Abstract Almost all the coal seams in China are characterized by high gas content, extremely low permeability, complicated geological structure, thus, natural disasters such as gas explosive and outburst easily occur. At present, gas extraction is an effective approach for preventing such disasters. In this paper, in order to improve gas extraction efficiency, the Multi-discharge CO2 fracturing system (Multi CO2 Frac) was proposed and tested in Changping coal mine. Through analysis of the waveforms from the explosions of 0.5 and 1 kg TNT dynamites and 1 kg liquid CO2, it can be obtained that the TNT equivalence of 1 kg CO2 is about 400–430 g, that is, the explosion of 1 kg liquid CO2 has same fracturing ability as 400–430 g TNT dynamites. Besides, field test shows that the effective drainage radius caused by Multi-CO2-Frac technique is about 12.5 m, regardless of the number CO2 discharging sets. However, the gas extraction concentration increases with the number of CO2 discharges. This phenomenon indicates that more cracks will be created along the axial direction in coal seam drilling boreholes by Multi CO2 Frac techniques. By comparing gas concentration before and after fracturing, it can be concluded that the Multi CO2 Frac techniques can change and maintain the uptrend of gas concentration at a higher level in a long period of time. Therefore, in order to extend the duration of high efficiency gas extraction, Multi CO2 Frac techniques can be used periodically at different position of coal seam. Meanwhile, after gas extraction, the maximum gas emission of coal seam was reduced from 5.5 m3/min to 3.48 m3/min and the driving footage was enhanced from 2.4 m/d to 5.7 m/d. In summary, Multi CO2 Frac techniques can effectively improve the coal seam permeability and further enhance the gas drainage efficiency, which can meet the requirement of gas extraction in high gas content thick and soft coal seam. Therefore, this technique has a promising application prospect because of its advantages of safety, environmental protection and economy.

中文翻译：

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高瓦斯厚煤层增产环保技术─多排CO2压裂系统

摘要 我国几乎所有煤层都具有含气量高、渗透率极低、地质构造复杂等特点，易发生瓦斯爆炸、突出等自然灾害。目前，天然气开采是预防此类灾害的有效途径。本文为提高瓦斯抽采效率，在昌平煤矿提出并试验了多排二氧化碳压裂系统（Multi CO2 Frac）。通过对0.5、1kg TNT炸药和1kg液态CO2爆炸波形的分析，可以得出1kg CO2的TNT当量约为400-430g，即1kg液态CO2的爆炸具有与 400-430 克 TNT 炸药相同的压裂能力。此外，现场试验表明，Multi-CO2-Frac技术产生的有效排水半径约为12。5 m，不考虑CO2 排放套数。但是，抽气浓度随着CO2 排放次数的增加而增加。这种现象表明，采用多二氧化碳压裂技术在煤层钻孔中沿轴向会产生更多的裂缝。通过对比压裂前后瓦斯浓度，可以得出结论：Multi CO2 Frac技术可以在较长时间内改变并保持瓦斯浓度在较高水平上的上升趋势。因此，为了延长高效瓦斯抽采的持续时间，可以在煤层的不同位置周期性地使用Multi CO2 Frac技术。同时，采气后煤层最大瓦斯排放量由5.5 m3/min降低至3.48 m3/min，掘进进尺由2.4 m/d提高至5.7 m/d。总之，多CO2压裂技术可有效提高煤层渗透率，进一步提高瓦斯抽采效率，满足高瓦斯厚软煤层瓦斯抽采的要求。因此，该技术具有安全、环保、经济等优点，具有广阔的应用前景。