Chemical solvents instead of pure water being as hydraulic fracturing fluid could effectively increase permeability and improve clean methane extraction efficiency. However, pore-fracture variation features of lean coal synergistically affected by solvents have not been fully understood. Ultrasonic testing, nuclear magnetic resonance analysis, liquid phase mass spectrometry was adopted to comprehensively analyze pore-fracture change characteristics of lean coal treated by combined solvent (NMP and CS). Meanwhile, quantitative characterization of above changing properties was conducted using geometric fractal theory. Relationship model between permeability, fractal dimension and porosity were established. Results indicate that the end face fractures of coal are well developed after CS and combined solvent treatments, of which, end face box-counting fractal dimensions range from 1.1227 to 1.4767. Maximum decreases in ultrasonic longitudinal wave velocity of coal affected by NMP, CS and combined solvent are 2.700%, 20.521%, 22.454%, respectively. Solvent treatments could lead to increasing amount of both mesopores and macropores. Decrease ratio of fractal dimension is 0.259%–2.159%, while permeability increases ratio of NMR ranges from 0.1904 to 6.4486. Meanwhile, combined solvent could dissolve coal polar and non-polar small molecules and expand flow space. Results could provide reference for solvent selection and parameter optimization of permeability-enhancement technology.

中文翻译：

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溶剂协同影响煤孔隙-裂缝变化及分形特征以提高清洁瓦斯抽采效果的实验研究


采用化学溶剂代替纯水作为水力压裂液，可以有效提高渗透率，提高甲烷清洁开采效率。然而，溶剂协同影响贫煤孔隙-裂缝变化特征尚未完全了解。采用超声检测、核磁共振分析、液相质谱分析等综合分析了联合溶剂（NMP和CS）处理贫煤的孔隙—裂隙变化特征。同时，利用几何分形理论对上述变化特性进行了定量表征。建立了渗透率、分形维数和孔隙度之间的关系模型。结果表明，经过CS和联合溶剂处理后，煤端面裂隙发育良好，端面盒数分形维数为1.1227~1.4767。 NMP、CS和组合溶剂对煤的超声纵波波速最大降低分别为2.700%、20.521%、22.454%。溶剂处理可能导致中孔和大孔数量增加。分形维数下降率为0.259%～2.159%，核磁共振渗透率增加率为0.1904～6.4486。同时，组合溶剂可以溶解煤极性和非极性小分子，扩大流动空间。研究结果可为增透技术的溶剂选择和参数优化提供参考。