Abstract
Coal and gas outbursts can lead to serious disasters in coal mines. The drilling of boreholes to predrain the gas is an effective measure for preventing such accidents. However, due to the complexity of the geological situation, the drilling trajectory often deviates from the design trajectory, resulting in poor gas extraction. To solve the problem of gas drainage borehole deflection, an analytic hierarchy process (AHP) model is established based on geological factors, technical factors, and human factors. The AHP model is used to rank the weights of various influencing factors, and the analysis is combined with a drilling model and engineering examples. Finally, the results show that soft and hard interlayers are the most important factors affecting the deflection of the borehole. The rock drilling model is mainly affected by the formation forces. The regularity of the change in the azimuth angle during drilling is not obvious when the angle of the encountered layer is less than some critical value. When the borehole is skewed downward, the deflection angle ranges from 0 to 4°, and the deflection of the borehole occurs mainly at the interface of the rock layers. When the angle of the encountered layer is greater than the critical value, the borehole is skewed upward, with a deflection angle of 0–6°, and the deflection occurs at the rock interface. The trajectory curve obtained by theoretical predictions from field data is found to be consistent with that of an actual project.
Highlights
Coal and gas outbursts can result in serious disasters in coal mines [1,2,3,4,5]
[20] analyzed the lateral penetration ability of the drill bit, which is different from its axial penetration ability. e inclination angle and lateral force of the drill bit were determined based on a weighted residual method
Half of the cases are skewed to the left with a deflection angle of 0–2°, and the other half are skewed to the right with a deflection angle of 0–1.7°, with an overall increasing trend
Summary
Coal and gas outbursts can result in serious disasters in coal mines [1,2,3,4,5]. In response to this problem, many scholars have conducted research related to rock mechanics and gas flow [6,7,8,9,10,11,12,13]. Erefore, it is necessary to study the deflection law of gas drilling in soft outburst-prone coal seams [18, 19]. Liu et al [25] established a linear elastic model of borehole collapse failure based on pore fluid seepage and obtained the attenuation law for the soft coal seam collapse pressure with gas seepage; the basic mechanical parameters of the coal seam and fluid seepage and the space of the borehole trajectory were analyzed. E influence of factors such as the azimuth angle of drilling on the borehole collapse provides theoretical support for soft rock formations and the deflection of boreholes in coal seams. E deflection law and main influencing factors of gas treatment boreholes are analyzed, and the borehole deflection trajectory equation is determined. Pumping provides a strong guarantee of preventing the occurrence of coal and gas outbursts and gas overruns and ensures a certain basis for the study and control of borehole deflection in high-gas mines and coal/gas outburst mines
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