无烟煤中甲烷和二氧化碳混合气吸附运移规律

煤中CH₄和CO₂的运移和竞争吸附,是煤中CO₂封存和提高煤层气采收率(CO₂-ECBM)项目注入优化设计、注入驱替效果评价的重要依据。针对沁水盆地南部3个高阶煤样品开展了煤岩煤质、孔隙结构测试(低温液氮和二氧化碳吸附实验)和纯甲烷、二氧化碳及其混合气(体积分数75%CH₄+25%CO₂,50%CH₄+50%CO₂,25%CH₄+75%CO₂)吸附试验。基于混合气吸附相密度计算,分析了煤中混合气吸附的绝对吸附量和气体分离因子随压力的变化规律;同时采用双孔隙扩散模型计算了吸附实验中每个压力段气体的等效扩散系数和大孔扩散占比;最后研究了煤岩煤质和孔隙结构对煤中CH₄,CO₂吸附运移和竞争吸附作用的影响。结果表明:①煤中超微孔比表面积较低温氮比表面积高1~2个数量级,是煤岩吸附的主要场所;②高阶煤气体吸附分离因子(S(CO₂-CH₄))随平衡压力和气体体积分数于3~20变化,一般随压力的升高而下降,与煤中微、小孔发育正相关;③双孔扩散模型可以较好的描述混合气体的扩散过程,总体上受基质膨胀效应影响煤样的大孔等效扩散率随压力升高呈幂函数降低,且在相同压力下,大孔等效扩散率随CO₂体积分数增加而增高。煤样的小孔等效扩散率与气体体积分数和压力关系不明显,大孔扩散占比随压力升高呈“V”型变化。项目成果有助于深入理解CH₄和CO₂的吸附解吸过程。

Adsorption and transport of methane and carbon dioxide mixture in anthracite

Deepening the understanding of the transport and competitive adsorption of methane and carbon dioxide in coal has important significance for the injection optimization design and injection displacement evaluation of the project that CO₂storage in coal and enhanced coalbed methane recovery CO₂-ECBM.The experiments were carried out containing the proximate analysis,pore structure analysis by low temperature liquid nitrogen and CO₂adsorption,and isotherm adsorption of pure methane,carbon dioxide,and their mixture(volume fraction 75%CH₄+25%CO₂,50%CH₄+50%CO₂,25%CH₄+75%CO₂)for three anthracite samples in the southern Qinshui Basin.The variation of absolute adsorption amount and gas separation factor of mixture adsorption in coal with pressure is analyzed based on the calculation of mixture adsorption phase density.Meanwhile,the equivalent diffusion coefficient and the proportion of macropore diffusion of gas in each pressure section in the adsorption experiment were calculated by using the bidisperes diffusion model.Finally,it was studied that the effects of coal composition and pore structure on gas coal migration and competitive adsorption in coal.The results show that①The specific surface area of super-micropores in coal is 1-2 orders of magnitude higher than that of micropores,which is the main place for gas adsorption.②The adsorption separation factor(S(CO₂-CH₄))of anthracite varies from 3 to 20 with equilibrium pressure and equilibrium molar volume fraction of gas components.Generally,S(CO₂-CH₄)decreases with the increase of pressure,and is positively related to the development of micropores in coal.③The bidisperes diffusion model can better describe the diffusion process of the mixed gas.In general,the macropore equivalent diffusivity of coal samples varies as a power function with pressure.Under the same pressure,the macropores equivalent diffusivity increases with the increase of the carbon dioxide content.The micropore equivalent diffusivity have no significant relationship with the gas volume fraction and pressure.And the proportion of macropore uptake changes in a“V”shape with the increase of pressure.The results will help to deeply understand the sorption processes of the methane and carbon dioxide mixture.