基于Ono-Kondo格子模型的页岩气超临界吸附机理探讨

页岩气吸附机理的研究对于页岩气成藏和储量评价具有重要意义.甲烷在地层温度和压力条件下处于超临界状态，页岩气的吸附实际上为超临界吸附，但其机理目前尚不明确.在建立Ono-Kondo格子模型的基础上，结合低温氮气吸附和高压甲烷等温吸附实验，对龙马溪组页岩的微观孔隙结构和超临界吸附曲线进行了分析.结果表明，页岩中发育的孔隙尺度较小，比表面积较大，吸附气主要赋存于微孔和中孔中；页岩的等温吸附曲线在压力较大时，必然存在下降的趋势，这并非异常现象，而是超临界甲烷过剩吸附量的本质特征.Ono-Kondo格子模型对页岩高压等温吸附曲线的拟合效果很好，相关系数均在0.99以上，说明该模型可以表征页岩纳米孔隙中超临界甲烷的吸附特征.基于拟合得到的吸附相密度可将过剩吸附量转换为绝对吸附量，并直接计算地层温度和压力下甲烷的吸附分子层数，计算层数均小于1，表明甲烷分子并没有铺满整个孔隙壁面.因此受流体性质、吸附剂吸附能力和孔隙结构3个方面的影响，页岩气的吸附机理为单层吸附，不可能为双层甚至多层吸附. 

Discussion on the Supercritical Adsorption Mechanism of Shale Gas Based on Ono-Kondo Lattice Model

Studies on the adsorption mechanism of shale gas are of great significance to shale gas accumulation and reserves evaluation. Methane is in supercritical state at conditions under formation temperature and pressure, so the adsorption of shale gas is supercritical adsorption, however, its mechanismis still controversial. Based on Ono-Kondo lattice model, micropore structure and supercritical adsorption curves of Longmaxi shales were analyzed, combining with low temperature N₂ adsorption and high pressure CH₄ adsorption experiments. The results show that its pore size is small but specific surface area is large, and adsorbed gas may mainly exist in the micro-mesopores. The excess adsorption capacity will reach a maximum value when the pressure reaches about 10 MPa, and then the excess adsorption capacity decreases with the increase of pressure. The decreasing phenomenon is not abnormal, but the essential characteristics of excess adsorption capacity of supercritical methane. The fitting effect of the high pressure isothermal adsorption curves by Ono-Kondo lattice model is very good with its correlation coefficient above 0.99, indicating that the model can characterize the process of supercritical methane adsorption. Based on the fitted absorbed phase density of methane, the excess adsorption was converted into absolute adsorption. Then the number of adsorbed molecular layers under formation temperature and pressure was calculated and they are all less than 1, showing that the methane molecules are not entirely covered on the whole pore wall. Considering the influence of supercritical fluid properties, adsorption capacity and pore structure of shale, the adsorption mechanism of shale gas should be monolayer adsorption instead of two-layer or multilayer adsorption.