Phase 12 — Clay Seal (Final Barrier and System Closure): Capillary–Permeability Control in a Dynamic Multiphase Framework

This study presents Phase 12 of a unified dynamic multiphase model describing subsurface fluid systems, focusing on clay sealing as the final barrier controlling fluid migration and enabling system closure. Clay seals are low-permeability geological formations, typically composed of shale or clay-rich materials, that act as highly effective barriers to fluid flow. As fluids migrate upward and progressively lose mobility through earlier phases, they eventually encounter formations characterized by extremely low permeability, small pore throat sizes, and high capillary entry pressures. These conditions prevent further upward movement of fluids and create a closed system. Clay-rich layers act as combined mechanical and capillary barriers, simultaneously restricting flow through low permeability and high capillary resistance. Fluid movement ceases when the driving pressure falls below the capillary threshold, expressed as Λ = P_flow / P_c < 1. This condition defines the transition to stable accumulation. This phase represents the final control point of the system, where dominance shifts completely from fluid-driven dynamics to rock-controlled behavior. The system becomes closed, stable, and capable of long-term hydrocarbon accumulation. By integrating sealing processes into the dynamic framework, this phase provides a mechanism for definitive trapping, linking micro-scale capillary effects with large-scale geological barriers. It also enables the evaluation of sealing efficiency based on permeability and capillary properties. This publication is part of the research series: “A Dynamic Multiphase Model for Hydrocarbon and Hydrothermal Systems” It represents Phase 12 in a structured 13-phase framework describing the evolution of subsurface fluid systems from deep energy generation to accumulation. This phase defines the final sealing mechanism of the system, where low permeability and high capillary resistance combine to terminate fluid migration and enable stable accumulation.