According to the studies on tight gas reservoirs, liquid CO2 (L- CO2) is much superior in hydraulic fracturing compared to conventional fracturing fluids. However, the applicability of this technique for hydraulic fracturing of coal seams has been hindered due to the lack of understanding. This paper investigates the superiority of L-CO2 as a fracturing fluid for coal seam gas extraction, in terms of break-down pressure and acoustic emission (AE) energy release during fracture propagation. The results reveal that L-CO2 induced break-down pressure is around 19.6% lesser than the water induced break-down pressure, whereas the time taken to break-down is 59.3% higher for L-CO2 compared to water. Importantly, due to the low compressibility, water injection pressure showed an exponential pressure development closer to the break-down, resulting a higher break-down pressure within a short time period. Conversely, the highly compressible L-CO2 exerted only a gradual increment in pressure in the sample over a considerable time, which caused a more controllable fracturing process. The observed AE suggests that the ability of low viscous L-CO2 to penetrate through the tiny pores in the coal mass has the potential to create a stable and dense fracture network, instead of the uncontrolled unstable sudden failure occurred under the water based fracturing. This well-developed fracture network can significantly enhance the rock mass permeability and the reservoir gas extraction. Overall, it can be concluded that the combined characteristics of L-CO2 (high compressibility, low viscosity) lead more controllable and effective coal seam hydraulic fracturing process compared with the water based fracturing.

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