Molecular simulations of adsorption, phase behavior, and reactivity in subsurface fluid-solid systems
Keywords: Engineering.
Mechanical Engineering.

Addressing the joint climate and energy crises requires a fundamental understanding of storage processes for energy carriers and greenhouse gases in the subsurface. Unconventional geological formations, such as shale and mafic rocks, offer promising storage sites both because of their prevalence around the world and because of their nanoporous structure. Inside nanopores, fluids, such as carbon dioxide (CO2) and light alkanes, interact with mineral surfaces. These interactions lead to physico-chemical properties that differ from those of bulk fluids, including layered density, shift in the phase envelope, and several different diffusion regimes. Understanding how fluid-rock interactions induce changes in adsorption, phase behavior, and reactivity remains an open research question. In this talk, we employ a suite of molecular simulation techniques along with theoretical analysis to address the following knowledge gaps: 1. How do bulk fluids self-organize at supercritical conditions, and what is the role of energetic interactions among molecules in this process? 2. How can one separate the fundamentally coupled influences of thermodynamics and pore geometry when describing the adsorption and phase behavior of nanoconfined fluids? 3. What is the corrosive influence of CO2 on crack propagation in quartz, and how does that crack growth mechanism compare to the mechanisms in other chemical environments? First, to understand the microstructural behavior and self-organization of fluids at the elevated temperature and pressure conditions of the subsurface, we utilize an energetic criterion to describe the fluid topology in terms of molecular clusters. By connecting fluid topology to thermodynamic conditions, we find that the structural response is well represented by a complex network whose dynamics, which arise from the energetics of isotropic molecular interactions, are described by a hidden variable network model. Next, with this knowledge of the fluid structure, we examine the impact of confinement. To decouple thermodynamic changes from the impact of pore geometry, we extend a Minkowski functionals framework to the study of real fluids in 3D quartz pores with surface roughness. This mathematical reconstruction agrees very well with molecular simulations data. Moreover, we identify the fluid molecular electrostatic moment as an important factor in the formation of adsorption layers along pore walls, where we demonstrate that the fluid undergoes a two-dimensional rearrangement. Finally, we focus on the role of the interfacial reactivity of CO2 in crack growth in quartz. By analyzing how the structural properties of quartz -- bond length distribution and crack tip shape -- evolve, we propose a crack growth mechanism for this environment. These results demonstrate that CO2 reduces the fracture toughness of quartz by 12.1% compared to quartz in vacuum, thereby promoting crack growth and enhancing fluid transport in the subsurface. The findings presented in this talk contribute to a more fundamental physical understanding of how fluids behave at elevated temperature and pressure conditions in nanoconfinement. This knowledge is a prerequisite to developing more accurate constitutive models and reduced-order upscaling techniques for fluid-rock interactions in the subsurface