基于核磁共振的煤岩注CO_(2)吸附置换CH4试验研究

Experimental study on CO_(2) adsorption and displacement of methane in coals:An NMR relaxation method

注CO_(2)强化煤层气开采(CO_(2)-ECBM)过程的实质是吸附态甲烷在CO_(2)-CH_(4)竞争吸附作用下相变为游离态的过程,研究煤岩注CO_(2)吸附置换甲烷的动态化相变过程对于理解CO_(2)-ECBM微观作用机理和现场工程实施具有重要的理论意义和工程价值。基于低场核磁共振技术开展一系列不同压力状态下游离态甲烷实验,首先建立游离态甲烷核磁共振标定系数;随后选取准噶尔盆地南缘长焰煤和沁水盆地无烟煤,基于自主研发搭建的低场核磁共振CO_(2)置换CH_(4)装置开展不同注入压力、不同温度约束条件下CO_(2)吸附置换CH_(4)核磁共振实验,建立吸附态甲烷核磁共振标定系数,并定量评价不同煤阶煤样CO_(2)注入压力及实验温度对置换甲烷微观作用机理。结果表明:煤岩甲烷吸附核磁共振T_(2)弛豫谱呈三峰分布,从左至右分别代表了吸附态甲烷(P1峰)、颗粒间游离态甲烷(P2峰)和自由空间游离态甲烷核磁共振响应(P3峰);随着CO_(2)注入气体压力的增大,煤岩吸附态甲烷核磁共振信号幅度明显减小;而随着CO_(2)置换甲烷实验温度的增加,游离态甲烷核磁共振信号幅度(P2和P3峰)呈下降趋势。煤岩注CO_(2)吸附置换CH_(4)过程中,最大甲烷吸附量随CO_(2)注入气体压力的变化呈明显两阶段分布特征:低压阶段,最大甲烷吸附量随CO_(2)压力明显下降;高压阶段最大甲烷吸附量的变化趋于平缓;同时,煤储层温度的升高会降低煤样CO_(2)置换甲烷的能力。

The essence of CO_(2) injection for enhancing coalbed methane recovery(CO_(2)-ECBM)is the transformation of adsorbed methane to free phased due to the CO_(2)-CH_(4) competitive adsorption characterizations in coals.The investigation of the real-time dynamic evolution of native adsorbed and rejected non-adsorbed methane during the process of CO_(2)-ECBM has a dual desirable benefit both in theoretical and engineering research.The authors first determined the nuclear magmatic resonance(NMR)calibration coefficient of free methane based on a series of free methane NMR experiments,without coals in the sample cell.After that,two contrasting long flame coal and anthracite collected from the Southern Junggar Basin and Qinshui Basin,respectively,were applied for exploring the characterizations of methane desorption and CO_(2) displacement under different CO_(2) injection pressures or different experimental temperatures using the self-designed NMR-based CO_(2) displace methane equipment.The NMR calibration coefficients of adsorbed methane were determined and further used for quantitatively characterizing the dynamic process of CO_(2) adsorption and displacement in coals.Results show that the NMR transverse relaxation time(T2)distributions of methane adsorption in coals exhibit three peaks,labeled from left to right as P1,P2,and P3,corresponding to the NMR relaxation of adsorbed methane,free methane in inter-particles,and free methane occurs at headspace of the sample cell.With the increase of CO_(2) injection pressure,the NMR amplitude of adsorbed methane presents a significant decrease trend.Meanwhile,the NMR amplitude of free methane(P2 and P3 peak)decreases with the increase of experimental temperature.In the process of methane desorption and CO_(2) displacement,the maximum methane adsorption capacity following increase CO_(2) injection pressure occurs in two stages:a rapid initial decline under low gas pressure stages,followed by a slower tail in high gas pressure stages.Additionally,the CO_(2) displacement capacity decreases with the increase of coal reservoir temperature.