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Fracking to approach permeability enhancement is indispensable to enhance coalbed methane recovery, which can significantly reduce greenhouse gas emissions and produce substantial clean energy. To access efficient fracking, samples of Permian low permeable coal reservoir were cracked into different sizes or scales of blocks and particles with liquid nitrogen cryogenic freeze fracturing (FF) by maximizing the underlying heterogeneity of coal. To investigate the crack efficacy, this study systematically examined the permeability evolution and mesoscopic cracking behaviors of coal with different water contents and cleat–fracture systems in the context of cryogenic FF. Results showed that the permeability enhancement and microcracking tended to occur with increasing water content of coal sample; and the efficacy of cryogenic FF on the tighter coal sample was more remarkable. However, the permeability does not strictly increase with the cycles of cryogenic FF and has a close relationship with water content, structural plane direction, efficacy of cryogenic FF, and porosity compaction. The mesoscopic cracking behaviors indicate that numerous smaller pores are iteratively, not strictly sequentially, cracked and become connective in this process. The permeability evolution of coal sample is identified as significantly associated with the mesoscopic cracking behaviors. Notably, the first cycle of cryogenic FF acted on the detected several scales of pores and micro fissures, and partially caused these structures to be opened and interconnected to be permeable. Two main changes were observed in the microcracking of the coal samples in this study: 1) nonuniform shrinkage deformation and micro fissure expansion; and 2) pores opening mostly in the macropore and mesopore scale. It is reasoned that thermal cracking and intermittent opening of seepage pores due to the phase transition of free water in pores or micro fissures ultimately contribute to the permeability enhancement in low permeable and heterogeneous coal. [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]