Migration Characteristics Of The Gas In Coal Reservoir And The Numerical Simulation Of CO₂-ECBM

Starting from coal pore structure and coal mechanical properties analyses and based on the adsorption diffusion and adsorption permeation experiments, the migration processes, from adsorption to diffusion to permeation, of gas in coal were studied. In addition, the dynamic variation of coal reservoir properties and the production characteristics in the carbon dioxide enhanced coalbed methane (CO₂-ECBM) process were studied by numeric simulation method. The resultses are helpful for further understanding the controlling factors of CBM production and the CBM production process for anthracite reservoir.The results of mercury porosimetry and nitrogen adsorption analysis show that the coal pore structure is dominated by micropores and transition pores, followed by macropores and mesopores. The volume fractal dimension obtained by mercury porosimetry analysis is an effective characterization for macropore-mesopore distribution. The specific surface area fractal dimension obtained on the stage that capillary condensation dominate the adsorption process in nitrogen adsorption analysis can reflect the adsorption capability of coal. Both of the two fractal dimensions are affected by the coal composition. Depending on the relationship of the accommodation space and peat accumulation, the coal facies play an important role on the coal reservoir development.The coal mechanical test results show that the Young's modulus and Poisson's ratio range from 2.25GPa?to?9.75?GPa?and 0.12 to 0.45, respectively for the anthracite in Jinchen area.The maximum adsorption amounts for carbon dioxide is slightly more than that for methane, and the D-A micropore filling model is better than Langmuire model in describing the gas adsorption isotherms for the coal studied. The diffusion of helium in anthracite is controlled by the critical pressure gradient, 2MPa/m. The diffusion coefficient keeps decline when the pressure gradient is higher than 2 MPa/m, and the diffusion coefficient keeps constant or zero when the pressure gradient is lower than 2 MPa/m. The diffusion coefficients of helium, methane and carbon dioxide are all small in anthracite ranging form 2.78×10^-8cm²/s to 2.22×10^-6cm²/s, and the diffusion of helium and methane, carbon dioxide and moisture can be described by single-pore model, dual-pore model and Langmuir type model very well, respectively.Langmuir equation can describe both the relationships of coal volumetric expansion with time and pore pressure. Under the confined conditions, the expansion strain induced by adsorption is much greather than the compaction caused by effective pressure change. After the adsorption expansion the anthracite became stronger. The effect of adsorption expansion and effective pressure on coal permeability has the same mechanism, which is the pores and fractures decrease in size or closed under both of the effects. The prediction of permeability under both of the effects using Shi-Durucan model is consistent with the experiment results.The CO₂-ECBM process of Jinchen anthracite under coal reservoir geological conditions was studied by numerical method with software SIMED Win, and the results show that 200 meters is the appropriate production-injection well spacing. When ECBM by mixed gas of nitrogen and carbon dioxide, the recommendation is that appropriately increase the nitrogen component in the mixed gas to improve the reservoir permeability in the early production stage, and then inject the pure carbon dioxide to mak ECBM and CO₂ sequestration more economical and effective.