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

Zhou Shangwen; Wang Hongyan; Xue Huaqing; Guo Wei; Li Xiaobo

doi:10.3799/dqkx.2017.543

Abstract：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 10MPa, 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.