Environmentally friendly techniques for high gas content thick coal seam stimulation─multi-discharge CO2 fracturing system

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 (MultiCO2Frac) 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 MultiCO2Frac techniques. By comparing gas concentration before and after fracturing, it can be concluded that the MultiCO2Frac 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, MultiCO2Frac 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, MultiCO2Frac 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.