Abstract
The internal collapse of deep seam drainage borehole and negative pressure loss represents a serious technical problem affecting gas drainage. To address this problem a creep model of coal around borehole was established based on the plastic softening characteristics of coal. The final collapse time of the borehole was determined and used to derive the three stages of the borehole collapse process. The model of negative pressure loss in drainage borehole was established according to the theory of fluid dynamics, the model of methane gas flow and the creep model of the coal around the borehole. The relationship between the negative pressure loss of drainage and the change of borehole aperture was derived, thereby revealing the main influencing factors of the negative pressure loss in the borehole. A drainage technique named “Full-hole deep screen mesh pipe” was introduced and tested to prevent the collapse of borehole and reduce the negative pressure loss. The result shows that after the borehole was drilled, the borehole wall was affected by the complex stress of the deep coal seam, the coal surrounding the borehole collapsed or presented the characteristics of creep extrusion towards the borehole. The “Full-hole deep screen mesh pipe drainage technology” could effectively control the collapse as well as the deformation of the borehole and reduced the negative pressure loss. Compared with the traditional drainage technology, the methane gas drainage concentration was increased by 101% and the gas flow was increased by 97% when the methane gas was drained for 90 days, the gas drainage efficiency increased significantly.
Highlights
Gas drainage is the fundamental method to reduce the pressure of methane gas and its content in coal seams, and it is a direct way to solve gas disasters and realize the transformation of gas resources into treasure [1,2,3,4,5,6]
Instability-negative pressure loss model of gas drainage borehole and prevention technique funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript
It is of great theoretical and engineering significance to study the deformation of gas drainage borehole and the rule of negative pressure loss in gas drainage boreholes to determine the appropriate drainage technology and improve the gas drainage efficiency
Summary
Gas drainage is the fundamental method to reduce the pressure of methane gas and its content in coal seams, and it is a direct way to solve gas disasters and realize the transformation of gas resources into treasure [1,2,3,4,5,6]. Instability-negative pressure loss model of gas drainage borehole and prevention technique funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Complex mechanical environment of “three high and one disturbance” (High geostress, high geothermal, high karst water pressure and strong mining disturbance) [7,8,9,10] Under this condition, during the coal seam gas drainage process, the borehole diameter will change due to the creep characteristics of the coal itself, and the borehole may even collapse in some cases [11, 12]. Based on the actual parameters of a coal mine, the surrounding rock stress of the roadway was divided, and the causes of instability collapse and negative pressure loss in the drainage borehole were thoroughly analyzed. The prevention technology for the phenomenon of borehole collapse and negative pressure loss was proposed, and the effect of the prevention technology was investigated, thereby providing an effective technical way for improving the gas drainage effect of borehole in deep coal seam and reducing the pressure loss
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