Nonequilibrium Phenomena in Multiphase Flow, Transport, and Phase Change in Porous Media: Pore-Level Physics, Network Modeling, and Upscaling

Abstract

Multiphase fluid flow, multicomponent transport, and thermodynamic phase change in porous media are important for many subsurface environmental and energy applications, including vadose zone contaminant transport and shale oil and gas production. Understanding these complex physical processes requires quantitative modeling approaches across scales---from pore-scale (e.g., nanometer to millimeter), to representative-elementary-volume (REV)-scale (e.g., centimeter to meter), to field-scale (e.g., meter to kilometer). To date, most field-scale models are built upon the thermodynamic equilibrium assumption at the REV-scale. However, this assumption is oftentimes violated due to the nonequilibrium physical processes in the complex pore spaces within each REV. This dissertation aims to test the equilibrium assumption and study the impact of pore-scale nonequilibrium physics on the REV-scale behaviors in subsurface environment and energy systems by developing novel pore-scale modeling frameworks. The first part of this dissertation studies the transport of a group of emerging contaminants named Per- and Polyfluoroalkyl Substances (PFAS) in the vadose zone. PFAS transport in vadose zone is complicated due to their adsorption at both bulk capillary air--water interfaces between soil grains and thin-water-film air--water interfaces on soil grain surfaces. So far, most state-of-art field-scale models assumed that PFAS can instantaneously reach chemical equilibrium in each REV and access all air--water interfaces therein. However, it remains unknown if all air--water interfaces are accessible by PFAS and how potential nonequilibrium mass exchange between the two types of air--water interfaces will control PFAS transport. To address this knowledge gap, I developed the first pore-scale model that represents the detailed physics controlling PFAS transport. The modeling results suggested that the pore-scale nonequilibrium mass-transfer processes can significantly impact PFAS transport in soil columns at the REV-scale. The second part of this dissertation investigates the flow, transport, and thermodynamic phase change behaviors of oil and gas in shale rocks. Shale oil and gas production has been challenging to characterize and predict due to two factors: (1) the abnormal thermodynamic phase change behaviors of hydrocarbon compounds in the nanometer-scale pore spaces within shale rocks, and (2) the nonequilibrium transport of oil/gas and hydrocarbon compounds in the complex pore spaces. To date, most field-scale models did not incorporate the abnormal thermodynamic phase change behaviors or the nonequilibrium transport of hydrocarbon compounds as controlled by the realistic nanometer-scale pore structures. To bridge this knowledge gap, I develop three novel pore-network models that represent the realistic pore structures and the aforementioned detailed physics. The modeling results revealed that the REV-scale transport and phase change behaviors of oil and gas are controlled by their abnormal phase change and nonquilibrium transport processes in the complex nanometer-scale pore spaces . Overall, this dissertation demonstrates the importance of considering pore-scale nonequilibrium physics in the field-scale modeling concepts for subsurface environment and energy problems.