Study on Gas-Phase State Evolution and Dynamic Mechanism of High-Temperature and -Pressure Microfluidic-Saturated CO2 Mixture System

Summary The pore-scale evolution of the gas phase during carbon dioxide (CO2) depressurization is a key control factor governing the storage efficiency and safety of CO2 geological storage and utilization [i.e., carbon capture, utilization, and storage (CCUS)]. However, the transient mechanisms underlying the sequential process of “gas precipitation-coalescence-temporary plugging” under reservoir conditions have not been fully elucidated, particularly in conventional sandstone media, which limits reliable prediction of CO2 distribution. In this work, a high-temperature, high-pressure (HTHP) microfluidic platform (80°C, 4–42 MPa) was developed to enable in-situ visualization and quantitative investigation of this phase evolution process. Three key stages were identified as (1) gas precipitation–dominated growth, where dissolved CO2 preferentially nucleates and expands along pore interfaces; (2) coalescence-induced temporary plugging, in which bubbles bridge pore throats and locally divert flow, governed by capillary force, with a critical rupture pressure difference of ≈500 Pa; and (3) microbubble-liquid (two-phase) migration/flow coordinated, where secondary nucleation and microbubble coalescence enhance gas connectivity and reshape the flow network. We further quantified the evolution of bubble size distribution and established a capillary-dominated stability criterion for temporary plugging. These findings provide mechanistic insight and a pore-scale basis for predicting CO2 transport and optimizing injection-production strategies in CCUS projects, particularly in conventional sandstone reservoirs.