Experimental investigation on coal pore-fracture variation and fractal characteristics synergistically affected by solvents for improving clean gas extraction

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 CS₂). 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 CS₂ 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, CS₂ 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 D_s 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.