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Outbursts comprise the predominant natural hazards in Chinese underground coal mines. Extracting high content gas within coal seams is one of the most effective ways to reduce the risk of triggering outbursts. However, with the depth increase of coal mining production, conventional gas drainage might be failed due to the low permeability of coal seams. Additional permeability enhancement methods should be adopted to improve the gas drainage efficiency. In this study, we proposed the application of liquid carbon dioxide fracturing for enhancing gas drainage in a underground coal mine in China with low permeability coal seams. By drilling cross-seam boreholes, liquid carbon dioxide fracturing was simultaneously performed in the two drilled fracturing boreholes. The effective damage radius of liquid carbon dioxide fracturing was further identified by continuously monitoring pure gas flow rates in different observation boreholes. After liquid carbon dioxide fracturing, there was no significant decrease in pure gas flow rate of observation boreholes for a period of 2 weeks. A conceptual model of liquid carbon dioxide fracturing was proposed for illustrating the mechanisms of improving gas drainage efficiency. Some methods for further optimizing gas drainage with the operation of liquid carbon dioxide fracturing under various thickness of coal seams and geological conditions were discussed. These results suggest that the application of liquid carbon dioxide fracturing are effective for gas control and hazards prevention in underground coal mines. [ABSTRACT FROM AUTHOR]

Although the production mechanism of CO 2 -enhanced coalbed methane (ECBM) recovery has been previously reported and corresponding models have been proposed, investigations on the effect of the injection pressure on the production efficiency considering the mass transfer between the coal fracture and matrix are lacking. In this study, laboratory core flooding experiments were conducted on the coal sample from South Sichuan Basin, China. Experimental results indicate that earlier breakthroughs and a higher initial CH 4 flow rate and cumulative CH 4 flow rate occurred under a higher injection pressure in the CO 2 -ECBM recovery. However, as CO 2 was continuously injected, the CH 4 flow rate and cumulative CH 4 flow rate under a lower injection pressure exceeded those at the higher injection pressure. Furthermore, a dual porosity model was used to perform numerical simulations. The simulation results under four injection pressures matched the experimental results well. The simulation results indicate that the dispersion in fractures and the mass transfer rate between matrices and fractures significantly affect the CH 4 flow rate. The injection pressure leads to changes in the experimental observations by affecting the dispersion and the mass transfer rate. [ABSTRACT FROM AUTHOR]

Because of the low or ultra-low permeability of coal seams, hydraulic fracturing technology has been widely used to improve the permeability of coal seams in both ground and underground extraction. Recently, liquid carbon dioxide (L-CO 2 ) as an environment-friendly fracturing fluid has drawn attention from researchers and engineers. In this study, a true tri-axial system was adopted to perform hydraulic and L-CO 2 fracturing experiments on coal specimens from the South Sichuan Basin in China. The effects of geological and engineering factors (horizontal stress ratio, injection rate, and fracturing fluid) on induced fracture propagation were investigated in this study. The various geological and engineering factors can synthetically, rather than unilaterally, impact the fractures propagation to generate three types of stimulation mechanism. Based on our experimental results, we succeeded in approximately predicting the stimulation mechanism for induced fracturing under different conditions. Generally, the effect of pre-existing natural fractures on the fracture propagation was enhanced, whereas the effect of the geo-stress was reduced, when L-CO 2 was adopted as the fracturing fluid. [ABSTRACT FROM AUTHOR]