Multiscale Digital Core Characterization Of Coal Reservoirs In Mabidong Block

China has abundant coalbed methane?CBM?resource,its exploitation and utilization are of great significance for improving energy structure,alleviating environmental pollution,and reducing gas explosion accidents.In this dissertation,multiple experiments including low-temperature N₂ adsorption/desorption,dual-resolution X-ray CT,nuclear magnetic resonance?NMR?,and numerical simulation were carried out on coal samples collected from No.3 coal seam,Mabidong block,Qinshui basin.In addition,quantitative characterization was carried out and fluid microscopic gas transport model were established.The results of low-temperature N₂ adsorption/desorption and non-local density functional theory reveal that the pore structure of coal sample from Well Ma-13 is dominated by micropores,and the pores of coal sample from wells Ma-13 and Ma-62are mostly open cylindrical or flat holes and parallel plate holes or sharp split holes closed at one end respectively.The pore structure of coal at different location of the same well shows a strong heterogeneity.Meanwhile,pores with diameter of 10¹00nm and 2¹0nm contribute most of the pore volume and specific surface area,respectively.Based on dual-resolution X-ray CT scanning experiments,the fracture system in multi-scale and connectivity characterization model of No.3 coal seam in the study area were established.At the same time,the development characteristics,spatial morphology,and distribution of pores at multi-scale were also explored in detail.The result of X-ray CT scanning experiments show No.3 coal seam has pores ranging from micron to millimeter.Furthermore,the overall distribution of pores is relatively scattered,and it is easy to form continuous fractures in local areas and the fractures in coal are filled with minerals on different scales.Moreover,the degree of axial distribution is extremely heterogeneous.NMR and dual-resolution X-ray CT scanning experiment were comprehensive used to establish multi-scale pore size distribution ranging from nanoscale to micrometer scale.The results show that the development of micropores and macropores in No.3 coal seam is conducive to the adsorption and transmission of CBM,but lacks of mesopores that communicate the adsorption and seepage spaces.Based on dust gas model?DGM?and generalized Maxwell-Stefan?GMS?model,the microscopic gas transport model was established and the contribution of different transport mechanism was also compared.The results reveal that viscous flow can be negligible under small pore size and low-pressure conditions while it dominates the gas transport under large pore size and high-pressure conditions.Knudsen diffusion and surface diffusion can be ignored under the same conditions.The sensitivity analysis shows that temperature,Langmuir constant,surface roughness,and tortuosity have a great influence on CBM transport.