高压下页岩吸附特性及吸附异常原因分析

Analysis of shale adsorption characteristics and adsorption abnormal cause under high pressure

从分子学角度出发,探讨了页岩吸附量随压力变化趋势可能的内在原因。并结合模拟实验数据,分析了页岩在吸附后期出现吸附异常现象的原因。研究认为,甲烷分子在页岩表面的吸附与分子间引斥力的相互作用密切相关。在吸附初期,页岩表面能相对较大,甲烷分子和页岩表面分子间作用力以吸引力为主且逐渐增大,页岩吸附量随压力升高而增大且增加较快;在吸附中期,页岩表面能减小,甲烷分子和页岩表面分子间作用力由吸引力向排斥力转变,页岩吸附量随压力升高增加变缓达到最大值,此时吸附速率与解吸速率动态平衡;在吸附后期的超高压下,页岩表面能趋于零,甲烷分子和页岩表面分子间作用力以排斥力为主且增加较快,页岩吸附量随压力的升高而降低,甚至出现负值。页岩吸附异常的原因主要有两点:一是页岩具有相对低的吸附量,而且甲烷分子直径大于氦气,致使实验所用自由体积值偏大,造成计算结果"负吸附";二是高压下页岩中甲烷处于超临界吸附且具有较大的分子间斥力。

In view of molecular ,the possible internal cause of shale adsorption trend with pressure change was discussed .And according to simulation data ,the causes of shale adsorption abnormal in the late adsorption were analyzed .Results show that methane molecules adsorbed on the surface of the shale is closed related to the interaction of gravitation and repulsion between molecules .Early in the adsorption ,shale surface energy is relatively large ,the intermolecular force between methane molecules and shale surface molecules is gradually increasing gravitation ,which leads shale adsorption to increasing rapidly with pressure increasing .In the middle of adsorption ,shale surface energy decreases ,the intermolecular force between methane molecules and shale surface molecules transforms from gravitation to repulsion ,which leads shale adsorption to increasing slowly to maximum .At this point the adsorption rate and desorption rate are dynamic equilibrium .Under super high pressure in the adsorption late ,shale surface energy tends to zero ,the intermolecular force between methane molecules and shale surface molecules is rapidly increasing repulsion , which leads shale adsorption to decreasing with pressure increasing even negative .There are two main reasons for shale adsorption abnormal ：First ,a relatively low shale adsorption and the greater methane molecular diameter than helium result in larger free volume value in the experiment ,which leads to the results of “negative absorption”;Second ,shale methane is in a supercritical adsorption under high pressure＆amp;nbsp;and intermolecular has larger repulsion .