From Joule-Thomson cooling to secondary hydrate risk: Mechanistic insights from a one-dimensional flow experiment

Secondary hydrate formation is a critical bottleneck in natural gas hydrate production. The Joule-Thomson (J-T) effect, a key cooling source from long-distance gas flow, is often overlooked in conventional experiments. To address this, we developed a 4.9 m large-scale flow apparatus specifically designed to isolate the J-T effect from confounding multi-physics and reveal its dominant role in forming the Hydrate Thermodynamic Favorable Zone (HTFZ) for secondary hydrate formation. Its cumulative nature along the flow path and localized intensification near the outlet with high pressure gradient establish it as the dominant factor reshaping the reservoir temperature field. The production pressure drop is the critical operational driving force, with its magnitude dictating the entire lifecycle of the HTFZ: larger pressure drops shorten emergence time, accelerate the expansion rate, and intensify the ultimate cooling effect. In contrast, initial reservoir pressure sets the thermodynamic potential for reformation by establishing the equilibrium temperature threshold. This study reveals a consistent location for HTFZ emergence, determined by an optimal balance between sufficient cooling and adequate pressure. We also identify a 'Cascading Amplification Risk', where localized ice plugging triggers an upstream pressure surge that dramatically elevates the hydrate equilibrium temperature, causing instantaneous, widespread HTFZ expansion across the pre-cooled reservoir. Crucially, dynamic pressure control effectively suppresses the J-T effect, promoting a system-wide thermal recovery that reversibly eliminates the HTFZ. This work provides critical mechanistic insights and a foundational framework for developing new efficient-production strategies.