be verified by the results of drilling penetration velocity analysis. To determine the optimum sealing depth, gas drainage holes with different sealing depths were drilled in the field. The field results revealed that the crushing zones were the main area for air leakage, and the stress concentration induced by roadway excavation assisted in the reduction of air leakage. Therefore, the optimized sealing depth should both cover the plastic zone and the stress concentration zone. The research achievements can provide a quantitative method for the determination of optimum sealing depth in cross-measure drainage boreholes.
Highlights
Coal mines in China have experienced the most serious coal and gas burst disasters in the world [1, 2]
Based on the results of tests and analysis, the following conclusions can be drawn: (1) Based on theoretical analysis presented, two parts of the fractures system surrounding the drainage borehole were proposed, i.e. the fractures induced by roadway excavation and the fractures induced by borehole drilling
Both parts can lead to air leakage of the drainage borehole. e first part consists of crushing zone and plastic zone
Summary
Coal mines in China have experienced the most serious coal and gas burst disasters in the world [1, 2]. Calculated the stress and physical state of the surrounding rock by FLAC3D simulation and proposed that the sealing depth for the borehole should be beyond the stress concentration zone as there are some macro-fractures in this area [37]. En, a numerical simulation was carried out to study the stress and deformation distribution characters and a new method was proposed to determine the sealing depth based on the rate of penetration of the drilling rig and in-situ crack zone test by ultrasonic detection. This method was tested through gas drainage experiment with different sealing depths
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