Compressibility Phenomenon in Coalbed Methane Reservoirs: Mechanisms, Experimental Methods, and Modeling Approaches for Green Exploitation of Deep Resources

Coalbed methane (CBM) reservoirs exhibit complex deformation behaviors during gas production, and reservoir rock compressibility plays a critical role in controlling permeability evolution and production performance. With the increasing development of deep CBM resources, a comprehensive understanding of different compressibility behaviors under complex geological conditions is essential for accurate reservoir evaluation and sustainable gas production. This study systematically reviews the recent advances in the characterization of the compressibility phenomena in CBM reservoirs, including bulk compressibility, fracture compressibility, cleat compressibility, pore-volume compressibility, matrix compressibility, and matrix shrinkage effect. The mechanisms, experimental characterization methods, and theoretical models associated with each type of compressibility are discussed and compared. Existing studies indicate that fracture and cleat closure can substantially reduce permeability, while pore-volume compression varies with coal rank, pore–fracture structure, and loading conditions. Meanwhile, gas desorption-induced matrix shrinkage can partially counteract stress-induced fracture closure and contribute to permeability recovery during the later stage of CBM production. Various experimental techniques, including triaxial mechanical testing, mercury intrusion porosimetry, nuclear magnetic resonance, X-ray computed tomography (X-ray CT), and in situ monitoring methods, have been widely applied to characterize reservoir compressibility and the resultant pore–fracture space evolution. Classical models have provided fundamental insights into the relationship between compressibility and permeability evolution. The research findings reveal that, despite significant progress, current studies still mainly focus on individual compressibility behaviors, while the coupled evolution mechanisms among different types of compressibility remain insufficiently understood. In addition, the effects of deep-reservoir conditions, including high temperature and high stress on compressibility evolution require further investigation. Future research should also focus on the multiscale and multiphysics characterization of coupled compressibility behaviors, the development of dynamic compressibility evaluation methods, and the integration of advanced imaging techniques and data-driven approaches to improve permeability prediction for CBM reservoirs.