Nitrogen treatment is a widely used coalbed methane (CBM) recovery technique for low permeability CBM reservoirs. We conducted a series of experiments to investigate the nitrogen injection and soaking (NI-n-S) induced coal structure alteration and, thus, the permeability variations. Two sets of experimental variations were investigated, which are the treatment pressure and the treatment duration. The experiments were carried out with nitrogen pressures of 6, 9, 12, and 15 MPa for 2, 12, 24, and 36 h, respectively. The core-scale permeability was employed as an indication of the gas flow enhancement capability and effectiveness. Both X-ray CT and field emission scanning electron microscopy (FESEM) were used for the imaging analyses for cross-comparison before and after NI-n-S treatments. The permeability of the coal samples after NI-n-S has shown a significant increase by 13.6–524.1%, with an average of 98.5%. The permeability increase ratio positively correlates with nitrogen treatment pressure; high-pressure nitrogen produced six types of damage modes on microfractures. The main mode is improving the microfracture opening width, and it increased by 4.4–166.7% (mean 45.1%) of the coal specimen with NI-n-S.

中文翻译：

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高压注氮浸渗无烟煤增透实验研究

氮气处理是一种广泛应用于低渗透煤层气储层的煤层气（CBM）采收技术。我们进行了一系列实验来研究注氮浸泡 (NI-nS) 引起的煤结构改变以及渗透率变化。研究了两组实验变量，即治疗压力和治疗持续时间。实验分别在6、9、12和15 MPa的氮气压力下进行2、12、24和36 h。采用岩心尺度渗透率作为气体流动增强能力和有效性的指标。 X 射线 CT 和场发射扫描电子显微镜 (FESEM) 均用于成像分析，以进行 NI-nS 治疗前后的交叉比较。经过NI-nS处理后的煤样渗透率显着提高了13.6%~524.1%，平均提高了98.5%。渗透率增加率与氮处理压力正相关；高压氮气对微裂纹产生六种损伤模式。主要方式是提高微裂缝开口宽度，与NI-nS煤样相比，微裂缝开口宽度增加了4.4%~166.7%（平均45.1%）。