基于低场核磁共振原位监测的煤岩甲烷吸附解吸动力学研究

Study on kinetics of methane adsorption and desorption in coal based on low-field nuclear magnetic resonance in situ monitoring

吸附态甲烷在深部煤岩储层微纳米孔隙中占主导地位,研究其在吸附解吸中的扩散特征对于煤层气开发的预测与现场实施具有重要指导意义。基于自主研发搭建的体积法吸附原位低场核磁试验系统,研究了沁水盆地无烟煤样品的吸附解吸动力学特性,实现了吸附态甲烷在扩散过程中的原位动态监测。结果表明:核磁法所测吸附态有效扩散系数与体积法所测微孔扩散系数呈现相同演化规律:即在吸附过程中,吸附态甲烷的扩散系数随压力的增加呈现先增加后减小的规律;在解吸过程中,其随压力的减小而单调递减。该演化规律表明微孔填充过程中气体扩散分为两个阶段:气体压力小于6 MPa时,分子扩散及努森扩散占主导地位;当气体压力超过6 MPa时,吸附相表面扩散占主导地位。该实验设计为煤层气高效开发提供了重要的实验方法及数据,对实验室核磁共振分析设备的教学以及针对特定科学问题进行的设备升级改造具有重要指导意义。

[Objective]Adsorbed methane dominates the micro-and nanopore spaces of deep coal reservoirs.The diffusion characteristics during adsorption and desorption are crucial for predicting and implementing coalbed methane development.This study employed an in situ low-field nuclear magnetic resonance(NMR)test system to investigate the kinetic characteristics of adsorption and desorption in anthracite samples from the Qingshui Basin,enabling dynamic monitoring of methane adsorption during diffusion.[Methods]The coal core sample was placed in a core holder at a constant temperature of 35℃,with a confining pressure applied to maintain a constant effective stress of 2 MPa during methane gas adsorption.The initial gas pressures were set at 1.69,2.99,5.08,8.7,11.41,and 13.50 MPa.The reference tank was then connected to the sample tank to conduct adsorption kinetics experiments.High-precision pressure sensors were used to track the pressure decay process of the adsorption system,while low-field NMR imaging continuously monitored the nuclear magnetic signal of methane in the coal core to generate T2 relaxation spectra.[Results]The results indicate that gas diffusion during the adsorption phase can be divided into two stages as the micropores are filled.When the pressure is less than approximately 6 MPa,gas molecules are captured by adsorption sites on the micropore surfaces under strong confinement.Despite weak van der Waals force,molecules in the micropore centers can still transfer mass via molecular and Knudsen diffusions.Intermolecular collisions become more frequent,and increased pressure leads to a reduced Knudsen number,thereby decreasing the collision frequency between molecules and microporous surfaces and reducing the diffusion coefficient of adsorbed methane.When the gas pressure exceeded approximately 6 MPa,surface diffusion became dominant,and as the molecular surface coverage increased,the gas diffusion coefficient also increased.Because of the strong confinement in the micropores,the free energy required for the desorption of gas molecules is much greater than the heat energy released during adsorption,making spontaneous desorption difficult under pressure drops.Consequently,the effective diffusion coefficient desorption was significantly greater than that during adsorption.Weakly adsorbed gas molecules diffuse first,and as the pressure decreases,the proportion of gas molecules at weak adsorption sites gradually diminishes,whereas it increases at strong adsorption sites.Therefore,the diffusion coefficient of methane in the adsorbed phase gradually decreased.[Conclusions]The experimental design provides important methods and data for coalbed methane development.This method holds significant value for teaching NMR analysis techniques in laboratories and for upgrading and reconstructing equipment tailored to specific scientific challenges.