Abstract
The specific surface area (SSA) is a crucial parameter for estimating the adsorption capacity of shale, significantly influencing its adsorption characteristics. The Brunauer-Emmett-Teller (BET) method was widely used to characterize the surface area of various porous materials. However, research on its applicability for characterizing shale surface areas, particularly concerning the adsorption mechanism of nitrogen in shale nanopores, remains limited. In this study, ultra-low-pressure nitrogen adsorption experiments and molecular simulation methods were employed to characterize the adsorption behavior of nitrogen on shale nanopore surfaces at 77 K. The results indicate that the assumptions of the classic BET isotherm model do not fully align with the state and microscopic mechanisms of nitrogen on shale surfaces. Nitrogen exhibits multilayer adsorption on shale surfaces represented by organic matter and Illite, but the initial pressure for multilayer adsorption varies with the rock phase surface. Calculating the specific surface area of organic matter in shale using the relative pressure range recommended by the classic BET theory results in a certain degree of error. Through analysis of isotherm adsorption curves, density field distributions, and intermolecular interactions, the adsorption mechanisms of nitrogen on shale pore surfaces were elucidated. It was found that for organic matter, a more suitable relative pressure range for BET calculations is 0.002-0.035, whereas for Illite, it is 0.035-0.2. This study provided crucial insights into the adsorption mechanisms of nitrogen on shale pore surfaces and the optimization of BET surface area characterization for shale nanopores, laying a theoretical foundation for predicting shale adsorption capacity and estimating in-situ natural gas in shale.