Experimental And Simulation Study On The Sorption, Diffusion And Seepage Characters In Different-ranked Coals

Three typical different-ranked Chinese coal samples, JCC-1, CZ-1and TCG-1takenfrom Jinchen area,Changzhi area of Qinshui Basin and Junggar Basin of Xinjiang area,respectively, were used to study the adsorption, diffusivity and permeability and theirchanging laws. Moreover, the dynamic permeability change and the prediction ofpermeability during ECBM process were also studied.In this study, four types of gases, H_e,N₂,CH₄and CO₂, were used to do theexperiment. For each type of gas, the adsorption amount, diffusivity and permeability weremeasured under the condition of three or four different pore pressure steps(Generally,1MPa,2.5MPa,4MPa and6MPa)and five different effective stres(s1MPa,2MPa,3MPa,4MPa and5MPa)and three different temperatures(35℃,40℃and45℃). The adsorptionamount was measured and the adsorption isothermals were obtained. The diffusivity andthe changing curves with time were drawn; and the permeability was tested and its relativecurves were also plotted in this study. Meanwhile, by means of swelling calculation and theexperiment of mercury injection and low temperature liquid nitrogen adsorption experiment,the analysis about the experiment phenomenon and its relationship with coal rank and porestructure distribution was performed.Through studying the adsorption characteristics, the following conclusions can bemade: for the same coal sample, the adsorptive capacity for different gas is different.Meanwhile, for the same gas, the adsorptive capacity for different ranked coal is alsodifferent. The capacity increases with the increasing of coal rank from low to high.Studying on the diffusivity and using experiment of mercury injection experiment and lowtemperature liquid nitrogen experiment, it is found that micropore makes up the majority ofthe pore space for the high rank coal. While for the middle-ranked coal, mesopore is themajority; for the low-ranked coal, mesopore is still the majority but macropore is also animportant part of the structure. According to the modeling result, bidisperse model is bestdiscripbing diffusion for all the three different ranked Chinese coal.Studying on the permeability for the three coals found that effective stress and porepressure both have significant effect on permeability. Moreover, the gas type also hasimportant effect on permeability. Permeability and cleat compressibility have very closerelationship. Cleat compressibility is related to effective stress, pore pressure and gas type.Combined with result of swelling calculation and depicting the dynamic permeability prediction curve, it is found that during the ECBM process, permeability presents certainrules with the changing of CO₂molar fraction: it first declines dramatically, then declinesslowly, and then keeps a certain value, finally rises slowly. The dynamic permeabilityprediction curves have significant guidance to the coalbed methane production duringECBM process.