Density Characterization of Adsorbed Gas in Shale: Insights from Molecular Simulation to Application

The density of adsorbed gas is one of the important basic adsorption parameters for shale gas, but the research on its variation features mainly focuses on microscopic-scale simulations and cannot yet be mathematically characterized on a macroscopic scale; therefore, it is always approximately treated as an undetermined coefficient or a fixed value when using. Therefore, based on the molecular simulation theory and adsorption theory, a research method is innovatively proposed to clarify the macroscopic density changes of adsorbed gas and a scale-up application from micro- to macroscale is simultaneously studied. The results show that the density characteristics of adsorbed gas are similar to those of free gas, but with the increase of pressure, the density of adsorbed gas is initially greater than that of free gas and then becomes smaller. To describe the macroscopic change of adsorbed gas density, a mathematical model with a simple structure and convenient application is successfully established by considering the intrinsic properties of gas density and the analysis of the force differences between adsorbed gas and free gas. This new model, as an empirical formula, retains the characteristics of gas density and can be applied to calculations within a wide range of temperatures and pressures.