Phase 6 — Behavior Change (Fluid Mobility Transition): From Free Flow to Restricted Regimes in a Dynamic Multiphase Fr

This study presents Phase 6 of a unified dynamic multiphase model describing subsurface fluid systems, focusing on the transition of fluid behavior from highly mobile to restricted flow conditions. This phase defines a critical transformation stage where fluid mobility decreases progressively due to changes in physical properties and geological constraints. As fluids migrate through the subsurface, cooling effects (Phase 5), layered geological control (Phase 3), and ongoing interactions with the surrounding medium collectively lead to increasing viscosity, reduced pressure gradients, and enhanced capillary resistance. These processes drive a continuous transition from a free-flow regime toward a restricted-flow regime. Fluid mobility, defined as M = k/μ, decreases as viscosity increases, directly reducing flow capacity. As the system evolves, the driving pressure approaches the capillary threshold, expressed through the parameter Λ = Pflow / Pc, which trends toward unity during this phase. This marks the transition zone where fluid motion becomes increasingly constrained and sensitive to geological controls. Physically, this phase represents a shift in system dominance: from fluid-driven behavior to rock-controlled dynamics. Fluids become slower, more confined, and increasingly prone to accumulation as their ability to penetrate smaller pore spaces diminishes. This phase introduces the concept of a continuous mobility transition zone rather than an abrupt boundary between migration and trapping. It provides a key mechanism linking thermal evolution, flow dynamics, and the onset of accumulation, enhancing the predictability of subsurface fluid behavior. This publication is part of the research series: “A Dynamic Multiphase Model for Hydrocarbon and Hydrothermal Systems” It represents Phase 6 in a structured 13-phase framework describing the evolution of subsurface fluid systems from deep energy generation to accumulation. This phase defines the transition from fluid-dominated flow to rock-controlled behavior, marking the onset of conditions leading toward accumulation.