Phase 13 — Accumulation (Reservoir Formation and Stabilization): Dynamic Equilibrium in a Multiphase Subsurface System

This study presents Phase 13, the final stage of a unified dynamic multiphase model describing subsurface fluid systems. This phase defines accumulation as the process through which migrating fluids become trapped and stored within porous and permeable rock formations beneath an effective seal, forming a stable reservoir. Following the progressive evolution of the system through the preceding phases—including mobility reduction, capillary control, barrier network formation, compositional modification, fluid–rock interaction, space development, and sealing—fluids reach conditions where further upward migration is no longer possible. At this stage, accumulation begins. Accumulation occurs when the driving pressure is insufficient to overcome capillary resistance (Λ = Pflow / Pc < 1), while storage capacity is available and fluid inflow may continue. The reservoir evolves into a dynamic equilibrium system in which inflow, outflow, and internal pressure balance over time. This phase introduces a mass balance framework in which accumulation depends on the relationship between inflow and outflow rates, as well as storage capacity defined by porosity and rock volume. The resulting system is not static but dynamically maintained, capable of long-term stability and potential replenishment. Accumulation represents the convergence of all prior processes in the model, transforming the subsurface into a fully developed reservoir system. It integrates flow dynamics, geological control, chemical interaction, and structural evolution into a unified framework that explains reservoir formation, variability, and persistence. This publication is part of the research series: “A Dynamic Multiphase Model for Hydrocarbon and Hydrothermal Systems” It represents Phase 13, the final stage in a structured 13-phase framework describing the complete evolution of subsurface fluid systems. This phase represents the final outcome of the multiphase system, where all preceding processes converge to form stable and dynamically maintained reservoirs.