Integrated Multifractal Framework for Characterizing Full-Scale Pore Architecture and Reservoir-Controlling...

Understanding pore heterogeneity and its control on gas occurrence and migration in deep coal seams remains a critical challenge in coalbed methane (CBM) exploration. In this study, an integrated multifractal framework is proposed to characterize the full-scale pore architecture and quantify reservoir-controlling mechanisms of deep low-rank coals from the Turpan-Hami Basin. Comprehensive pore-structure characterization was conducted using high-pressure mercury intrusion, low-temperature N₂ adsorption, and CO₂ adsorption. Three coupled multifractal modelsthe pore-size distribution-fractal spectrum correlation model, the pore volume-fractal dimension coupling model, and the function-structure synergistic fractal modelwere constructed to resolve multiscale heterogeneity, dominant pore types, and their effects on adsorption-seepage functionality. Results reveal a distinct "micropore-macropore dual-dominant" system, where micropores (>85% of surface area; 34.7-64.8% of pore volume across all samples) act as major adsorption sites, macropores (26.6-60.9% of pore volume) control seepage capacity, and mesopores (<5% of pore volume) function as transitional pathways. Multifractal analysis demonstrates scale-dependent heterogeneity and connectivity: macropores exhibit the widest singularity spectrum (Δα = 0.821-2.672) but poor connectivity (H = 0.638-0.904), whereas micropores show more uniform distribution (Δα = 0.935-2.695, H = 0.878-0.955). The integrated fractal dimension (D _a) indicates that vitrinite-inertinite interactions produce high-complexity "micropore-macropore composite" systems with dual adsorption-seepage potential. Synergistic fractal dimensions (D _syn,i) effectively differentiate reservoir functional domains, where D _syn,3 > 0.8 represents superior adsorption zones and D _syn,1 > 0.6 marks high-efficiency seepage regions. This multifractal framework provides a strengthened theoretical and quantitative basis for evaluating deep coal reservoir quality and optimizing CBM production strategies.