Molecular Simulation Analysis of Methane Adsorption Micromechanisms and the Impact of Water Saturation on Methane Adsorption in Transitional Shale

Abstract

Abstract Adsorption of gas is one of the ways that shale gas exists in the transitional shale (TS). The adsorption mechanism and the impact of water saturation on methane adsorption affect the estimated accuracy of the gas in place (GIP). This work constructs a TS molecular model comprising type III kerogen, kaolinite, illite, and quartz by using the molecular simulation (MS) according to the physical properties of the TS components. This is verified by the gas adsorption experiments of the TS and its components. The study establishes the methane adsorption model on the TS surface, the impact of the TS with varying water saturation on methane adsorption, and microscopic mechanisms of the TS methane adsorption. The experimental and simulated results show that it is feasible to construct a TS molecular model based on TS’s realistic porosity and density. Methane molecules in different mineral components of the TS are not characterized by monomolecular layer adsorption, mainly including micropore filling, transitional phase adsorption, and monomolecular layer adsorption. The transitional phase layer includes the strong adsorption layer (SAL), adsorption layer trough (ALT), and weak adsorption layer (WAL). Micropore filling and transitional phase adsorption coexist in the kerogen of the TS, and transitional phase adsorption primarily occurs in the illite of the TS. Kaolinite and quartz of the TS are featured with monomolecular layer adsorption. Moreover, quartz does not increase the pore space under high pressure because of stable silicon-oxygen chemical bonds. The chemical bonds in the kerogen and clay of the TS are relatively plastic, yielding the larger pore space under high pressure. Moreover, when water saturation exceeds 30% in the TS, water molecules cover the TS pore surface, affording the minimal methane adsorption capacity (MAC). In short, the micromechanisms of methane adsorption are determined by the TOC, adsorption model, water saturation, pressure, and isosteric heat of the TS components. The TS model can improve the accuracy of methane adsorption simulation and better understand the micromechanisms of methane adsorption. The establishment of the TS model and the impact of water saturation on methane adsorption also improves the estimated accuracy of the GIP.