Evaluation of Superiority of Liquid CO₂ as a Non-Aqueous Fracturing Fluid for Coal Seam Gas Extraction

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.