Abstract 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.

中文翻译：

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低渗非均质煤液氮低温冷冻压裂渗透率演化及细观裂化行为

摘要 提高渗透率的压裂技术对于提高煤层气采收率必不可少，可显着减少温室气体排放，生产大量清洁能源。为了获得有效的压裂，二叠纪低渗透煤储层的样品通过液氮低温冷冻压裂 (FF) 将煤的潜在非均质性最大化，将其裂解成不同大小或尺度的块体和颗粒。为了研究裂缝效果，本研究系统地研究了低温 FF 背景下不同含水量和割理-裂缝系统的煤的渗透率演变和细观裂缝行为。结果表明，随着煤样含水量的增加，渗透率增强和微裂纹趋于发生；低温FF对较致密煤样效果更显着。然而，渗透率并不严格随着低温FF的循环而增加，而是与含水量、结构面方向、低温FF的功效和孔隙压实有密切关系。细观开裂行为表明，在这个过程中，许多较小的孔隙是迭代的，而不是严格的顺序，破裂并变得连通。煤样的渗透率演化被确定为与细观开裂行为显着相关。值得注意的是，低温 FF 的第一个循环作用于检测到的几个尺度的孔隙和微裂缝，并部分导致这些结构被打开并相互连接以具有渗透性。本研究中煤样的微裂纹主要有两个变化：1）不均匀收缩变形和微裂隙扩展；2) 孔隙主要在大孔和中孔尺度上开放。认为由于孔隙或微裂隙中自由水的相变导致的热裂解和渗流孔的间歇打开最终有助于低渗透和非均质煤的渗透率提高。