Mechanism of adsorption-desorption hysteresis of shale gas in kerogen and influencing factors analysis

The adsorption behavior of methane in kerogen is of great significance for shale gas reserve evaluation and production prediction. However， there is still a lack of clear understanding of the hysteresis phenomenon and its micro-causes in the process of methane desorption. Therefore， based on the real kerogen model， the grand canonical Monte Carlo and molecular dynamics （GCMC-MD） coupling method was used to systematically investigate the adsorption-desorption behavior of methane under different conditions and the pore structure evolution characteristics of kerogen. The results show that the volumetric strain of kerogen shows a continuous growth during the methane adsorption process. This positive feedback mechanism of “adsorption， expansion， and readsorption” effectively expands the available pore space and significantly improves the methane storage capacity of the kerogen. Under the same pressure conditions， the methane’s absolute adsorption amount in the depressurization stage is higher than that in the pressurization stage， thus showing a significant hysteresis loop on the isothermal adsorption-desorption curve. The fundamental mechanism is that the kerogen skeleton undergoes partial irreversible structural deformation， making the thermodynamic path of the adsorption-desorption process do not coincide. The increase of temperature will weaken the interaction between methane molecules and pore wall， reduce the adsorption capacity， and alleviate the hysteresis in the desorption process. Under high temperature conditions， the overall deformation response ability of kerogen decreases. Compared with the type III-A kerogen， the type II-A kerogen has a higher proportion of micropores， which leads to a higher energy barrier to methane in the desorption process， thus aggravating the difficulty of desorption.