Multicomponent Fluid (H2/CO2-CH4-H2O) Occurrence Characteristics in Shale Nanopores: Insights from MD Simulations

Abstract

Abstract Due to the emissions of greenhouse gasses, the climate change has greatly threatened people's living environment, urging to reduce CO2 in the atmosphere and promote carbon-free energy structures. Large-scale underground H2/CO2 storage emerges as a promising technology for H2 seasonal supply and CO2 reduction. The depleted shale gas reservoir presents a favorable site due to its abundance of nanopores, which inherently prevents leakage and ensures long-term storage. However, the coexistence of H2/CO2 with the original water in the nanopore results in the formation of a ternary mixture system (H2/CO2-CH4-H2O), as water is ubiquitous within organic-rich shale. This transformation leads to the primary CH4-H2O binary mixture interaction evolving into a H2/CO2-CH4-H2O ternary mixture interaction, introducing complexity to the intermolecular interactions within the nanopores and rendering the occurrence characteristics of multicomponent fluids uncertain. In this study, we constructed a shale organic nanopore using a realistic kerogen model and conducted molecular dynamics (MD) simulations to gain insights into the occurrence characteristics of multicomponent fluids within the organic nanopore. The results reveal that, due to their inherent affinities towards the kerogen, these two ternary mixture systems exhibit distinct occurrence characteristics. In the H2-CH4-H2O system, H2 predominantly exists near the pores without the formation of an adsorption dense layer near the walls, resulting in a relatively high diffusion capacity. However, in the CO2-CH4-H2O system, CO2 tend to occupy adsorption sites and strips CH4 into the bulk free region due to the more attractive interaction upon kerogen, greatly restricting CO2 diffusion capacity due to limited vacancies along the dense layer. Our work is expected to provide a deeper insight into multicomponent fluid occurrence characteristics within shale organic nanopores, which could serve as a guideline for large-scale underground H2/CO2 storage evaluations.