The No. 3 coal seam in the Yuxi Coal Mine has a measured maximum gas content of 25.59 m3/t, along with a maximum gas pressure of 2.9 MPa, indicating its high risk to gas and outbursts. To mitigate outburst risks of the coal seam, the 1301 working face has been implemented gas pre-drainage measures by grid boreholes from underlying roadways. After one year of extraction, it was confirmed that the gas content at all 33 test sites was below 8 m3/t, meeting the outburst prevention standards. However, during subsequent coal tunnel excavation, the gas desorption index K1 value frequently exceeded the standard, resulting in numerous occurrences of abnormal gas emission or small-scale outbursts. To tackle the challenges associated with safe excavation following the first round regional outburst prevention measures, a research and industrial trial of CO2 gas fracturing(CO2-Frac) technology for secondary outburst prevention and rapid excavation was completed. Results show that the dual-hole and high-pressure (185MPa) CO2-Frac considerably contribute to outburst prevention. K1 exceedances per hundred meters of tunnel excavations were from average 2.54 without CO2-Frac to average 0.28 after the new technology being implemented, leading to a eight fold reduction. Additionally, the monthly excavation footage increased from average 81.64 m without CO2-Frac to average 162.42 m with CO2-Frac, resulting in a 2-fold improvement. The dual-hole and high-pressure CO2-Frac is an advanced technology for safe and efficient excavation for secondary outburst elimination in highly outburst-prone coal seams in the Yuxi Coal Mine, with potential for widespread application in similar coal seam conditions. 

Study on Safety Tunneling Technology of Secondary Outburst Elimination by CO2 Gas Fracturing in High Outburst Coal Seam

The No. 3 coal seam in the Yuxi Coal Mine has a measured maximum gas content of 25.59 m3/t, along with a maximum gas pressure of 2.9 MPa, indicating its high risk to gas and outbursts. To mitigate outburst risks of the coal seam, the 1301 working face has been implemented with gas pre-drainage measures by grid boreholes from underlying roadways. After one year of extraction, it was confirmed that the gas content at all 33 test sites was below 8 m3/t, meeting the outburst prevention standards. However, during subsequent coal tunnel excavation, the gas desorption index K1 value frequently exceeded the standard, resulting in numerous occurrences of abnormal gas emission or small-scale outbursts. To tackle the challenges associated with safe excavation following the first-round regional outburst prevention measures, a research and industrial trial of CO2 gas fracturing (CO2-Frac) technology for secondary outburst prevention and rapid excavation was completed. The results show that the dual-hole and high-pressure (185 MPa) CO2-Frac considerably contributes to outburst prevention. K1 exceedances per hundred meters of tunnel excavations were from an average of 2.54 without CO2-Frac to an average of 0.28 after the new technology was implemented, leading to an eight-fold reduction. Additionally, the monthly excavation footage increased from an average of 81.64 m without CO2-Frac to an average of 162.42 m with CO2-Frac, resulting in a two-fold improvement. The dual-hole and high-pressure CO2-Frac is an advanced technology for safe and efficient excavation for secondary outburst elimination in highly outburst-prone coal seams in the Yuxi Coal Mine, with potential for widespread application in similar coal seam conditions. 

Study on Safety Tunneling Technology of Secondary Outburst Elimination by CO2 Gas Fracturing in High-Outburst Coal Seam

Crossing borehole is an effective means to eliminate the outburst risk of coal and gas. The influence of borehole inclination angle, borehole diameter, and drainage time on the effective radius of gas drainage are studied by numerical simulation and engineering example verification. The study shows that the effective radius changes in a “U” shape with the increase in borehole inclination angle. When the angle α of the borehole and coal seam plane decreases, the effective radius increases. Furthermore, the coal mass around the borehole is broken by shear deformation, which is consistent with the results of the inner peephole. The effective radiuses are different in coal seam dip X and strike Y. When α is small, the effective radius in the X direction is generally wider. When α is large (α ≤ 90°), the effective radiuses in the X and Y directions are close. The effective radius is positively correlated with the borehole diameter and is more significantly influenced by increasing borehole diameter when α is small. The effective radius increases as a negative exponential function with time and eventually converges to a constant. The study has practical implications for the design of crossing borehole in the coal seam floor.

/pdf/numerical-investigation-on-potential-influencing-factors-1mtw4km3.pdf

Numerical Investigation on Potential Influencing Factors Affecting Drainage Effective Radius of Crossing Borehole

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Numerical Investigation on Potential Influencing Factors Affecting Drainage Effective Radius of Crossing Borehole