基于低场核磁共振法的甲烷扩散特征研究

Study on Methane Diffusion Features Based on Low-field Nuclear Magnetic Resonance(LF-NMR)Method

以准噶尔盆地与沁水盆地的低、高煤级样品为研究对象,以低场核磁共振法(NMR)等温吸附-扩散实验为主,辅以体积法扩散实验,研究了不同气体扩散模型下甲烷吸附-扩散特性;通过开展场发射扫描电镜、低温氮吸/脱附实验以及压汞实验对样品进行多尺度微纳米孔结构表征,分析了不同孔隙结构对煤层气扩散的贡献。结果表明:甲烷在煤储层多尺度孔隙中表现出初期快速扩散、中期的过渡性扩散以及后期慢速扩散的特征,且甲烷多孔扩散模型适用性高于双孔扩散模型。煤储层中微孔、过渡孔和大孔的比表面占比与其对应的扩散系数比例有很好的相关性。低场核磁共振法用于计算甲烷扩散系数的适用性较好,但其扩散结果略低于体积法。

Taking low,high ranked coal samples from the Junggar and Qinshui basins as subjects investigated,using mainly LF-NMR and isothermal adsorption-diffusion experiment,supplemented by volumetric method diffusion experiment have studied methane adsorption-diffusion features under different gas diffusion models.Through FE-SEM,low-temperature nitrogen adsorption/desorption and mercury intrusion experiments have carried out sample multiscale micropore,nanopore structural characterization,analyzed contribution of different pore structures on CBM diffusion.The results have shown that methane in coal reservoir multiscale pores shows initial stage fast diffusion,medium stage transitional diffusion and later stage slow diffusion features,and adaptability of methane multi-pore diffusion model is higher than diplo-pore diffusion model.Coal reservoir micropore,transitional pore and macropore specific surface area proportions have good correlation with corresponding diffusion coefficient ratios.LF-NMR method used to compute methane diffusion coefficient has better adaptability,but its diffusion result is slightly lower than from volumetric method.