Adsorption-Desorption Hysteresis in Shale Formation: New Insights into the Underlying Mechanisms

The production from shale gas formation is a key contributor to meet the increasing demand for energy. The gas contained in shales takes both free and sorbed forms. While the free amount exhibits transport mechanisms similar to that of the conventional gas, the sorbed form is of different nature with the adsorption isotherms being invoked to describe the sorption capacity. Adsorption-desorption hysteresis has been experimentally observed and mostly was attributed to the capillary condensation effect. However, hysteresis was also observed for low critical temperature gases that do not undergo condensation at surface and subsurface conditions such as methane and carbondioxide, which cannot be linked to the capillary effect. Hence, some additional mechanisms are taking place. In this study, the adsorption-desorption hysteresis of methane in the organic matter of shales is studied through a molecular simulation approach that combines both the Grand Canonical Monte Carlo simulation and molecular dynamics to capture both sorption and organic matter-methane interactions. It was observed that sorption caused internal structural changes and swelling in organic matter. Such changes in the structure led to variation in the pore space available for sorption during adsorption and desorption paths and eventually to hysteresis. The assessment of the mechanical behavior of the organic matter revealed high vulnerability of the structure to swell upon changes in the pressure, hinting that swelling could serve as a potential mechanism of adsorption hysteresis. The observed hysteresis could lead to some uncertainty in the interpretation of therecoverable amount of sorbed gas by 10-30% for the considered cases. The reported findings cast more light into the underlying mechanisms behind hysteresis for better asset exploitation and management.

Automated summary

This study investigates the adsorption-desorption hysteresis of methane in shale through molecular simulation and reveals that adsorption causes structural changes and swelling in the organic matter, which may serve as a potential mechanism for hysteresis. The observed hysteresis leads to 10-30% uncertainty in the interpretation of recoverable adsorbed gas.