Coalbed Methane Adsorption And Transport Behaviors In High Rank Coal: A Molecular Simulation Study

Coalbed methane(CBM)reservoir is a typical kind of heterogeneous organic rock.The development of pore structure has a key role in controlling the gas storage and migration.At present,the quantitative analysis experiments of pore structure as well as the internal fluid migration have been rapidly developed at home and abroad.However,affected by the magmatic hydrothermal fluids,the high-rank CBM reservoirs in Qinshui Basin are characterized by the high thermal evolution and low proportions of micron-pores and fractures.A systematic multi-scale analysis focused on the nano adsorption pores,which are the dominant part of the pore structure,is extremely required.In this paper,the advanced experiment of nuclear magnetic resonance cryoporometry(NMRC)was constructed based on 14 high-rank coal mines in Qinshui Basin for the quantitative characterization of nano-pores and related evolution.Furthermore,conventional characterization tests including field emission-scanning electron microscope(FE-SEM),focused ion beam-scanning electron microscope(FIB-SEM),CO₂/N₂ adsorption/desorption and nuclear magnetic resonance(NMR)experiments were also combined.On the other hand,the molecular simulation techniques were applied to expound the control mechanism of the heterogeneous structure on CBM adsorption and flow behaviors.The high-rank CBM reservoirs in the northern blocks of Qinshui Basin were taken as the main research object.Based on the characterization tests corresponding to different scales,the structure and geological genesis of nano-pore system were analyzed in two and three dimensions.The study was investigated from two aspects of visual observation and quantitative physical analysis,respectively.The evolution of pore structure during the coalification jump from semi-anthracite to anthracite were detailly studied,as well as the genetic control of the magmatic hydrothermal and plutonic metamorphisms.Furthermore,the study revealed the correlation between the heterogeneity degree and the type of pore structure by fractal analysis.The controlling mechanisms of heterogeneity on CBM adsorption were also discussed.Meanwhile,the advantages of NMRC experiment in the detecting precision of adsorption pores compared with the conventional tests were summarized through comparison and verification.In order to make up for the limitations of laboratory testing conditions,grand canonical Monte Carlo(GCMC)and molecular dynamics(MD)simulations were performed to systematically study the adsorption and transport mechanisms of CH₄ molecules under reservoir condition.A novel strategy,named the diamond nanocluster filling method,was proposed for the construction of closed pores of anthracite.The evolution of CBM adsorption mode from homogeneous filling to monolayer and multilayer adsorptions was simulated.The corresponding coupling effects of temperature,pressure and chemical fields was concluded,which could provide a theoretical basis for the enrichment and reserves prediction of CBM.A combined matrix and nano-fracture system based on the traditional double-porosity model was constructed.We aimed at providing a molecular-level insight into the gas adsorption and diffusion behaviors in the open systems through simulated study.The interference among different stages was investigated.And the controls of structure parameters and environmental variables on methane transport were detailly analyzed.To sum up,the molecular simulation study could effectively provide a theoretical support for an efficient exploitation of CBM.