The effect of coalification jumps on petrophysical properties, and the evolution of porosity and permeability of metamorphic coals are poorly understood, which significantly influences coalbed methane extraction. We estimated pore throat diameter, specific surface area, connectivity, moveable fluid space, heterogeneity, porosity and permeability in a series of 41 coal samples (maximum reflectance of vitrinite in 0.34–4.24%) over six coalification jumps, by processing low temperature nitrogen adsorption, mercury intrusion porosimetry and nuclear magnetic resonance (NMR) measurements. Each coalification jump generally leads to abrupt change of petrophysical properties from dehydration to graphitization. Connectivity parameters (efficiency mercury withdrawal and the ratio of movable fluid to bounded fluid) and fractal dimensions (DNA1, DNA2, DMIP, DNMRS and DNMRM) present binomial function with vitrinite reflectance. Generations of thermogenic gas and fractures growth are attributed to increasing pore-fracture connectivity in bituminization and debituminization. Fractures begin healing and compaction, as well as reduced connectivity, at the fourth jump in graphitization. Heterogeneous pore-structures (high DMIP and DNMRM) usually have low connectivity. Moveable fluid space and its porosity from NMR are negatively correlated with the increase in coal rank. The evolution and origin of porosity and permeability (<1 mD) during coalification is proposed. Coal permeability (>1 mD) has no relation with coal rank and is related to fracture characteristics. Unlike the origin of porosity, which intrinsically inherits from progressive coalification, the origin of permeability is attributed to both progressive coalification and tectonic stresses. This study reveals the complex pore-fracture structures variation and the effect of stages in coal maturation on petrophysical properties of coalbed methane reservoirs.
All Science Journal Classification (ASJC) codes
- Energy Engineering and Power Technology