Evolution of Coal Reservoir Properties with Burial Depth: Insights from Multiscale Pore Structure Characterization

The evolution of the coal seam pore-fracture system and mechanical properties with burial depth directly impacts the accurate assessment of coalbed methane content and the stimulation and development of reservoir permeability. This study selected coal samples from six mining areas with varying burial depths in the southern Junggar Basin. A systematic analysis was conducted on the vertical evolution of pore structure and mechanical properties in the coal reservoirs using low-pressure CO2 adsorption, low-temperature N2 adsorption, high-pressure mercury intrusion porosimetry, and determination of porosity and permeability of rock under overburden pressure. This study reveals a “U-shaped” relationship between total pore volume and burial depth across the investigated area, with a transition occurring at 800 m. Specifically, the inflection points for micropores and mesopores are near 1600 m, and for macropores near 700 m. Below 1100 m, the storage space for free gas increases. The pore compressibility coefficient remains relatively stable above 800 m, then decreases rapidly beyond this depth. The stress sensitivity coefficient shows an initial rapid decrease followed by stabilization. These findings demonstrate that shallow samples are not suitable substitutes for deep samples in predicting the evolution of deep coal reservoir permeability. Correcting pore-fracture volumes in deep coal reservoirs using a constant compressibility coefficient underestimates the actual space; a variable compressibility coefficient should be applied instead. This study provides a theoretical underpinning for the accurate evaluation and efficient development of deep coalbed methane reservoirs.