Gas Injection And Displacing Coalbed Methane Mechanism And Numerical Simulation In Deep Coal Seam

This paper studies 3#Coal of Shanxi Formation in the Shizhuang North Block of southern Qinshui basin, through data research, field exploration, systematic sampling, experiments analysis and other work, analyses the coal, porosity, permeability and isothermal adsorption and desorption experiments, makes stratigraphic interval partition segment, sequence and sedimentary facies comparison, and through software simulation to instruct CO₂ and mixed gas injection seam to enhance coalbed methane recovery.As used herein, the coal samples collects 3#Coal of Shanxi Group in Changping Coal Mine（CP）, Zhaozhuang Mine（ZZ） and Shizhuang Mine（SZ）. Coal samples are native structure of coal, industrial coal sample analysis found that the coal samples are low ash coal, fixed carbon content is more than 75%. Tests find the type of organic matter is Ⅲ kerogen, have a strong ability to produce gas, vitrinite content of the sample exceeded 65%, provide a naural place for gas recovery and CO₂ sequestration. Mercury-injection experiments find that CP, SZ coal porosity, permeability and connectivity is better than ZZ coal. Seam thickness is 33-72 m, generally around 46 m. The lower part of Shanxi Formation mainly consists of fine sand, muddy siltstone, shale and coal seam components. The upper part of the formation consists of mudstone, sandy mudstone, gray sandstone and mudstone, siltstone and shale interbedded coal line. Sedimentary facies is lower delta plain facies of delta sedimentary system, sedimentary microfacies are mainly distributary bay, distributary channel and peat bog, vertical distributary channel, distributary bay interaction occurs.Through the three coal samples single component adsorption-desorption analysis, adsorption capacity of coals is CO₂>CH4>N₂, and obtaines different gas langmuir volume and langmuir pressure. Through binary mixed gas（75%CO₂+25%CH4, 50%CO₂+50%CH4, 25%CO₂+75%CH4） isothermal adsorption and desorption experiments and gas chromatography experiment at the point of equilibrium, the results show that at the first adsorption pressure point, adsorption capacity of the CO₂ is high, in free gas the CO₂ reducese and achieves saturation, absorption slows down, as the pressure increased, the CO₂ gradually increased and stabilized; and CH4 ratio in the free gas increases, then decreases with the pressure increases and remains stable. In the desorption process, with the decrease of desorption pressure, CH4 is the overall increase- decrease process, CO₂ is down- up process.Use Simed Win software for ECBM numerical simulation before the injection, by different mesh distribution, wells, wells group, different proportions of mixed gas simulation find that the gas injection displacement can improve CBM recovery. Different mesh distribution group settings and wells find that double-well simulation is more effective than single-well stimulation, but gas production cycle is longer. With the increasing distance between wells, gas production decreased, but this doesn’t mean that the smaller the gas production well spacing is, the better. N₂ displacing is more effective than CO₂, but from the economic and environmental benefits consideration, 75%CO₂+25%N₂ is the best choice for ECBM.